Mediation by 5-HT2 Receptors of 5-Hydroxytryptamine-Induced Contractions of Human Placental Vein

• 1.In isolated human placental chorionic vein segments, 5-hydroxytryptamine (5-HT; 10⁻⁸ to 5×10⁻⁵ M) elicited concentration-dependent contractions with EC₅₀=5.5 (5.2–5.7)×10⁻⁸ M) and E_max=93.1±7.3% of 75 mM KCl-induced contraction.
• 2.The agonist of 5-HT₂ receptors, α-methyl-5-hydroxytryptamine, and the selective agonist of 5-HT₁ receptors, N,N-dipropyl-5-carboxamidotryptamine and 5-carboxamidotryptamine, induced pronounced concentration-related contractions, which reached 71.1±6.0%, 53.0±5.0% and 75.0±7.8% at the highest dose tested, respectively. The agonist of 5-HT₃ receptor, 2-methyl-5-hydroxytryptamine, reached a maximum averaging 36.7±5.1% of the maximal response to KCl.
• 3.The 5-HT₁ and 5-HT₃ receptor antagonists, methiothepin and metoclopramide (10⁻⁷ to 10⁻⁶ M) did not alter the response to 5-HT. However, ketanserin (10⁻⁷ to 10⁻⁶ M), a 5-HT₂ receptor antagonist, induced significant inhibition of the concentration–response curve to 5-HT.
• 4.Contractile responses to 5-carboxamidotryptamine and 2-methyl-5-hydroxytryptamine were not affected by methiothepin and metoclopramide, respectively, whereas ketanserin significantly attenuated the contractile response to these agonists.
• 5.In conclusion, our study shows that 5-HT₂ receptors mediate contraction of the human placental vein with no obvious role for 5-HT₁-like, or 5-HT₃ receptors.

     

Characterization of 5-Hydroxytryptamine receptors in goat cerebral arteries

• 1.1. In isolated goat middle cerebral artery segments, 5-hydroxytryptamine (5-HT, 10⁻⁸-3 × 10⁻⁵ M) elicited concentration-dependent contractions with EC₅₀ = 2.1 (1.9−2.5) × 10⁻⁷ M and E_max = 64 ± 2% of 50 mM KCl-induced contraction.
• 2.2. Several 5-HT receptor agonists were used: (a) the agonist of 5-HT₂ receptors α-methyl-5-hydroxytryptamine (10⁻⁷ -3 × 10⁻⁴ M) induced strong contraction (51± 6%); (b) the selective agonists of 5-HT_1A receptors sumatriptan (10⁻⁸ - 10⁻⁵ M) and 5-carboxamidotryptamine (10⁻⁹ - 10⁻⁴ M) and the agonist of 5-HT_1A receptors 8-hydroxy-2-(di-n-propylamino)tetralin (10⁻⁷ - 3 × 10⁻⁵ M) induced weak contractions (8, 18 and 14%, respectively); and (c) the agonist of 5HT₃ receptors 2-methyl-5-hydroxytryptamine (3 × 10⁻⁶ - 10⁻⁴ M) induced almost negligible contraction.
• 3.3. Pretreatment with the antagonist of 5-HT_1A and 5-HT_1B receptors cyanopindolol (10⁻⁸, 10⁻⁶ M), the antagonist of 5-HT₁/5-HT₂ receptors methysergide (10⁻¹¹, 10⁻⁹ M) and the antagonist of 5-HT₂ receptors ketanserin (10⁻¹¹, 10⁻⁹ M) induced non-competitive inhibition of the concentration-response curve to 5-HT. The antagonist of 5-HT₃ receptors 3-trophanyl-3,5-dichlorobenzoate (10⁻⁷, 10⁻⁵ M) did not inhibit the contractile curve to 5-HT.
• 4.4. These results suggest that 5-HT contracts the goat middle cerebral artery by acting mainly on 5-HT₂ receptors.

