Concentration- and rate-dependent electrophysiological effects of restacorin on isolated canine purkinje fibres

The cellular electrophysiological effects of restacorin, a new antiarrhythmic agent were studied using conventional microelectrode techniques in isolated dog cardiac Purkinje fibres. Restacorin (1-30 mumol/l) decreased the maximum rate of rise of the action potential upstroke and action potential amplitude while action potential duration measured at 90% of repolarization was shortened in a concentration-dependent manner during pacing at a constant basic cycle length of 500 ms. The effect of 10 mumol/l restacorin on maximal rate of rise of the action potential upstroke and on action potential duration measured at 90% of repolarization were also studied while varying the constant pacing cycle length between 300 and 5000 ms. The results of these studies indicated a rate-dependent effect of restacorin on the action potential characteristics examined. After abrupt changes in cycle length, 10 mumol/l restacorin slowed the fast component of the relation for restitution of action potential duration from 155.3 +/- 5.2 ms (control, n = 6) to 217.1 +/- 17.8 ms (n = 6, P less than 0.05). In the presence of restacorin (10 mumol/l), a second slow component for recovery of maximal action potential upstroke rising velocity was expressed having a time constants of 8.5 +/- 1.2 s. The range of premature action potential durations was significantly decreased (by 57.1%, P less than 0.01) by 10 mumol/l restacorin. These results indicate that the cellular electrophysiological effects produced by restacorin in dog cardiac Purkinje fibres best resemble those produced by recognized class Ic antiarrhythmic drugs.     

Electrophysiological effects of dridocainide on isolated canine,guinea-pig and human cardiac tissues

The cellular electrophysiological effects of dridocainide (EGIS-3966), a novel class I antiarrhythmic agent, was studied using conventional microelectrode techniques in canine cardiac Purkinje fibres and papillary muscle preparations obtained from humans and guinea-pigs. In each preparation, dridocainide (0.6–2 mol/l) decreased the maximum velocity of action potential upstroke (V_max) in a frequency-dependent manner, although marked differences were observed in its effects in Purkinje fibre and ventricular muscle preparations. In canine Purkinje fibres, action potential duration measured at 50% and 90% of repolarization was decreased, while action potential duration measured at 10% of repolarization was increased by dridocainide. In addition, the plateau of the action potential was depressed by the drug. These changes in action potential configuration were not observed in guinea pig or human papillary muscles. The offset kinetics of the dridocainide-induced V _max block were different in Purkinje fibres and in ventricular muscle: the slow time constant of recovery of V _max was estimated to be 2.5 s in dog Purkinje fibre and 5–6 s in human and guinea-pig papillary muscle. In guinea-pig papillary muscle, the rate of onset of the V _max block was 0.15 and 0.2 per action potential in the presence of 0.6 and 2 mol/l dridocainide, respectively. Dridocainide also decreased the force of contraction in this preparation. On the basis of the present results, dridocainide appears to posess mixed class LC and LA properties, with LC predominance in human and guinea-pig ventricular muscle. Present results also indicate that results of conventional classification of class I drugs may depend on the parameters chosen, as well as on the preparation selected.     

Agonist-antagonist properties of fluorescent opioid probes in the guinea-pig myenteric plexus-longitudinal muscle preparation

Summary The behavioral consequences of -adrenoceptor subsensitivity were investigated by determining whether a physiological response that is mediated by -receptors, isoproterenol-induced drinking (IID), would be reduced by subacute antidepressant/ ₂-antagonist treatmentThe coadministration of typical (e.g., imipramine) or atypical (e.g., mianserin) antidepressants with yohimbine or piperoxan twice daily for four consecutive days reduced IID. Both the time course as well as the magnitude of -adrenoceptor subsensitivity could be behaviorally demonstrated. In addition, the reduction in IID observed after coadministration of imipramine with yohimbine was a centrally mediated effect since it was observed after systemic (subcutaneous) and central (intraventricular) administration of isoproterenol. These results provide evidence that IID is an appropriate behavioral model to demonstrate -adrenoceptor subsensitivity following subacute antidepressant/ ₂-antagonist treatment.Paper presented in part at FASEB, St. Louis, MO, USA (Abstract: Fedn Proc 43:941, 1984)     

Biochemical and pharmacological effects of fluperlapine on noradrenaline and acetylcholine systems in some rodent,bovine and crustacean preparations

