Effects of dopamine on L-type Ca2+ current in single atrial and ventricular myocytes of the rat

1. The effects of dopamine on the L-type Ca²⁺ current (I_Ca,L) of both atrial and ventricular single myocytes and on the force of contraction of atrial trabeculae in rat heart were investigated.
2. Dopamine increased atrial I_Ca,L at concentrations higher than 1 μM, but had little or no effect on I_Ca,L at lower concentrations. The increase in I_Ca,L at high concentrations was reversed by propranolol and acetylcholine, but not by phentolamine. Activation and inactivation kinetics of I_Ca,L were not altered by dopamine.
3. In rat ventricular myocytes in which the D₄ receptor mRNA does not express, dopamine (20–100 μM) also increased the I_Ca,L amplitude and propranolol reversed this effect.
4. Clozapine, a potent D₄ receptor antagonist, blocked the augmenting effect of dopamine on I_Ca,L. However, this effect could be explained by β-antagonism, since clozapine also inhibited the isoprenaline effect.
5. In the atrial trabeculae, the increase in contraction by dopamine (1 to 30 μM) was reversed by 1 μM propranolol, but not by 2 μM phentolamine. Low doses of dopamine (0.01 to 0.3 μM) did not affect the contraction in the controls or during a modest stimulation of the β-adrenoceptor with 0.01 μM isoprenaline.
6. These results indicate that the positive inotropic action of dopamine is mediated through direct stimulation of the β-adrenoceptor in both atrial and ventricular myocytes. Involvement of D₄ receptor appears unlikely in the regulation of the atrial contraction.

     

Dopamine stimulation of cardiac β-adrenoceptors: the involvement of sympathetic amine transporters and the effect of SKF38393

1. Mechanisms underlying β-adrenoceptor stimulation by dopamine were examined on guinea-pig Langendorff-perfused hearts and isolated cells from the right atrium, by using the chronotropic effects and the enhancement of L-type Ca²⁺ current (I_Ca,L) in the presence of prazosin as indicators of β-adrenoceptor stimulation. Dopamine-induced overflow of noradrenaline (NA) concentrations was measured by high-performance liquid chromatography.
2. Dopamine caused positive chronotropic effects with an EC₅₀ of 2.5 μM and induced NA overflow with a similar EC₅₀ (1.3 μM). The chronotropic effect of dopamine was abolished by bisoprolol (1 μM).
3. The effects of dopamine were maintained during prolonged application, whereas the effects of tyramine faded with time. Dopamine (3 μM) restored the chronotropic effects and the NA release suppressed by pretreatment with tyramine, suggesting a de novo synthesis of NA during the exposure to dopamine.
4. Dopamine (3 μM)-induced NA release was not affected by tetrodotoxin, ω-conotoxin, rauwolscine, ICI118551 or sulpiride, but was inhibited by desipramine, a NA uptake inhibitor (IC₅₀ ∼1 μM). It was also not affected by GBR12909 and bupropion, dopamine uptake inhibitors in the central nervous system.
5. {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393, a D₁ receptor partial agonist, potently inhibited the 3 μM dopamine-induced release of NA (IC₅₀ ∼0.1 μM). D₁ receptors are not involved in the DA-induced release of NA, since {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 (3 μM), a potent D₁ antagonist, inhibited the NA release only slightly, and dihydrexidine (1 μM) and chloro-APB (1 μM), full D₁ agonists, caused no significant NA release.
6. {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393 inhibited tyramine-induced overflow of NA, and potentiated the field stimulation-induced NA release. {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393 and desipramine retarded the decay of the stimulation-induced tachycardia in a similar manner. These results indicate that {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393 is a potent monoamine transport inhibitor and a useful tool for the functional evaluation of indirectly-acting sympathomimetic agonists in the heart. In the presence of {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393 (10 μM), dopamine at 1 μM showed no chronotropic effect.
7. Voltage clamp experiments with isolated atrial cells revealed that dopamine is a weak partial agonist. The EC₅₀ for I_Ca,L stimulation by dopamine was high (13 μM). As a result, dopamine at 1 μM did not affect I_Ca,L. Bisoprolol abolished the stimulation of I_Ca,L by dopamine (30 μM), and dihydrexidine (1 μM) did not affect I_Ca,L.
8. It was concluded that the cardiac effects of dopamine at clinically relevant concentrations (<1 μM) result almost exclusively from the indirect effect of β adrenoceptor stimulation, involving the release of NA from sympathetic nerve terminals. The roles of the direct stimulation of β adrenoceptors by dopamine at these concentrations and the stimulation of postjunctional D₁ receptors seem negligible. The desipramine- and {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393-sensitive monoamine transporter mediates the release of NA.

     

Involvement of 5-hydroxytryptamine7 receptors in inhibition of porcine myometrial contractility by 5-hydroxytryptamine

1. 5-Hydroxytryptamine (5-HT; 1 nM–100 μM) concentration-dependently inhibited the amplitude and frequency of spontaneous contractions in longitudinal and circular muscles of the porcine myometrium. The circular muscle (EC₅₀; 68–84 nM) was more sensitive than the longitudinal muscle (EC₅₀; 1.3–1.44 μM) to 5-HT. To characterize the 5-HT receptor subtype responsible for inhibition of myometrial contractility, the effects of 5-HT receptor agonists on spontaneous contractions and of 5-HT receptor antagonists on inhibition by 5-HT were examined in circular muscle preparations.
2. Pretreatment with tetrodotoxin (1 μM), propranolol (1 μM), atropine (1 μM), guanethidine (10 μM) or L-NAME (100 μM) failed to change the inhibition by 5-HT, indicating that the inhibition was due to a direct action of 5-HT on the smooth muscle cells.
3. 5-CT, 5-MeOT and 8-OH-DPAT mimicked the inhibitory response of 5-HT, and the rank order of the potency was 5-CT>5-HT>5-MeOT>8-OH-DPAT. On the other hand, oxymethazoline, α-methyl-5-HT, 2-methyl-5-HT, cisapride, BIMU-1, BIMU-8, ergotamine and dihydroergotamine had almost no effect on spontaneous contractions, even at 10–100 μM.
4. Inhibition by 5-HT was not decreased by either pindolol (1 μM), ketanserin (1 μM), tropisetron (10 μM), MDL72222 (1 μM) or {"type":"entrez-nucleotide","attrs":{"text":"GR113808","term_id":"238362519","term_text":"GR113808"}}GR113808 (10 μM), but was antagonized by the following compounds in a competitive manner (with pA₂ values in parentheses): methiothepin (8.05), methysergide (7.92), metergoline (7.4), mianserin (7.08), clozapine (7.06) and spiperone (6.86).
5. Ro 20-1724 (20 μM) and rolipram (10 μM) significantly enhanced the inhibitory response of 5-HT, but neither zaprinast (10 μM) nor dipyridamole (10 μM) altered the response of 5-HT.
6. 5-HT (1 nM–1 μM) caused a concentration-dependent accumulation of intracellular cyclic AMP in the circular muscle.
7. From the present results, the 5-HT receptor, which is functionally correlated with the 5-HT₇ receptor, mediates the inhibitory effect of 5-HT on porcine myometrial contractility. This inhibitory response is probably due to an increase in intracellular cyclic AMP through the activation of adenylate cyclase that is positively coupled to 5-HT₇ receptors.