     

5-HT receptors on identified Lymnaea neurones in culture: Pharmacological characterization of 5-HT3 receptors

• 1.1. The selective agonist, 1-(m-chlorophenyl)-biguanide (m-CPBG) and antagonist, 3-tropanyl-3,5-dichlorobenzoate (MDL 72222) were used to characterize the 5-HT₃ receptors in cultured identified neurones; the serotonin-containing cerebral giant cells (CGCs) and some follower neurones in the buccal ganglia of Lymnaea stagnalis.
• 2.2. 5-HT and its agonists were pressure ejected, while the 5-HT antagonists were bath applied.
• 3.3. Although m-CPBG evoked mostly depolarizing responses, hyperpolarizing responses were sometimes evoked.
• 4.4. At 10⁻⁴ M, m-CPBG failed to mimic the responses of 5-HT, but at a concentration higher. 10⁻³ M, pressure-ejected m-CPBG mimicked most 5-HT responses.
• 5.5. The 5-HT₂ antagonist ketanserin failed to block the m-CPBG-evoked responses, whilst partially blocking the 5-HT responses.
• 6.6. These results suggest the presence of 5-HT₃ receptors similar to those found in mammalian neurones, and that multiple subtypes of these receptors may be present in Lymnaea neurones.

     

Investigation into the effect of 5-hydroxytryptamine on fluid transport in the rat small intestine

• 1.1. 5-Hydroxytryptamine (5-HT) has been shown to cause a consistent secretory effect in the rat small intestine only when administered luminally or by close intraarterial infusion. Intraluminal 5-HT-induced secretion is possibly mediated by 5-HT₄ receptors. Therefore, it was decided to investigate the effect of 5-HT and selective 5-HT₄ receptor agonists (SC 53116 and DAU 6236) on intestinal fluid transport in rat jejunum and ileum. The study also investigated the effect of a selective 5-HT₄ receptor antagonist (GR 113808) against the intraluminally administered 5-HT.
• 2.2. 5-HT receptor agonists and antagonists were administered intraluminally in pentobarbitoneanesthetized rats. Changes in intestinal fluid transport across the intestinal wall were measured by a single pass technique.
• 3.3. Intraluminal 5-HT produced significant antiabsorptive effects is both the jejunum and ileum. The 5-HT-induced responses were blocked by intraluminal administration of the 5-HT₄ receptor antagonist GR 113808. The 5-HT₄ agonist SC 53116 induced antiabsorptive effects in both regions of the small intestine, but DAU 6236 did not affect the rates of fluid transport.
• 4.4. The results indicate that a 5-HT₄ receptor has a role in the luminal 5-HT-induced antiabsorptive effect on intestinal fluid transport in the rat.

     

5-HT3 receptor blocking properties of the antiparkinsonian agent,talipexole

• 1.1. Talipexole showed moderate displacement activity of ³H-GR 65630 binding to 5-HT₃ receptors in both rat cortical and intestinal membrane fractions with Ki values of 0.35 μM and 0.22 μM, respectively.
• 2.2. Bromocriptine failed to displace the binding activity in either experimental system even at a concentration of 10 μM.
• 3.3. Both talipexole and tropisetron were found to significantly inhibit 5-HT₃ receptor-mediated effects of 5-HT in isolated guinea-pig ileum or atrium; however, the effect of talipexole was weaker than that of tropisetron.
• 4.4. Bromocriptine, in contrast, had no antagonistic effects on 5-HT₃ receptor-mediated activity in guinea-pig ileum or atrium.
• 5.5. It was concluded that talipexole might act as an antagonist on 5-HT₃ receptors in both brain and intestinal tissues.