Summary Fluperlapine was compared with clozapine, chlorpromazine, haloperidol and imipramine regarding its effects on some cholinergic and noradrenergic animal systems.Fluperlapine and clozapine showed the most pronounced anticholinergic effects. Fluperlapine was equipotent with clozapine in displacing [³H]-QNB from muscarinic receptors of the calf cerebral cortex (IC₅₀ about 15 nM). In the mydriasis test in the mouse and in the crayfish hindgut bioassay the differences between fluperlapine and clozapine were small.Like the other antischizophrenic drugs tested, fluperlapine displayed a marked affinity for ₁-adrenoceptors (calf cerebral cortex: IC₅₀ about 10 nM) but a neglible affinity for ₂-adrenoceptors in the same tissue. Only clozapine showed a weak affinity for the latter receptor type.Fluperlapine was as effective as imipramine in antagonizing tetrabenazine-induced ptosis in the rat, the antiptotic effect remaining constant after up to ten daily drug administrations. Still, imipramine was stronger than fluperlapine as an inhibitor of the accumulation of [³H]-noradrenaline ([³H]-NA) in rat cerebral cortex slices. Fluperlapine's effects on the spontaneous and the electrically-induced release of [³H]-NA from rat cerebral cortex slices, with and without protriptyline, showed it to be an inhibitor of the reuptake of NA.The results indicate that the pharmacological profile of fluperlapine is similar to that of clozapine, with additional antidepressant properties.     

Evidence for adenosine receptor-mediated isoprenaline-antagonistic effects of the adenosine analogs PIA and NECA on force of contraction in guinea-pig atrial and ventricular cardiac preparations

Summary The effects of the adenosine agonists (–)-N⁶-phenylisopropyladenosine (PIA) and 5-N-ethylcarboxamideadenosine (NECA) on force of contraction, adenylate cyclase activity and normal as well as slow action potentials were studied in guinea-pig isolatedatrial (left auricles) andventricular preparations (papillary muscles).Inauricles PIA and NECA exerted concentration-dependent negative inotropic effects with similar potenticies (mean EC₅₀:0.05 mol l^–1 for PIA and 0.03 mol l^–1 for NECA). Similar results were obtained in the presence of isoprenaline.Inpapillary muscles PIA and NECA alone had no effect on force of contraction but produced negative inotropic effects in the presence of isoprenaline (mean EC₅₀:0.19 mol l^–1 for PIA and 0.10 mol l^–1 for NECA).In both preparations, the negative inotropic effects of PIA and NECA in the presence of isoprenaline were antagonized by the adenosine receptor antagonist 8-phenyltheophylline.In both preparations, PIA and NECA did not affect adenylate cyclase activity, both in the absence and presence of isoprenaline.Inauricles the negative inotropic effects of both nucleosides were accompanied by shortening of the action potential. This effect was also observed in the presence of isoprenaline. Inpapillary muscles the adenosine analogs did not detectably alter the shape of the normal action potential. Ca²⁺-dependent slow action potentials elicited in potassium-depolarized preparations also remained unaltered in the presence of PIA or NECA alone. However, the isoprenaline-induced enhancement of the maximal rate of depolarization of slow action potentials was attenuated by PIA or NECA.It is concluded that in guinea-pig atrial and ventricular cardiac preparations the adenosine analogs PIA and NECA exert isoprenaline-antagonistic effects on force of contraction via adenosine receptors the existence of which can thus be shown in a functional way. These receptors are not detectably coupled to the adenylate cyclase. The negative inotropic effect in theauricle is most likely due to a shortening of the action potential resulting from an activation of potassium channels, which in turn indirectly reduces the Ca²⁺ influx during the action potential. In theventricle the adenosine receptor is either not linked to these potassium channels or adenosine-sensitive potassium channels do not exist in the ventricle. Instead the activation of the receptor causes a decrease of the slow Ca²⁺ inward current but this effect is observed only when the slow Ca²⁺ inward current had previously been enhanced by a cyclic AMP-dependent mechanism.     