     

Characterization of 5-HT receptors mediating constriction of porcine carotid arteriovenous anastomoses; involvement of 5-HT1B/1D and novel receptors

1. It was previously shown that porcine cranial arteriovenous anastomoses (AVAs) constrict to 5-hydroxytryptamine (5-HT), ergotamine, dihydroergotamine, as well as sumatriptan and that sumatriptan acts exclusively via 5-HT_1B/1D receptors. The present study was devoted to establish the contribution of 5-HT_1B/1D receptors in the constriction of AVAs elicited by 5-HT (in presence of 0.5 mg kg⁻¹ ketanserin), ergotamine and dihydroergotamine in anaesthetized pigs.
2. Intracarotid infusion of 5-HT (2 μg kg⁻¹ min⁻¹) and intravenous doses of ergotamine (2.5–20 μg kg⁻¹) and dihydroergotamine (3–100 μg kg⁻¹) reduced AVA and increased nutrient blood flows and vascular conductances. The vasodilator response to 5-HT, observed mainly in the skin and ear, was much more prominent than that of the ergot alkaloids.
3. Treatment with the 5-HT_1B/1D receptor antagonist {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 (0.5 mg kg⁻¹, i.v.) significantly attenuated both ergot-induced AVA constriction and arteriolar dilatation, whereas {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 only slightly affected the carotid vascular effects of 5-HT.
4. The results suggest that 5-HT constricts carotid AVAs primarily via receptors, which seem to differ from those (5-HT_1B/1D) stimulated by sumatriptan. The ergot alkaloids produce AVA constriction for a substantial part via 5-HT_1B/1D receptors, but also stimulate unidentified receptors. Both these non-5-HT_1B/1D receptors may be targets for the development of novel antimigraine drugs.
5. The moderate vasodilator response to the ergot derivatives seems to be mediated, at least in part, by 5-HT_1B/1D receptors, whereas the arteriolar dilatation caused by 5-HT may be mediated by other, possibly 5-HT₇ receptors.

     

Characterization of the endothelin receptor subtype mediating epithelium-derived relaxant nitric oxide release from guinea-pig trachea

1. The endothelin (ET) receptor subtype that mediates niric oxide (NO)-dependent airway relaxation in tracheal tube preparations precontracted with carbachol and pretreated with indomethacin was investigated. The release of NO induced by ET from guinea-pig trachea using a recently developed porphyrinic microsensor was also measured.
2. ET-1 (1 pM–100 nM) contracted tracheal tube preparations pretreated with the NO-synthase inhibitor, L-NMMA, and relaxed, in an epithelium-dependent manner, preparations pretreated with the inactive enantiomer D-NMMA. The effect of L-NMMA was reversed by L-Arg, but not by D-Arg.
3. The selective ET_B receptor agonists, IRL 1620 or sarafotoxin S6c, both (1 pM–100 nM) contracted tracheal tube preparations in a similar manner either after treatment with D-NMMA or with L-NMMA. In the presence of the ET_A receptor antagonist, {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317 (10 μM), ET-1 administration resulted in a contraction that was similar after either L-NMMA or D-NMMA. In the presence of the ET_B receptor antagonist, BQ788 (1 μM), ET-1 relaxed and contracted tracheas pretreated with D-NMMA and L-NMMA, respectively.
4. Exposure of tracheal segments to ET-1 (1–1000 nM) caused a concentration-dependent increase in NO release that was reduced by L-NMMA. IRL1620 (1 μM) did not cause any significant NO release. {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317 (10 μM), but not, BQ788 (1 μM), inhibited the NO release induced by ET-1.
5. These results demonstrate that in the isolated guinea-pig trachea activation of ET_B receptors results in a contractile response, whereas activation of ET_A receptors cause both a contraction, and an epithelium-dependent relaxation that is mediated by NO release.

     

Effects of the Aconitum alkaloid songorine on synaptic transmission and paired-pulse facilitation of CA1 pyramidal cells in rat hippocampal slices

1. The present study investigated the electrophysiological effects of songorine (1–100 μM), an alkaloid occurring in plants of the Aconitum genus, in rat hippocampal slices.
2. Songorine (10–100 μM) evoked a concentration-dependent increase in the amplitude of the orthodromic population spike and in the slope of the field e.p.s.p. The enhancement was long-lasting and was not reversed by up to 90 min of washout. Songorine failed to affect size and shape of the presynaptic fiber spike which represents the compound action potential of the Schaffer collaterals. This indicates that enhancement of the synaptic response is no consequence of an increased afferent excitability.
3. The antidromically evoked population spike was not affected by songorine at concentrations up to 100 μM suggesting that the enhancement of the orthodromic population spike and of the field e.p.s.p. was not due to an increase in pyramidal cell excitability.
4. The input-output curve for the postsynaptic population spike was shifted to the left implying that a presynaptic fiber spike of the same size elicited a larger postsynaptic response, indicating a decrease in threshold for generation of the population spike.
5. The songorine-evoked increase in excitability was not affected by the NMDA receptor antagonist, D-AP5. However, the effect of songorine was completely abolished by the selective dopamine D₂ receptor antagonist sulpiride (0.1 μM) as well as by haloperidol (10 μM) and was mimicked by application of the dopamine releaser, amantadine (100 mM). In contrast, the selective D₁ receptor antagonist, {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390, did not block the action of songorine.
6. The results indicate that the plant alkaloid songorine enhances excitatory synaptic transmission which may be due to an agonistic action at D₂ receptors.