     

Modulatory mechanisms in the isolated internally perfused ventricle of the whelk Busycon canaliculatum

• 1.1. Isolated cannulated ventricles commenced spontaneous beating on application of perfusion pressure of 10 cm water. Complete hearts showed a fast patterned cyclical rhythm, whereas ventricles devoid of atrial material showed a continuous slow rhythm.
• 2.2. Perfused ventricles were inhibited by ACh with a threshold at 10^∞ mol 1⁻¹ and arrested at 10⁻⁷ mol 1⁻¹ and ventricles under stimulation by 5HT could be arrested by ACh at this concentration.
• 3.3. Perfused ventricles were stimulated by 5HT, with threshold at 10⁻⁹ mol 1⁻¹ and maximum at 10⁻⁵ mol 1⁻¹. Metoclopramide was without affect on 5HT responses, but metitipine and methysergide did inhibit such responses suggesting that the 5HT receptor present possessed mixed properties of the vertebrate 5-HT₁ and 5-HT₂ receptor subtypes.
• 4.4. Ventricles were very sensitive to the excitatory actions of FMRFamide in the 10^-9 to 10⁵ mol 1^-1 range. Preparations were insensitive to GAPFLRFamide, but SCP-B was modestly excitatory (threshold 10⁻⁷ mol l⁻9.
• 5.5. Preparations were not significantly affected by adenosine, ATP, and guanosine, but GTP was strongly excitatory at 10⁻⁷ mol 1⁻¹
• 6.6. 5HT and FMRFamide responses were additive. Preparations responded strongly to the adenylate cyclase activator forskolin and dibutyryl cAMP enhanced spontaneous contractions and 5HT responses, suggesting that the 5HT receptor may operate via a cAMP secondary mechanism.
• 7.7. The IP₃ inhibitor lithium (10 mmol l⁻¹), caused slight inhibition of FMRFamide responses, suggesting that the receptor to this peptide may operate via IP₃ as a second messenger.
• 8.8. Neuromodulation in this preparation would appear to involve ACh as inhibitor, 5HT and FMRFamide as upregulators, with no clear roles for FMRFamide-related peptides and GTP.

     

Pre- and postjunctional effects of 5-hydroxytryptamine in dog cephalic veins of neonates

• 1.1. This investigation was undertaken to determine the existence of pre- and post junctional effects of 5-hydroxytryptamine (5-HT) in the cephalic vein of newborn dogs. For the sake of comparison, some experiments were also carried out on the veins of adult animals.
• 2.2. 5-HT reduced, in a concentration-dependent manner the overflow of tritium evoked by electrical stimulation of tissues previously loaded with ³H-noradrenaline for the same range of concentrations (0.01–1μM 5-HT) in both neonates and adults. The maximal reduction of tritium overflow caused by 5-HT was 36±6% and 77±5% (n=4; P<0.001) in neonates and adults, respectively.
• 3.3. Postjunctionally, 5-HT caused concentration-dependent contractions of vessels from both neonates and adults and its EC₅₀ values were not significantly different at the two ages. The maximal effect of 5-HT was smaller in newborn (0.69±0.03 g/mg) than in adult (1.16±0.06 g/mg) animals (n=9; P<0.01).
• 4.4. Metitepin antagonized 5-HT effects while ketanserin was ineffective, both pre- and postjunctionally, in newborn and adult dogs.
• 5.5. It is concluded that there are 5-HT receptors at pre- and postjunctional level at birth and that these receptors probably belong to the 5-HT₁ type.

     

Different calcium dependencies of contractile activity of prostatic and epididymal portions of rat vas deferens