Effects of prostaglandin I2 synthesized in the endothelium and in the smooth muscle on mechanical properties of the canine thoracic aorta

Summary In circular-cut strips prepared from canine thoracic aorta, acetylcholine (ACh) and A23187 relaxed endothelium-intact tissues [E(+) preparations] pre-contracted with noradrenaline or excess concentrations of K. These relaxations were associated with marked increases in the amount of 6-keto PGF₁. After removal of the endothelium [E(–) preparations] the relaxation ceased, and the amounts of 6-keto PGF₁ were markedly reduced. In E(+) preparations, application of indomethacin attenuated the increase in 6-keto PGF₁ induced by ACh or A23187 in the presence of noradrenaline or high K, but not the endothelium-dependent relaxations. In E(–) preparations, ACh (0.1–10 M) neither increased the amount of 6-keto PGF₁ nor produced a contraction. In dispersed single endothelial cells, A23187 markedly increased but 118 mM K did not modify the amount of 6-keto PGF₁. Both noradrenaline and high K increased the production of 6-keto PGF₁ in the E(–) preparations but to a lesser extent than that in the E(+) preparations. This action was attenuated by indomethacin. The amplitude of the noradrenaline-and K-induced contractions was enhanced with indomethacin pretreatment in both E(+) and E(–) tissues. PGI₂-Na (10 nM), reduced the amplitude of noradrenaline-induced contractions, concentration dependently and to the same extent in both E(+) and E(–) preparations. These results indicate that synthesis of PGI₂ in the endothelium is not causally related to the endothelium dependent relaxation. PGI₂ synthesized in the endothelium may not act directly on the muscle tissue, but PGI₂ synthesized in the smooth muscle tissue may produce an inhibition of contraction.     

Effects of antiarrhythmic drugs on the extraneuronal accumulation of isoprenaline in perfused rat hearts

Summary The effects of class I, II, III and IV antiarrhythmic drugs (as classified by Vaughan Williams 1974), tetrodotoxin and ₂-adrenoceptor antagonists on the extraneuronal accumulation of isoprenaline were examined in isolated rat hearts perfused with ³H-isoprenaline (1 mol/l) and tropolone (100 mol/l) for 30 min at a constant flow rate (6.5 ml/min) at 40°C. Quinidine (class I), verapamil (IV), diltiazem (IV), dilazep (IV), nifedipine (IV), tetrodotoxin and butoxamine, at a concentration of 10 mol/l, significantly decreased the extraneuronal accumulation of isoprenaline.The present study demonstrated that quinidine (class I) and all of the calcium channel blockers (class IV) had potent inhibitory effects on the extraneuronal accumulation of isoprenaline. The concentrations of these drugs needed for this decrease were nearly comparable to those needed to suppress isoprenaline-tropolone-induced ventricular fibrillation (Sone et al. 1985a). The antiarrhythmic effects of quinidine and calcium channel blockers in this experimental model may be partly due to a decrease in the extraneuronal accumulation of isoprenaline.This study was supported in part by a Grant-in-Aid for Scientific Research (59570980) from the Ministry of Education, Science and Culture, Japan     

Apomorphine contracts and relaxes circular smooth muscle of guinea-pig stomach via action on adrenoceptor mechanisms

Summary Nanomolar concentrations of apomorphine caused contractions of the circular smooth muscle from the body region of the guinea-pig stomach, the response showing rapid tachyphylaxis. These contractions were antagonised by yohimbine but not by prazosin, haloperidol, propranolol or methysergide. Higher concentrations of apomorphine caused concentration-related relaxations of the stomach body which were not subject to tachyphylaxis. These were antagonised by propranolol but not by prazosin, yohimbine or haloperidol. Dopamine-induced contractions of the circular smooth muscle from the stomach body were antagonised by apomorphine in nanomolar concentration; acetylcholine-induced contractions and isoprenaline-, dopamine- and phenylephrine-induced relaxations were unaffected by apomorphine. Thus, it is concluded that the contraction of circular smooth muscle from the stomach body to apomorphine is mediated via an adrenoceptor with characteristics of the ₂-type, and that a partial agonist-antagonists action prevents subsequent contractile responses to apomorphine and dopamine. Relaxation caused only at higher concentrations of apomorphine is mediated via an adrenoceptor with characteristics of the -type.     

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.