     

κ1- and κ2-opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum

1. The effects of selective opioid receptor agonists and antagonists on N-methyl-D-aspartate (NMDA, 10 μM)-induced release of [³H]-dopamine and [¹⁴C]-acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional κ-opioid receptor subtypes in rat brain.
2. The κ receptor agonists (−)-ethylketocyclazocine ((−)-EKC), {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 and the endogenous opioid peptide dynorphin A_1–13 caused a naloxone-reversible inhibition of NMDA-induced [³H]-dopamine release, with pD₂ values of about 9, 8.5 and 8.2, respectively, whereas both the μ agonist Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) and theδ agonist D-Pen²-D-Pen⁵-enkephalin (DPDPE) were ineffective in this respect. The inhibitory effect of submaximally effective concentrations of dynorphin A_1–13, {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 and (−)-EKC on NMDA-induced [³H]-dopamine release were not changed by the δ₁/δ₂-opioid receptor antagonist naltrindole (up to a concentration of 1 μM), but reversed by the κ receptor antagonist nor-binaltorphimine (nor-BNI), with an IC₅₀ as low as 0.02 nM, indicating the involvement of {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593-sensitive κ₁-opioid receptors.
3. NMDA-induced [¹⁴C]-ACh release was reduced in a naloxone-reversible manner by DPDPE (pD₂ about 7.2), dynorphin A_1–13 (pD₂ 6.7) and EKC (pD₂ 6.2), but not by {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 and DAMGO. The inhibitory effect of a submaximally effective concentration of DPDPE, unlike those of dynorphin A_1–13 and (−)-EKC, on NMDA-induced [¹⁴C]-ACh release was antagonized by naltrindole with an IC₅₀ of 1 nM, indicating the involvement of δ-opioid receptors in the inhibitory effect of DPDPE. On the other hand, the inhibitory effects of dynorphin A_1–13 and (−)-EKC on [¹⁴C]-ACh release were readily antagonized by nor-BNI with an IC₅₀ of about 3 nM. A 100 fold higher concentration of nor-BNI also antagonized the inhibitory effect of DPDPE, indicating the involvement of {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593-insensitive κ₂-opioid receptors in the inhibitory effects of dynorphin A_1–13 and (−)-EKC.
4. Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative κ₃-opioid receptor, antagonized the inhibitory effects of dynorphin A_1–13 and (−)-EKC on [³H]-dopamine and [¹⁴C]-ACh release as well as that of {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 on [³H]-dopamine release, it displayed a low apparent affinity (IC₅₀ about 100 nM) in each case.
5. In conclusion, whereas activation of κ₁-opioid receptors causes presynaptic inhibition of NMDA-induced dopamine release, κ₂ receptor activation results in inhibition of ACh release in rat neostriatum. As such, this study is the first to provide unequivocal in vitro evidence for the existence of functionally distinct κ-opioid receptor subtypes in the brain.

     

Modulation of dopamine release from rat striatum by protein kinase C: interaction with presynaptic D2-dopamine-autoreceptors

1. Interactions between dopamine receptors and protein kinase C (PKC) have been proposed from biochemical studies. The aim of the present study was to investigate the hypothesis that there is an interaction between protein kinase C and inhibitory D₂-dopamine receptors in the modulation of stimulation-induced (S-I) dopamine release from rat striatal slices incubated with [³H]-dopamine. Dopamine release can be modulated by protein kinase C and inhibitory presynaptic D₂ receptors since phorbol dibutyrate (PDB) and (−)-sulpiride, respectively, elevated S-I dopamine release.
2. The protein kinase C inhibitors polymyxin B (21 μM) and chelerythrine (3 μM) had no effect on stimulation-induced (S-I) dopamine release. However, when presynaptic dopamine D₂ receptors were blocked by sulpiride (1 μM), an inhibitory effect of both PKC inhibitors on S-I dopamine release was revealed. Thus, sulpiride unmasks an endogenous PKC effect on dopamine release which suggests that presynaptic D₂ receptors normally suppress endogenous PKC activity. This is supported by results in striatal slices which were pretreated with PDB to down-regulate PKC. In this case the facilitatory effect of sulpiride was completely abolished.
3. The inhibitory effect of the dopamine D₂/D₃ agonist quinpirole on S-I dopamine release was partially attenuated by PKC down-regulation. Since the effect of sulpiride was completely abolished under the same conditions, this suggests that exogenous agonists may target a PKC-dependent as well as a PKC-independent pathway. The inhibitory effect of apomorphine was not affected by either polymyxin B or PKC down-regulation, suggesting that it operated exclusively through a PKC-independent mechanism.
4. These results suggest that there are at least two pathways involved in the inhibition of dopamine release through dopamine receptors. One pathway involves dopamine receptor suppression of protein kinase C activity, perhaps through inhibition of phospholipase C activity and this is preferentially utilized by neuronally-released dopamine. The other pathway which seems to be utilized by exogenous agonists does not involve PKC.

     

Tesofensine, a Novel Triple Monoamine Reuptake Inhibitor, Induces Appetite Suppression by Indirect Stimulation of ��1 Adrenoceptor and Dopamine D1 Receptor Pathways in the Diet-Induced Obese Rat

Tesofensine is a novel monoamine reuptake inhibitor that inhibits both norepinephrine, 5-HT, and dopamine (DA) reuptake function. Tesofensine is currently in clinical development for the treatment of obesity, however, the pharmacological basis for its strong effect in obesity management is not clarified. Using a rat model of diet-induced obesity (DIO), we characterized the pharmacological mechanisms underlying the appetite suppressive effect of tesofensine. DIO rats treated with tesofensine (2.0 mg/kg, s.c.) for 16 days showed significantly lower body weights than vehicle-treated DIO rats, being reflected by a marked hypophagic response. Using an automatized food intake monitoring system during a 12 h nocturnal test period, tesofensine-induced hypophagia was investigated further by studying the acute interaction of a variety of monoamine receptor antagonists with tesofensine-induced hypophagia in the DIO rat. Tesofensine (0.5–3.0 mg/kg, s.c.) induced a dose-dependent and marked decline in food intake with an ED₅₀ of 1.3 mg/kg. The hypophagic response of tesofensine (1.5 mg/kg, s.c.) was almost completely reversed by co-administration of prazosin (1.0 mg/kg, α₁ adrenoceptor antagonist) and partially antagonized by co-administration of {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 (0.03 mg/kg, DA D₁ receptor antagonist). In contrast, tesofensine-induced hypophagia was not affected by RX821002 (0.3 mg/kg, α₂ adrenoceptor antagonist), haloperidol (0.03 mg/kg, D₂ receptor antagonist), NGB2904 (0.1 mg/kg, D₃ receptor antagonist), or ritanserin (0.03 mg/kg, 5-HT_2A/C receptor antagonist). Hence, the mechanism underlying the suppression of feeding by tesofensine in the obese rat is dependent on the drug''s ability to indirectly stimulate α₁ adrenoceptor and DA D₁ receptor function.     