• 1.1. The effects of some organic calcium entry blockers and different concentrations of extracellular calcium on electrically-evoked contractions of isolated epididymal and prostatic portions of rat vas deferens were investigated.
• 2.2. Both epididymal and prostatic parts of rat vas deferens responded to single pulse or train electrical field stimulation, with twitch contractions of submaximal amplitude.
• 3.3. Verapamil showed a biphasic action on the contractions produced by single pulse electrical stimulation. In concentrations < 10⁻⁵M, it potentiated the responses of both portions, but at higher concentrations, the excitatory action was overcome by a concentration-dependent inhibitory effect.
• 4.4. Nifedipine reduced the amplitude of electrically-evoked contractions of both portions in a concentration-dependent manner. The ED₅₀ of nifedipine was 3.6 × 10⁻⁸M and 2.1 × 10⁻⁶M in prostatic and epididymal portions, respectively.
• 5.5. Dantrolene sodium reduced the amplitude of electrically-evoked contractions of both portions in a concentration-dependent manner. The ED₅₀ of dantrolene was 1.55 × 10⁻⁴M and 9.1 × 10⁻⁴M in prostatic and epididymal portions, respectively.
• 6.6. Reduction of Ca²⁺ concentration in medium reduced the amplitude of contractions of both portions significantly. This calcium dependence was more apparent in low frequencies of electrical stimulation.

     

Central and peripheral 5-HT receptors in the leech (Hirudo medicinalis) redefined

• 1.1. 5-Hydroxytryptamine (5-HT) receptors in the Retzius neurones (R cells) and longitudinal body wall muscle strips of the leech H. medicinalis were studied using a range of selective agonists developed for mammalian receptors.
• 2.2. All agonists induced hyperpolarisation of R cells: the order of their potency was 5-HT > 5-CT > 2-Me 5HT > α-Me-5-HT > > CGS-12066B=5MeOT > PAPP > Buspirone. This receptor is most like a mammalian 5-HT_1c/2 receptor.
• 3.3. 5-HT induces relaxation of body wall strips, preceded at higher doses by contraction or an increase in spontaneous contractions. The relaxing effect was best mimicked by 5-CT and α-Me-5-HT, suggesting a 5-HT_1c/2 receptor. The contractile effect was best mimicked by 5-MeOT suggesting a 5-HT₄-like receptor. The overall potency of agonists on the body wall muscle was 5-MeOT > 5-CT > 5-HT > α-Me-5-HT > 2-Me 5-HT > PAPP > Buspirone > > CGS-12066B.
• 4.4. Results suggest that either the sites of 5-HT action possess several different receptors or the receptors themselves are more complex with multiple properties.

     

Effects of tandospirone on second messenger systems and neurotransmitter release in the rat brain

• 1.1. We studied the effects of tandospirone, a novel serotonin (5-HT)_1A receptor-related anxiolytic, on the intracellular second messenger systems and neurotransmitter release.
• 2.2. Tandospirone inhibited forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes by activation of 5-HT_1A receptors and had high efficacy comparable to 5-HT_1A receptor agonists such as 5-HT and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT).
• 3.3. Tandospirone suppressed carbachol-stimulated phosphatidyl-inositol metabolism (PI response), which was shown to be a 5-HT_1A receptor-mediated event.
• 4.4. Tandospirone did not affect the release of 5-HT, norepinephrine (NE), dopamine (DA) and acetylcholine (ACh) from rat brain slice preparations.
• 5.5. These findings suggested that tandospirone shows high agonistic efficacy on the postsynaptic 5-HT_1A receptors but does not affect the presynaptic autoreceptors located on nerve endings. The modulation of the second messenger system via postsynaptic 5-HT_1A receptors might be involved in the anxiolytic efficacy of tandospirone.

     

Effects of cirsiliol,a flavone isolated from Achillea fragrantissima,on rat isolated ileum