     

Evaluation of P2-purinoceptor antagonists at two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli

The guinea-pig taenia coli possesses two relaxation-mediating receptors for nucleotides: a prototypic P_2Y-purinoceptor, which is activated by adenosine 5-O-(2-thiodiphosphate) (ADPßS), and a separate receptor for ,-methylene ATP (,-MeATP). Effects of several as yet incompletely characterized P₂-purinoceptor antagonists at these receptors were examined.The concentration-relaxation curve of ADPßS was shifted to the right by reactive blue 2, suramin, 8-(3,5-dinitro-phenylenecarbonylimino)-1,3,5-naphthalenetrisulphonic acid (XAMR0721; at 1000 M only), pyridoxalphosphate-6-azophenyl-2,5-disulphonic acid (iso-PPADS), pyridoxal 5-phosphate, trypan blue and Evans blue (at 320 M only). Schild plots for the antagonism of reactive blue 2, suramin, iso-PPADS and pyridoxal 5-phosphate against ADPßS had slopes <1. The concentration-relaxation curve of ,-MeATP was shifted to the right by reactive blue 2, suramin, XAMR0721, iso-PPADS, pyridoxal 5-phosphate and trypan blue but not by Evans blue (320 M). Schild plots for the antagonism of suramin, XAMR0721 and iso-PPADS against ,-MeATP had slopes >1. Only XAMR0721 differed clearly in potency against the two nucleotides: it was considerably more potent against ,-MeATP than against ADPßS. 2-Methylthio ATP (MeSATP; 1 M) and ATP (100 M) were degraded by pieces of taenia coli. All antagonists except trypan blue attenuated the degradation of either or one of the two nucleotides.The selective effect of XAMR0721 against ,-MeATP confirms the existence of two relaxation-mediating P₂-purinoceptors in guinea-pig taenia coli. Comparison of the apparent affinities of the antagonists for the two taenia coli receptors with affinities for the P_2X-purinoceptor of the rat vas deferens shows that reactive blue 2, suramin, iso-PPADS, pyridoxal 5-phosphate and trypan blue have little selectivity for any of the three receptors. XAMR0721, which has been shown to possess relatively high affinity for the P_2Y-purinoceptor in turkey erythrocytes, was very weak at the P_2Y-receptor of the taenia, thus supporting the existence of pharmacologic P_2Y-receptor subtypes. Evans blue, with little effect in the taenia coli but a marked effect in the rat vas deferens, is the most selective P_2X-(versus P_2Y-) purinoceptor antagonist presently known, although its effect on the degradation of nucleotides must be kept in mind.     

Peripheral effects of opioid drugs on capsaicin-sensitive neurones of the guinea-pig bronchus and rabbit ear

Summary The effect of a potent opioid agonist, [d-Met², Pro⁵]-enkephalinamide was investigated on two responses involving capsaicin-sensitive afferent neurones, namely, atropine-resistant contractions of the guinea-pig bronchus evoked by electrical field stimulation and the nociceptor stimulation to intraarterial injections of acetylcholine or capsaicin into the vascularly isolated rabbit ear. The hypotheses to be tested were whether (a) opioid receptor activation may inhibit mediator release from primary afferent neurones and (b) the opioid could exert an analgesic effect at a peripheral site of action. Non-cholinergic contractions of the guinea-pig isolated main bronchi due to electrical stimulation were concentration-dependently inhibited by [d-Met², Pro⁵]-enkephalinamide (10 nM–1 M). This effect was abolished by naloxone (1 M). Naloxone alone induced no change in the stimulation-evoked contractions of the bronchus, indicating that no endogenous opioid control was present. Substance P and neurokinin A induced bronchial contractions that were not influenced by [d-Met², Pro⁵]-enkephalinamide. This indicates that [d-Met², Pro⁵]-enkephalinamide inhibits electrically-evoked bronchial contractions by reduced mediator release from capsaicin-sensitive sensory nerve endings, since these contractions are most probably brought about by tachykinins, released from afferent neurones. Capsaicin-induced bronchial contractions were in contrast to electrical stimulation not influenced by [d-Met², Pro⁵]-enkephalinamide which suggests a different site of action. The activation of sensory neurones in the rabbit ear by i. a. injection of acetylcholine and capsaicin was not reduced under infusion of [d-Met², Pro⁵]-enkephalinamide (1 and 10 M) or lofentanil (1 and 10 M). The enhancement of the effect of acetylcholine by infusion of prostaglandin E₂ (0.15 M) also remained unchanged under infusion of 10 M [d-Met², Pro⁵]-enkephalinamide. A peripheral analgesic action of the two opioid agonists studied is therefore not indicated. Send offprint requests to F. Lembeck     

Effects of NRA0045, a novel potent antagonist at dopamine D4, 5-HT2A,and alpha1 adrenaline receptors,and NRA0160, a selective D4 receptor antagonist,on phencyclidine-induced behavior and glutamate release in rats