Mechanism of actions of sumatriptan on coronary flow before and after endothelial dysfunction in guinea-pig isolated heart

1. The mechanism of action of sumatriptan on coronary flow was examined before and after two different forms of endothelial ablation in guinea-pig isolated hearts. The mechanism was assessed in terms of the influence of the integrity of the coronary endothelium, the role of release of nitric oxide (NO) from the endothelium, and the receptor subtypes mediating the effects.
2. Continuous perfusion with sumatriptan reduced coronary flow, but the concentration-response curve was v-shaped. Sumatriptan (0.001–0.1 μM) caused a concentration-dependent decrease in coronary flow with the maximum effect achieved at 0.23±0.04 μM. The pEC₅₀ was 8.49±0.07. At higher concentrations (0.1–10 μM) there was a concentration-dependent diminution of the vasoconstrictor effect. Endothelial ablation by saponin removed the diminution in the vasoconstrictor effect. In contrast, pretreatment with N^G-nitro L-arginine methyl ester (L-NAME) (100 μM, 45 min perfusion) did not affect it. This was despite both saponin and L-NAME being effective in reducing basal release of NO into the coronary effluent (measured by chemiluminescence) to the same extent (71±3 and 73±2%, respectively).
3. {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935, a selective 5-hydroxytryptamine_1D (5-HT_1D) receptor antagonist (3 and 10 nM), which by itself had no effect on coronary flow or NO release, antagonized the vasoconstrictor response to sumatriptan and unmasked a sumatriptan-induced concentration-dependent increase in coronary flow and NO release. These increases in coronary flow and NO release were abolished by pretreatment with either saponin or L-NAME.
4. Mesulergine, a 5-HT₂ receptor antagonist which had no effect by itself on basal coronary flow or NO release, inhibited the vasodilator response to sumatriptan that occurred in the presence of {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935, and actually enhanced the vasoconstrictor response, increasing the maximum fall in coronary flow from −3.9±0.4 to −5.2±0.4 ml min⁻¹ g⁻¹ (P<0.05). The diminution of vasoconstrictor effect of sumatriptan was abolished by mesulergine and by pretreatment with saponin, but not by L-NAME.
5. In conclusion, guinea-pig coronary arteries constrict to low concentrations of sumatriptan, causing a reduction in coronary flow. This effect appears to be caused by 5-HT_1D agonism with the receptors located on the coronary vascular smooth muscle. With higher concentrations of sumatriptan this is partially offset by a weaker vasodilator effect, which is caused by low affinity 5-HT₂ agonism. Although this effect is endothelium-dependent, it is not caused by the release of NO. Interestingly, when the vasoconstrictor effect of sumatriptan was inhibited by the 5-HT_1D antagonist {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935, a high affinity vasodilator effect of sumatriptan was unmasked. This is 5-HT₂ receptor mediated and is caused by release of NO from the coronary endothelium.
6. In man, sumatriptan and 5-HT may both be capable of causing pathogenic coronary vasoconstriction. The implications of the present data are that the scope for this may depend greatly on (i) the extent of underlying endothelial dysfunction, (ii) the extent of endothelial 5-HT₂ receptor-mediated release of vasodilator autacoids (which include NO) and (iii) the extent of smooth muscle 5-HT_1D receptor-mediated vasoconstriction.

     

The role of dopamine D<Subscript>3</Subscript> compared with D<Subscript>2</Subscript> receptors in the control of locomotor activity: a combined behavioural and neurochemical analysis with novel,selective antagonists in rats

Background The role of dopamine D₃/D₂ receptors in the control of locomotion is poorly understood.Objectives To examine the influence of selective antagonists at D₃ or D₂ receptors on locomotion in rats, alone and in interaction with the preferential D₃ versus D₂ receptor agonist, PD128,907.Methods Affinities of ligands at rat D₂ and cloned, human hD₃, hD_2S, hD_2L and hD₄ sites were determined by standard procedures. Locomotion was monitored automatically in rats pre-habituated for 30 min to an open-field environment. Extracellular levels of dopamine (DA) were determined by dialysis in the nucleus accumbens and striatum. Drugs were given acutely via the systemic route.Results PD128,907, which preferentially recognised D₃ versus D₂ sites, biphasically reduced and enhanced locomotion at low (0.01–0.63 mg/kg) and high (2.5–10 mg/kg) doses, respectively. L741,626 and S23199, which behaved as preferential D₂ versus D₃ receptor antagonists, enhanced the reduction in locomotion evoked by the low dose of PD128,907, blocked the increase provoked by the high dose and suppressed spontaneous locomotion alone. Analogous findings were obtained with haloperidol and raclopride which showed equilibrated affinity at D₂ and D₃ receptors. UH232 and AJ76, which showed a mild preference for D₃ versus D₂ sites, did not modify the effect of a low dose of PD128,907, slightly enhanced the hyperlocomotion elicited by the high dose and exerted little influence on locomotion alone. S14297 and U99194, which acted as preferential D₃ versus D₂ receptor antagonists, abolished the reduction in locomotion elicited by a low dose of PD128,907, potentiated the induction of locomotion by a high dose, and failed to influence locomotion alone. The actions of S14297 were stereoselective inasmuch as they were mimicked by the racemic form, S11566, but not by the inactive enantiomer, S17777. In contrast to S14297, S11566 and U99194, however, S33084, SB269,652, GR218,231 and N-[-4-[-(1-naphtyl)piperazine-1-yl]butyl] anthracene-2-carboxamide (NGB-1), highly selective D₃ versus D₂ receptor antagonists, were inactive under all conditions. PD128,907 (0.01–10.0 mg/kg) suppressed dialysate levels of DA in the nucleus accumbens and striatum, actions blocked by L741,626 and haloperidol, yet unaffected by S14297 and S33084.Conclusions The facilitatory influence of a high dose of PD128,907 upon locomotion is mediated by postsynaptic D₂ receptors and, possibly, countered by their D₃ counterparts. Correspondingly, selective blockade of D₂ but not of D₃ receptors alone suppresses motor function. The reduction in locomotion provoked by a low dose of PD128,907 may be mediated by D₂ autoreceptors, but a role of postsynaptic D₃ receptors cannot be excluded. Finally, mechanisms underlying the contrasting influence of chemically diverse D₃ receptor antagonists upon locomotion remain to be elucidated.     