• 1.1. In concentrations from 10⁻⁸ M to 3 × 10⁻⁴ M, cirsiliol caused concentration-dependent relaxation of rat isolated ileum.
• 2.2. Phentolamine (10⁻⁶ M) or phentolamine and propranolol (10⁻⁶ M) had no significant effects on the concentration-effect curves or on the EC₅₀ of cirsiliol on the ileum.
• 3.3. Cirsiliol shifted to the right the acetylcholine (Ach) concentration-effect curves on ileum and significantly inhibited the maximum contractions induced by Ach.
• 4.4. In Ca²⁺-free, depolarizing solution, cirsiliol shifted to the right the CaCl₂ concentration-effect curves and inhibited the maximum contractions induced by CaCl₂ on ileum.
• 5.5. Large concentrations (10⁻⁴ M, 3 × 10⁻⁴ M) of cirsiliol induced relaxation followed by contraction of the ileal segments incubated in Ca²⁺-free solution.
• 6.6. In Ca²⁺-free solution, cirsiliol (10⁻⁴ M, 3 × 10⁻⁴ M) caused concentration-dependent potentiation of the ileal contractions induced by 3 × 10⁻³ M Ach when the latter was added 2–3 min after cirsiliol. When Ach was added 15–20 min after cirsiliol, the latter compound inhibited the Ach-induced contractions.
• 7.7. These observations suggest that cirsiliol inhibits Ca²⁺ influx but stimulates Ca²⁺ release from intracellular stores. Furthermore, they suggest that cirsiliol utilizes the same Ca²⁺ source used by acetylcholine in Ca²⁺-free solution.

     

Neuromodulation in molluscan smooth muscle: the action of 5-HT,FMRFamide and purine compounds

• 1.1. The RR, OR, RS and RP muscles of Buccinum did not respond directly to 5-HT, but this monoamine converted their normally tonic ACh responses to fast twitch contractions with lowered tonic force. This action was not accompanied by significant membrane potential changes.
• 2.2. Pre-treatment with dibutyryl cAMP potentiated ACh responses and enhanced 5-HT modification of the responses.
• 3.3. All muscles responded strongly to FMRFamide with twitch contractions but this was not accompanied by significant membrane potential changes.
• 4.4. FMRFamide enhanced ACh contracture force and converted the responses into fast twitch activity. FMRFamide responses were dramatically inhibited by 5-HT with loss of all tonic force and fast twitch activity.
• 5.5. While dibutyryl cAMP did not affect FMRFamide responses, the IP₃ inhibitor lithium, at very high concentrations, caused a significant diminution of FMRFamide responses.
• 6.6. All four muscles were unresponsive to adenosine and ATP but all except the RP responded in a dose-dependent manner to GTP and GTP-γ-S over the 10⁻⁷-10⁻⁴ mol 1⁻¹ range. The responses showed moderate fast twich activity which was unaccompanied by action potential discharges. Guanosine was without effect, except at very high concentrations where it inhibited FMRFamide responses.
• 7.7. ACh and GTP acted additively to increase muscle force and to enhance ACh-induced depolarization. Similarly both GTP and GTP-γ-S acted additively, considerably enhancing FMRFamide responses.
• 8.8. It is proposed that 5-HT, FMRFamide and GTP may, via their separate receptors or by possible interaction with ion channels, activate secondary messenger systems to modify the calcium released by ACh-induced depolarization to modulate excitation-contraction coupling and force generation in these muscles.

     

Positive inotropic and chronotropic effects of beraprost sodium,a stable analogue of prostacyclin,in isolated guinea pig myocardium

• 1.1. Inotropic and chronotropic effects of beraprost sodium (beraprost), a chemically stable prostacyclin analogue, were examined in isolated myocardial preparations of guinea pigs.
• 2.2. In the left atria, 10⁻⁹-10⁻⁷ M beraprost had no significant effect on the contractile force, but 10⁻⁶ and 10⁻⁵ M produced a concentration-dependent positive inotropic response. This effect was antagonized by S-145, a potent and selective thromboxane A₂ (TXA₂) receptor antagonist, but not by propranolol.
• 3.3. Beraprost, from 10⁻⁹ to 10⁻⁵ M, had no significant inotropic effect in the right ventricular papillary muscles.
• 4.4. In the right atria, 10⁻⁹ and 10⁻⁸ M beraprost had no significant effect, but 10⁻⁷-10⁻⁵ M caused an increase in beating rate; this effect was not affected by S-145.
• 5.5. The present study demonstrates that beraprost has positive inotropic and chronotropic effects at high doses on isolated guinea pig atria. The inotropic effect may be mediated by the TXA₂ receptor, but some mechanism other than the TXA₂ receptor is responsible for the chronotropic effect.