Rationale Administration of phencyclidine (PCP) to animals produces abnormal behavior such as hyperlocomotion, stereotyped behavior, and ataxia; this abnormal behavior is only weakly blocked by dopamine D₂ receptor antagonists. This study examined the effects of a novel thiazole derivative, NRA0045 which potently antagonizes not only dopamine D₄ receptors but also 5-HT_2A and ₁ adrenaline receptors, and NRA0160, a selective dopamine D₄ receptor antagonist, on PCP-induced abnormal behavior, and accompanying increases in extracellular levels of glutamate in the medial prefrontal cortex. Furthermore, this study compared the effects of these drugs with those of clozapine and haloperidol.Methods To study the effects of NRA-drugs, atypical and typical antipsychotics, we measured locomotor activity with an infra-red sensor, and stereotypy and ataxia on a rating scale. Extracellular glutamate levels were measured by in vivo microdialysis.Results NRA0045 (1 or 3 mg/kg) or clozapine (1 mg/kg) attenuated hyperlocomotion, stereotypy, and ataxia induced by PCP (7.5 mg/kg) without affecting behavior after saline injection. Although haloperidol (0.1 or 1 mg/kg) attenuated or inhibited PCP-induced behavior, this drug also affected behavior after saline injection. NRA0160 (0.1, 1, or 3 mg/kg) had no effect on behavior induced by PCP or saline. NRA0045 (3 mg/kg), but not NRA0160, inhibited PCP-induced increases in glutamate levels in the medial prefrontal cortex. PCP-induced hyperlocomotion correlated with the PCP-induced increases in glutamate levels in this brain region.Conclusions These results suggest that the effects of NRA0045 on PCP-induced abnormal behavior are similar to those of the atypical antipsychotic clozapine. NRA0045 probably attenuates PCP-induced abnormal behavior by inhibiting the PCP-induced increase in glutamate levels in the medial prefrontal cortex; this inhibition may be mediated via the blockade of 5-HT_2A receptors.     

Corelease of noradrenaline and ATP by brief pulse trains in guinea-pig vas deferens

Contractions and overflow of tritium and ATP elicited by single electrical pulses or short pulse trains were studied in the guinea-pig isolated vas deferens preincubated with [³H]-noradrenaline. ATP was measured using the luciferase technique.A single pulse caused only a small contraction and minimal tritium and ATP overflow. In contrast, trains of 6 pulses elicited marked contractions as well as tritium and ATP overflow. In experiments with 6 pulses/100 Hz, prazosin 0.3 M reduced the contraction by 73 %, did not change the evoked overflow of tritium, and reduced the evoked overflow of ATP by 85%. Suramin 300 M reduced the contraction by 69% but changed neither the evoked overflow of tritium nor that of ATP. The combination of prazosin 0.3 gM and suramin 300 M abolished the contraction, did not change the evoked overflow of tritium, and reduced the evoked overflow of ATP by 70%. When 6 pulses were applied at frequencies of 1, 2, 10 or 100 Hz, all responses increased with frequency up to a maximum at 10 Hz, but contractions and the evoked overflow of ATP increased with frequency to a greater extent than the evoked overflow of tritium. A similar frequency overflow relationship was observed when the medium contained prazosin 0.3 M and suramin 300 M (and evoked ATP overflow was greatly reduced). Yohimbine 1 M did not affect the overflow of tritium evoked by 6 pulses/100 Hz but increased that evoked by 6 pulses/10 Hz.The results demonstrate an overflow of both noradrenaline and ATP in response to short pulse trains. As observed previously for prolonged pulse trains, the major part of the evoked overflow of ATP was derived from non-neural cells. The ATP overflow remaining during ₁-adrenoceptor blockade by prazosin and P₂-purinoceptor blockade by suramin is likely to reflect neural release of ATP. The results support the view that release of ATP increases with frequency to a greater extent than release of noradrenaline. The latency for the onset of prejunctional ₂-autoinhibition in guinea-pig vas deferens is between 50 and 500 ms. Correspondence to: I. von Kügelgen at the above address     

Studies on the effect of two angiotensin-converting enzyme inhibitors,captopril and cilazapril,on platelet and vascular prostaglandin metabolism in vivo