Selective antagonism of the GABAA receptor by ciprofloxacin and biphenylacetic acid

1. Previous studies have shown that ciprofloxacin and biphenylacetic acid (BPAA) synergistically inhibit γ-aminobutyric acid (GABA)_A receptors. In the present study, we have investigated the actions of these two drugs on other neuronal ligand-gated ion channels.
2. Agonist-evoked depolarizations were recorded from rat vagus and optic nerves in vitro by use of an extracellular recording technique.
3. GABA (50 μM)-evoked responses, in the vagus nerve in vitro, were inhibited by bicuculline (0.3–10 μM) and picrotoxin (0.3–10 μM), with IC₅₀ values and 95% confidence intervals (CI) of 1.2 μM (1.1–1.4) and 3.6 μM (3.0–4.3), respectively, and were potentiated by sodium pentobarbitone (30 μM) and diazepam (1 μM) to (mean±s.e.mean) 168±18% and 117±4% of control, respectively. 5-Hydroxytryptamine (5-HT; 0.5 μM)-evoked responses were inhibited by MDL 72222 (1 μM) to 10±4% of control; DMPP (10 μM)-evoked responses were inhibited by hexamethonium (100 μM) to 12±5% of control, and αbMeATP (30 μM)-evoked responses were inhibited by PPADS (10 μM) to 21±5% of control. Together, these data are consistent with activation of GABA_A, 5-HT₃, nicotinic ACh and P2_X receptors, respectively.
4. Ciprofloxacin (10–3000 μM) inhibited GABA_A-mediated responses in the vagus nerve with an IC₅₀ (and 95% CI) of 202 μM (148–275). BPAA (1–1000 μM) had little or no effect on the GABA_A-mediated response but concentration-dependently potentiated the effects of ciprofloxacin by up to 33,000 times.
5. Responses mediated by 5-HT₃, nicotinic ACh and P2_X receptors in the vagus nerve and strychnine-sensitive glycine receptors in the optic nerve were little or unaffected by ciprofloxacin (100 μM), BPAA (100 μM) or the combination of these drugs (both at 100 μM).
6. GABA (1 mM)-evoked responses in the optic nerve were inhibited by bicuculline with an IC₅₀ of 3.6 μM (2.8–4.5), a value not significantly different from that determined in the vagus nerve. Ciprofloxacin also inhibited the GABA-evoked response with an IC₅₀ of 334 μM (256–437) and BPAA (100 μM) potentiated these antagonist effects. However, the magnitude of the synergy was 48 times less than that seen in the vagus nerve.
7. These data indicate that ciprofloxacin and BPAA are selective antagonists of GABA_A receptors, an action that may contribute to their excitatory effects in vivo. Additionally, our data suggest that the molecular properties of GABA_A receptors in different regions of the CNS influence the extent to which these drugs synergistically inhibit the GABA_A receptor.

     

Investigation into the contractile response of melatonin in the guinea-pig isolated proximal colon: the role of 5-HT4 and melatonin receptors

1. The interaction of melatonin (N-acetyl-5-methoxytryptamine) with 5-hydroxytryptamine₄ (5-HT₄) receptors and/or with melatonin receptors (ML₁, ML₂ sites) has been assessed in isolated strips of the guinea-pig proximal colon. In the same preparation, the pharmacological profile of a series of melatonin agonists (2-iodomelatonin, 6-chloromelatonin, N-acetyl-5-hydroxytryptamine (N-acetyl-5-HT), 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT)) was investigated.
2. In the presence of 5-HT_1/2/3 receptor blockade with methysergide (1 μM) and ondansetron (10 μM), melatonin (0.1 nM–10 μM), 5-HT (1 nM–1 μM) and the 5-HT₄ receptor agonist, 5-methoxytryptamine (5-MeOT: 1 nM–1 μM) caused concentration-dependent contractile responses. 5-HT and 5-MeOT acted as full agonists with a potency (−log EC₅₀) of 7.8 and 8.0, respectively. The potency value for melatonin was 8.7, but its maximum effect was only 58% of that elicited by 5-HT.
3. Melatonin responses were resistant to atropine (0.1 μM), tetrodotoxin (0.3 μM), and to blockade of 5-HT₄ receptors by SDZ 205,557 (0.3 μM) and GR 125487 (3, 30 and 300 nM). The latter antagonist (3 nM) inhibited 5-HT-induced contractions with an apparent pA₂ value of 9.6. GR 125487 antagonism was associated with 30% reduction of the 5-HT response maximum. Contractions elicited by 5-HT were not modified when melatonin (1 and 10 nM) was used as an antagonist.
4. Like melatonin, the four melatonin analogues concentration-dependently contracted colonic strips. The rank order of agonist potency was: 2-iodomelatonin (10.8) >6-chloromelatonin (9.9) ⩾ N-acetyl-5-HT (9.8) ⩾5-MCA-NAT (9.6) >melatonin (8.7), an order typical for ML₂ sites. In comparison with the other agonists, 5-MCA-NAT had the highest intrinsic activity.
5. The melatonin ML_1B receptor antagonist luzindole (0.3, 1 and 3 μM) had no effect on the concentration-response curve to melatonin. Prazosin, an α-adrenoceptor antagonist possessing moderate/high affinity for melatonin ML₂ sites did not affect melatonin-induced contractions at 0.1 μM. Higher prazosin concentrations (0.3 and 1 μM) caused a non-concentration-dependent depression of the maximal response to melatonin without changing its potency. Prazosin (0.1 and 1 μM) showed a similar depressant behaviour towards the contractile responses to 5-MCA-NAT.
6. In the guinea-pig proximal colon, melatonin despite some structural similarity with the 5-HT₄ receptor agonist 5-MeOT, does not interact with 5-HT₄ receptors (or with 5-HT_1/2/3 receptors). As indicated by the rank order of agonist potencies and by the inefficacy of luzindole, the most likely sites of action of melatonin are postjunctional ML₂ receptors. However, this assumption could not be corroborated with the use of prazosin as this ‘ML₂ receptor antagonist'' showed only a non-concentration-dependent depression of the maximal contractile response to both melatonin and 5-MCA-NAT. Further investigation with the use of truly selective antagonists at melatonin ML₂ receptors is required to clarify this issue.

     

Differential influence of D1 and D2 dopamine receptors on acute opiate withdrawal in guinea-pig isolated ileum

1. The effects exerted by D₁ and D₂ dopamine agonists and antagonists on the acute opiate withdrawal induced by μ- and κ-receptor agonists were investigated in vitro.
2. Following a 4 min in vitro exposure to morphine (moderately selective μ-agonist), [D-Ala², Me-Phe⁴, Gly-ol⁵]enkephalin (DAMGO, highly selective μ-agonist) or U-50488H (highly selective κ-agonist) the guinea-pig isolated ileum exhibited a strong contracture after the addition of naloxone.
3. The non-selective dopamine receptor antagonist haloperidol when added before or after the opioid agonists, was able dose-dependently to prevent or to reverse the naloxone-induced contracture after exposure to μ- (morphine and DAMGO) and κ- (U-50488H) opioid agonists. The non-selective dopamine receptor agonist, apomorphine, was able to exert the same effects only at the highest concentration used.
4. The selective D₂ dopamine receptor antagonist, sulpiride, was also able to reduce dose-dependently both μ- and κ-opioid withdrawal, whereas the D₁-receptor selective antagonist SCH 23390 did not affect either μ- or κ-opioid withdrawal.
5. Bromocriptine, a D₂ selective dopamine receptor agonist was able to increase significantly, and in a concentration-dependent manner, the naloxone-induced contracture by μ- and κ-opioid agonists, whereas SKF 38393, a D₁ selective dopamine receptor agonist, increased only the withdrawal after morphine or U50-488H.
6. Our data indicate that both D₁ and D₂ dopamine agonists and antagonists are able to influence opiate withdrawal in vitro, suggesting an important functional interaction between the dopaminergic system and opioid withdrawal at both the μ- and κ-receptor level.
7. Furthermore, the ability of sulpiride to block strongly opiate withdrawal when compared to SCH 23390, as well as the effect of bromocriptine to increase opiate withdrawal suggest that D₂ dopamine receptors may be primarily involved in the control of opiate withdrawal.

     

Pharmacological characterization of endothelin-induced contraction in the guinea-pig oesophageal muscularis mucosae

1. In the oesophageal muscularis mucosae, we examined the effects of endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3) and sarafotoxin S6c (SX6c) as agonists, and {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317, BQ-123 and RES-701-1 as endothelin receptor antagonists.
2. All of the endothelins produced tonic contractions which were frequently superimposed on rhythmic motility in a concentration-dependent manner. The order of potency (−log EC₅₀) was ET-1 (8.61)=SX6c (8.65)>ET-2 (8.40)>ET-3 (8.18).
3. {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317 (1–3 μM) and BQ-123 (1 μM) caused parallel rightward shifts of the concentration-response curve to ET-1, but at higher concentrations caused no further shift. RES-701-1 (3 μM) caused a rightward shift of the concentration-response curve to ET-1, while RES-701-1 (10 μM) had no additional effect. RES-701-1 (0.1–1 μM) concentration-dependently caused a rightward shift of the concentration-response curve to SX6c. The contraction to ET-1 (10 nM) in preparations desensitized to the actions of SX6c was greatly inhibited by pretreatment with {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317 (10 μM).
4. Modulation of the Ca²⁺ concentration in the Krebs solution caused the concentration-response curve to ET-1 or SX6c to shift to the right and downward as external Ca²⁺ concentrations decreased. Verapamil (30 μM) abolished rhythmic motility induced by ET-1 or SX6c. Ni²⁺ (0.1 mM) weakly inhibited ET-1- or SX6c-induced tonic contraction. SK&F 96365 (60 μM) completely inhibited ET-1-induced contractions.
5. We conclude that there are two types of ET-receptors, excitatory ET_A- and ET_B-receptors in the oesophageal muscularis mucosae. These receptors mediate tonic contractions predominantly by opening receptor-operated Ca²⁺ channels (ROCs) and partly by opening T-type Ca²⁺ channels, and mediate rhythmic motility by opening L-type Ca²⁺ channels.

     

??-Amphetamine and Antipsychotic Drug Effects on Latent Inhibition in Mice Lacking Dopamine D2 Receptors

Drugs that induce psychosis, such as 𝒟-amphetamine (AMP), and those that alleviate it, such as antipsychotics, are suggested to exert behavioral effects via dopamine receptor D₂ (D₂). All antipsychotic drugs are D₂ antagonists, but D₂ antagonism underlies the severe and debilitating side effects of these drugs; it is therefore important to know whether D₂ is necessary for their behavioral effects. Using D₂-null mice (Drd₂−/−), we first investigated whether D₂ is required for AMP disruption of latent inhibition (LI). LI is a process of learning to ignore irrelevant stimuli. Disruption of LI by AMP models impaired attention and abnormal salience allocation consequent to dysregulated dopamine relevant to schizophrenia. AMP disruption of LI was seen in both wild-type (WT) and Drd₂−/−. This was in contrast to AMP-induced locomotor hyperactivity, which was reduced in Drd₂−/−. AMP disruption of LI was attenuated in mice lacking dopamine receptor D₁ (Drd₁−/−), suggesting that D₁ may play a role in AMP disruption of LI. Further supporting this possibility, we found that D₁ antagonist {"type":"entrez-protein","attrs":{"text":"SKF83566","term_id":"1157390490","term_text":"SKF83566"}}SKF83566 attenuated AMP disruption of LI in WT. Remarkably, both haloperidol and clozapine attenuated AMP disruption of LI in Drd₂−/−. This demonstrates that antipsychotic drugs can attenuate AMP disruption of learning to ignore irrelevant stimuli in the absence of D₂ receptors. Data suggest that D₂ is not essential either for AMP to disrupt or for antipsychotic drugs to reverse AMP disruption of learning to ignore irrelevant stimuli and further that D₁ merits investigation in the mediation of AMP disruption of these processes.     