Summary We have studied in 12 healthy volunteers the effects of two angiotensin-converting enzyme (ACE) inhibitors, captopril and cilazapril, on vascular and platelet prostaglandin metabolism, in a double-blind, placebo-controlled, randomized cross-over study. Formation of 6-keto-prostaglandin F₁ (6-keto-PGF₁) and thromboxane B₂ (TxB₂) was measured locally at the site of a microvascular injury. Similar amounts of TxB₂ and 6-keto-PGF₁ were generated following administration of either ACE inhibitor as compared to placebo. It is concluded that neither captopril nor cilazapril significantly influence vascular and platelet prostaglandin metabolism.Abbreviations ACE angiotensin-convertin enzyme - PGI₂ prostacyclin - TxA₂ thromboxane A₂ - 6-keto-PGF₁ 6-keto-protaglandin - F₁ TxB₂, thromboxane B₂     

Alpha2-adrenoceptor antagonism and other pharmacological antagonist properties of some substituted benzoquinolizines and yohimbine in vitro

Summary Comparison of pA₂ values for antagonism of clonidine induced inhibition of the electrically evoked contraction of the rat isolated vas deferens ( ₁-adrenoceptors) and antagonism of contractions to methoxamine on the rat isolated anococcygeus ( ₂-adrenoceptors) showed a group of substituted benzoquinolizines (Wy 25309, 26392 and 26703) to be more potent and more selective ₂-adrenoceptor antagonists than yohimbine.The benzoquinolizines and yohimbine enhanced stimulation-evoked overflow of tritium from rabbit isolated pulmonary arteries preloaded with [³H]-noradrenaline, as expected for ₂-adrenoceptor antagonists. In contrast to the results on the rat vas deferens, yohimbine was more potent than the benzoquinolizines. At higher concentrations all the -adrenoceptor antagonists reduced the stimulation-evoked contraction of the pulmonary artery.The benzoquinolizines were competitive antagonists of 5-hydroxytryptamine on the rat isolated ileum. Wy 25309 showed only weak activity (pA₂=5.21) whereas Wy 26703 was more potent (pA₂=7.25). Yohimbine was a potent antagonist of 5-hydroxytryptamine.Wy 26703 was the only compound to have histamine antagonist effects in the guinea pig isolated ileum and to antagonise the chronotropic effect of isoprenaline on the isolated atria of the guinea pig and in both instances activity was weak (pA₂ values 5.3 and 5.5 respectively). Yohimbine reduced the spontaneous beating of the atria at 3×10^–6 M. No compound at 10^–5 M antagonised acetylcholine on the guinea pig ileum. These novel substituted benzoquinolizines should be useful experimental compounds for the study of -adrenoceptors mediated responses.     

Behavioural evidence that different neurochemical mechanisms underly stretching-yawning and penile erection induced in male rats by SND 919, a new selective D2 dopamine receptor agonist

The behavioural effects induced in male Wistar rats by SND 919, a new drug reputed to have selective agonistic activity at D₂ dopamine (DA) receptors, were studied. The following aspects of behaviour were considered: motor activity, stretching-yawning (SY), penile erection (PE) and stereotyped behaviour (SB). Intraperitoneal injection (IP) of the drug (0.01–20 mg/kg) induced an SY syndrome in the form of a bell-shaped dose-response curve, the effect being maximal at the dose of 0.1 mg/kg and disappearing completely at 10 mg/kg. SND 919 also potently elicited PE; this latter effect, however, was not coincident with SY induction, being maximal at 1 mg/kg and persisting at 10 and 20 mg/kg. SND 919-induced SY was potently antagonized by pretreatment not only with the D₂ antagonist,l-sulpiride (20 mg/kg), but also with the ₂ antagonist, yohimbine (1, 3 mg/kg), and the more selective ₂ antagonist, idazoxan (1, 2 and 5 mg/kg). While sulpiride also decreased SND 919-induced PE, idazoxan at all doses and yohimbine at 1 mg/kg did not affect this behaviour. Inhibition of motor activity was induced by the D₂ agonist at low doses (0.05, 0.1 mg/kg), while at high doses (1, 10 and 20 mg/kg), it was actually replaced by a form of SB characterized by downward sniffing and licking. When, for comparison, the D₂ agonist, RU 24213 (0.1–20 mg/kg IP), was tested for PE, SY, motor activity and SB, it displayed a behavioural pattern very similar to that obtained with SND 919. Idazoxan (2 mg/kg), administered before RU 24213 (10 mg/kg), significantly antagonized the drug-induced SY, but not PE. The discussion centres on the specific neurochemical mechanisms presumably underlying the various forms of SND 919-induced behaviour and, in particular, PE and SY, which seem to differ, at least with respect to ₂ involvement.     