Effects of clozapine on rat striatal muscarinic receptors coupled to inhibition of adenylyl cyclase activity and on the human cloned m4 receptor

1. Clozapine has recently been claimed to behave as a selective and full agonist at the cloned m4 muscarinic receptor artificially expressed in Chinese hamster ovary (CHO) cells. In the present study we have investigated whether clozapine could activate the rat striatal muscarinic receptors coupled to the inhibition of adenylyl cyclase activity, considered as pharmacologically equivalent to the m4 gene product. In addition, we have examined the effect of the drug on various functional responses following the activation of the cloned m4 receptor expressed in CHO cells.
2. In rat striatum, clozapine (1 nM–10 μM) caused a slight inhibition of forskolin-stimulated adenylyl cyclase activity, which was not counteracted by 10 μM atropine. On the other hand, clozapine antagonized the inhibitory effect of acetylcholine with a pA₂ value of 7.51. Moreover, clozapine (1 μM) failed to inhibit dopamine D₁ receptor stimulation of adenylyl cyclase activity, but counteracted the inhibitory effect of carbachol (CCh). Clozapine displaced [³H]-N-methylscopolamine ([³H]-NMS) bound to striatal M₄ receptors with a monophasic inhibitory curve and a pK_i value of 7.69. The clozapine inhibition was not affected by the addition of guanosine-5′-O-(thio)triphosphate (GTPγS).
3. In intact CHO cells, clozapine inhibited forskolin-stimulated cyclic AMP accumulation with an EC₅₀ of 31 nM. This effect was antagonized by atropine. CCh produced a biphasic effect on cyclic AMP levels, inhibiting at concentrations up to 1 μM (EC₅₀=50 nM) and stimulating at higher concentrations (EC₅₀=7 μM). Clozapine (0.3–5 μM) antagonized the CCh stimulation of cyclic AMP with a pK_i value of 7.47. Similar results were obtained when the adenylyl cyclase activity was assayed in CHO cell membranes.
4. In CHO cells pretreated with the receptor alkylating agent 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (10 μM), the maximal inhibitory effect of clozapine on cyclic AMP formation was markedly reduced, whereas the CCh inhibitory curve was shifted to the right with no change in the maximum.
5. As in rat striatum, in CHO cell membranes the displacement of [³H]-NMS binding by clozapine yielded a monophasic curve which was not affected by GTPγS.
6. Clozapine (10 nM–10 μM) had a small stimulant effect (∼20%) on the binding of [³⁵S]-GTPγS to CHO cell membranes, whereas CCh caused a 250% increase of radioligand binding. Moreover, clozapine (50 nM–5 μM) antagonized the CCh-stimulated [³⁵S]-GTPγS binding with a pA₂ value of 7.48.
7. These results show that at the striatal M₄ receptors clozapine is a potent and competitive antagonist, whereas at the cloned m4 receptor it elicits both agonist and antagonist effects. Thus, clozapine behaves as a partial agonist, rather than as a full agonist, at the m4 receptor subtype, with intrinsic activity changing as a function of the coupling efficiency of the receptor to effector molecules.

     

Characterization of putative 5-HT7 receptors mediating tachycardia in the cat

1. It has been suggested that the tachycardic response to 5-hydroxytryptamine (5-HT) in the spinal-transected cat is mediated by ‘5-HT₁-like'' receptors since this effect, being mimicked by 5-carboxamidotryptamine (5-CT), is not modified by ketanserin or MDL 72222, but it is blocked by methiothepin, methysergide or mesulergine. The present study was set out to reanalyse this suggestion in terms of the IUPHAR 5-HT receptor classification schemes proposed in 1994 and 1996.
2. Intravenous (i.v.) bolus injections of the tryptamine derivatives, 5-CT (0.01, 0.03, 0.1, 0.3, 1, 3, 10 and 30 μg kg⁻¹), 5-HT (3, 10 and 30 μg kg⁻¹) and 5-methoxytryptamine (3, 10 and 30 μg kg⁻¹) as well as the atypical antipsychotic drug, clozapine (1000 and 3000 μg kg⁻¹) resulted in dose-dependent increases in heart rate, with a rank order of agonist potency of 5-CT >> 5-HT > 5-methoxytryptamine >> clozapine.
3. The tachycardic effects of 5-HT and 5-methoxytryptamine were dose-dependently antagonized by i.v. administration of lisuride (30 and 100 μg kg⁻¹), ergotamine (100 and 300 μg kg⁻¹) or mesulergine (100, 300 and 1000 μg kg⁻¹); the highest doses of these antagonists used also blocked the tachycardic effects of 5-CT. Clozapine (1000 and 3000 μg kg⁻¹) did not affect the 5-HT-induced tachycardia, but attenuated, with its highest dose, the responses to 5-methoxytryptamine and 5-CT. However, these doses of clozapine as well as the high doses of ergotamine (300 μg kg⁻¹) and mesulergine (300 and 1000 μg kg⁻¹) also attenuated the tachycardic effects of isoprenaline. In contrast, 5-HT-, 5-methoxytryptamine- and 5-CT-induced tachycardia were not significantly modified after i.v. administration of physiological saline (0.1 and 0.3 ml kg⁻¹), the 5-HT_1B/1D receptor antagonist, {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 (500 μg kg⁻¹) or the 5-HT_3/4 receptor antagonist, tropisetron (3000 μg kg⁻¹).
4. Intravenous injections of the 5-HT₁ receptor agonists, sumatriptan (30, 100 and 300 μg kg⁻¹) and indorenate (300 and 1000 μg kg⁻¹) or the 5-HT₄ receptor (partial) agonist cisapride (300 and 1000 μg kg⁻¹) were devoid of effects on feline heart rate per se and failed to modify significantly 5-HT-induced tachycardic responses.
5. Based upon the above rank order of agonist potency, the failure of sumatriptan, indorenate or cisapride to produce cardioacceleration and the blockade by a series of drugs showing high affinity for the cloned 5-ht₇ receptor, the present results indicate that the 5-HT receptor mediating tachycardia in the cat is operationally similar to other putative 5-HT₇ receptors mediating vascular and non-vascular responses (e.g. relaxation of the rabbit femoral vein, canine external carotid and coronary arteries, rat systemic vasculature and guinea-pig ileum). Since these responses represent functional correlates of the 5-ht₇ gene product, the 5-HT₇ receptor appellation is reinforced. Therefore, the present experimental model, which is not complicated by the presence of other 5-HT receptors, can be utilized to characterize and develop new drugs with potential agonist and antagonist properties at functional 5-HT₇ receptors.