Pharmacological characterization of a nonpeptide bradykinin B2 receptor antagonist,FR165649, and agonist,FR190997

1. The nonpeptide bradykinin (BK) B₂ receptor antagonist, {"type":"entrez-nucleotide","attrs":{"text":"FR165649","term_id":"258310103","term_text":"FR165649"}}FR165649 (8-[2,6-dichloro-3-[N-[(E)-4-(N- methylcarbamoyl)cinnamidoacetyl] -N-methylamino] benzyloxy] -2 - methylquinoline), and agonist, {"type":"entrez-nucleotide","attrs":{"text":"FR190997","term_id":"257934014","term_text":"FR190997"}}FR190997 (8-[2,6-dichloro-3-[N-[(E)-4-(N-methylcarbamoyl) cinnamidoacetyl]-N-methylamino]benzyloxy]-2-methyl-4-(2-pyridylmethoxy)quinoline) have been identified. These compounds have a common chemical structure, and the 2-pyridylmethoxy group is the only structural difference between them.
2. Both {"type":"entrez-nucleotide","attrs":{"text":"FR165649","term_id":"258310103","term_text":"FR165649"}}FR165649 and {"type":"entrez-nucleotide","attrs":{"text":"FR190997","term_id":"257934014","term_text":"FR190997"}}FR190997 displaced [³H]-BK binding to B₂ receptors in guinea-pig ileum membranes, with an IC₅₀ of 4.7×10⁻¹⁰ M and 1.5×10⁻⁹ M, respectively. They also displaced [³H]-BK binding to B₂ receptors in human lung fibroblast IMR-90 cells, with an IC₅₀ of 1.6×10⁻⁹ M and 9.8×10⁻¹⁰ M, respectively.
3. In guinea-pig isolated ileum-preparations, {"type":"entrez-nucleotide","attrs":{"text":"FR165649","term_id":"258310103","term_text":"FR165649"}}FR165649 had no agonistic effect on contraction and caused parallel rightward shifts of the concentration-response curves to BK on contraction. Analysis of the data produced a nominal pA₂ value of 9.2±0.1 (n=5) and a slope of 1.4±0.1 (n=5). On the other hand, {"type":"entrez-nucleotide","attrs":{"text":"FR190997","term_id":"257934014","term_text":"FR190997"}}FR190997 induced concentration-dependent contraction of guinea-pig ilea with a pD₂ of 7.9±0.2 and the contraction was inhibited by a specific peptide bradykinin B₂ receptor antagonist, Hoe 140 (D-Arg-[Hyp³, Thi⁵, D-Tic⁷, Oic⁸]BK) in a non-competitive manner.
4. In IMR-90 cells, {"type":"entrez-nucleotide","attrs":{"text":"FR165649","term_id":"258310103","term_text":"FR165649"}}FR165649 had no agonistic effect on phosphatidyl inositol (PI) hydrolysis and caused parallel rightward shifts (approximately 200 fold shift at 10⁻⁷ M) of the concentration-response curves to BK on PI hydrolysis. {"type":"entrez-nucleotide","attrs":{"text":"FR190997","term_id":"257934014","term_text":"FR190997"}}FR190997 induced concentration-dependent PI hydrolysis in IMR-90 cells with a pD₂ of 8.4±0.1, and this effect was inhibited by Hoe 140.
5. These results indicate that {"type":"entrez-nucleotide","attrs":{"text":"FR165649","term_id":"258310103","term_text":"FR165649"}}FR165649 and {"type":"entrez-nucleotide","attrs":{"text":"FR190997","term_id":"257934014","term_text":"FR190997"}}FR190997 are, respectively, a potent bradykinin B₂ receptor antagonist and agonist, and that the agonistic activity depends on the small part of the nonpeptide ligand. {"type":"entrez-nucleotide","attrs":{"text":"FR165649","term_id":"258310103","term_text":"FR165649"}}FR165649 and {"type":"entrez-nucleotide","attrs":{"text":"FR190997","term_id":"257934014","term_text":"FR190997"}}FR190997 may be useful tools for studying the relationship between ligands and 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.

     

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.