     

Involvement of 5-HT1B/1D and 5-HT2A receptors in 5-HT-induced contraction of endothelium-denuded rabbit epicardial coronary arteries

1. The receptors responsible for 5-hydroxytryptamine (5-HT)-mediated contraction of rabbit isolated epicardial coronary artery denuded of endothelium was examined by bioassay.
2. A variety of 5-HT mimetics caused concentration-dependent contractions. The rank order of agonist potency was 5-carboxamidotryptamine (5-CT)>5-HT>(±)-α-methyl-5-hydroxytryptamine ((±)-α-me-5-HT)=sumatriptan. This was not consistent with relative potencies at any single recognized 5-HT receptor, suggesting the presence of a mixed receptor population. In one subset of preparations precontracted with U46619 (10–30 nM) with the endothelium intact, none of the agonists caused a relaxation.
3. Contractions to 5-HT were antagonized by ketanserin, a 5-HT_2A-selective antagonist, but the displacement of concentration-response curves was inconsistent with an interaction between 5-HT and a single receptor population; the slope of regression between antagonist log M concentration and agonist log (concentration-ratio −1) was shallow (0.57). Responses to 5-HT were also antagonized by the 5-HT_1B/1D-receptor antagonist {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 and, again, the slope of regression was shallow (0.68). These data suggest a possible involvement of 5-HT_2A and 5-HT_1B or 5-HT_1D receptors in the response to 5-HT.
4. Contractions to (±)-α-me-5-HT, which is selective for 5-HT_2A over 5-HT_1B and 5-HT_1D receptors, were competitively antagonized by low concentrations of ketanserin. The regression between antagonist log M concentration and agonist log (concentration-ratio −1) fitted the Schild equation with a slope that was not significantly different from unity (0.95), giving a pA₂ value of 9.0. {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 (3–30 nM), had no effect on the contractile response to (±)-α-me-5-HT. These data establish, unequivocally, the presence of 5-HT_2A receptors in the tissue.
5. Sumatriptan, a relatively selective 5-HT_1B/1D-receptor agonist, induced contractions that were antagonized competitively by {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 (3–30 nM), although there was a reduction in the maximum response when concentrations of {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 exceeded 3 nM. The apparent pA₂ (estimated by imposing a unit slope on the log agonist (concentration-ratio −1) value in the presence of 3 nM {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935) was 8.92. Contractions to sumatriptan were not affected by low (5-HT_2A receptor-selective) concentrations of ketanserin, but were antagonized in a competitive manner at higher concentrations (pA₂ 6.5). These data appear to confirm the presence of 5-HT_1B and/or 5-HT_1D receptors in the tissue.
6. Antagonism of 5-HT responses by {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 was reassessed after blockade of 5-HT_2A receptors with 1 μM ketanserin. Under these conditions, {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 was able to antagonize 5-HT-induced contractions fully. The slope of regression between log M antagonist concentration and log agonist (concentration-ratio −1) fitted the Schild equation with a slope not significantly different from unity (1.1) (albeit there was still a reduction in maximum response when {"type":"entrez-nucleotide","attrs":{"text":"GR127935","term_id":"238377770","term_text":"GR127935"}}GR127935 concentration exceeded 3 nM). The apparent pA₂ value was 8.8. This reinforces the evidence that 5-HT_1B and/or 5-HT_1D receptors contribute to the effects of 5-HT in the tissue.
7. In conclusion, in endothelium denuded rabbit epicardial coronary arteries, 5-HT activates 5-HT_2A and 5-HT_1D and/or 5-HT_1B receptors to cause contraction. This appears to be similar to the situation in man.

     

Modulation of AT1 receptor-mediated contraction of rat uterine artery by AT2 receptors

1. The aim of this study was to characterize the angiotensin II receptors in isolated uterine arteries from non pregnant and pregnant rats, since it has been reported from binding studies that ovine uterine arteries contain AT₂ receptors.
2. Uterine arterial segments were obtained from virgin, non-pregnant and late pregnant (18–21 days) Sprague-Dawley rats and mounted in small vessel myographs. Concentration-response curves were constructed to angiotensin II (1 nM–10 μM) in the absence and presence of various angiotensin II receptor subtype selective compounds. These included losartan (AT₁ antagonist; 1, 10 and 100 nM), PD 123319 (AT₂ antagonist; 1 μM) and CGP 42112 (AT₂ agonist; 1 μM). Responses to angiotensin II were measured as increases in force (mN) and expressed as a per cent of the response to a K⁺ depolarizing solution.
3. Losartan (1, 10 and 100 nM) caused significant concentration-dependent rightward shifts of the angiotensin II concentration-response curve in uterine arteries from non-pregnant and pregnant rats. The pA₂ values calculated from these data were 9.8 and 9.2, respectively, although the slope of the Schild plot in the non-pregnant group was less than unity.
4. PD 123319 (1 μM) caused significant 6- and 3 fold leftward shifts of the angiotensin II concentration-response curve in uterine arteries from non-pregnant and pregnant rats, respectively. In vessels from pregnant rats, PD 123319 also significantly increased the maximum response to angiotensin II.
5. CGP 42112 (1 μM) attenuated the response to angiotensin II of uterine arteries from non-pregnant rats. This was reflected by a 14 fold rightward shift of the angiotensin II concentration-response curve and a decrease in the maximum response. In uterine arteries from pregnant rats, CGP 42112 (1 μM) caused a 3 fold rightward shift of the angiotensin II concentration-response curve, but had no effect on the maximum response.
6. PD 123319 (1 μM) and CGP 42112 (1 μM) had no effect on the concentration-response curves to phenylephrine (PE) of uterine arteries from non-pregnant or pregnant rats. In addition, CGP 42112 (1 nM–1 mM) had no vasodilator effect on tissues precontracted with phenylephrine.
7. These results suggest that the contractile responses of the rat uterine artery are mediated by the AT₁ receptor. Furthermore, in this vascular preparation, the AT₂ receptor appears to inhibit the response mediated by the AT₁ receptor, although, this is not uniform between the non-pregnant and pregnant states.