     

Potentiation of the hypoxic contraction of guinea-pig isolated pulmonary arteries by two inhibitors of superoxide dismutase

• 1.1. Isolated proximal and distal extralobar branches of the pulmonary artery of the guinea-pig develop slow and well-sustained contractions in response to hypoxia (P_O2 11–15 mm Hg) without prior stimulation with an agonist. These contractions are readily reversible by readministration of oxygen.
• 2.2. Incubation of these preparations with diethyldithiocarbamic acid (DETCA, 5 mM for 30 min), an inhibitor of superoxide dismutase, significantly increased the hypoxic contractions whether DETCA was added before the challenge with hypoxia or after the hypoxic contraction had reached a plateau. This treatment also reduced the oxygen-induced relaxation.
• 3.3. Similarly, incubation with triethylenetetramine (TETA, 5 mM for 30 min), another inhibitor of superoxide dismutase, produced larger potentiation of the hypoxic contraction in the two preparations and reduced the oxygen-induced relaxation.
• 4.4. Furthermore, addition of H₂O₂ (10⁻⁵ M −3 × 10⁻⁴M) caused concentration-dependent relaxation of the hypoxic contraction while larger concentrations (10⁻³M and 3 × 10⁻³M) caused contraction that did not respond to readministration of oxygen.
• 5.5. These observations suggest that during hypoxic stress, the accumulation of superoxide anions may participate in the hypoxia-induced contraction and that the metabolism of these radicals into H₂O₂ by superoxide dismutase maintains the relaxed state during normoxia.

     

Effects of the selective 5-HT receptor agonists 8-OHDPAT and DOI on behavior and brain biogenic amines of rats

• 1.1. The behavioral responses, as well as the biogenic amines and metabolite contents in discrete brain areas were determined in male rats subcutaneously treated with a 5-HT_1A (8-OHDPAT) or 5-HT_2A (DOI) agonist at doses (0.5-2 mg/kg) sufficient to produce the typical effects of the stimulation of these brain receptor subtypes.
• 2.2. Besides the expected effects (i.e., forepaw treading, flat body posture and inhibition of 5-HT release and turnover), 8-OHDPAT displayed signs of increased dopaminergic transmission.
• 3.3. DOI increased dopamine turnover and provoked stereotypical behavior, in addition to head shakes and body twitches.
• 4.4. Moreover, DOI induced both forepaw treading and flat body posture, which are believed to be typical responses to the stimulation of brain 5-HT_1A receptors.
• 5.5. This finding cannot be explained on the basis of actual knowledge, because the affinity of DOI for 5-HT_1A receptor has been found to be very low, whereas indirect mechanisms of activation of this receptor subtype triggered by stimulation of 5-HT_2A receptor are actually unknown.

     

Effect of naringin and naringenin on contractions induced by noradrenaline in rat vas deferens—I. Evidence for postsynaptic alpha-2 adrenergic receptor

• 1.1. Naringin at all doses (2 × 10⁻⁶, 5 × 10⁻⁷, and 1 × 10⁻⁷ M) significantly increased contractions induced by noradrenaline in rat vas deferens but the increments of maximal contraction were not concentration-dependent.
• 2.2. In a medium containing 1 × 10⁻⁶ M yohimbine (a selective blocker of α₂-adrenoceptor) and naringin, the curve constructed with noradrenaline decreased below the control curve.
• 3.3. Naringenin (aglycone of naringin) (2 × 10⁻⁶ and 1 × 10⁻⁷ M) increased the contractile effect of noradrenaline and the maximal effect evoked was related to the maximal dose of naringenin.
• 4.4. The α₂ antagonism produced by yohimbine in the naringenin-noradrenaline association were retained at two doses of naringenin tested and we noticed a similar behaviour when we used clonidine-noradrenaline.

     

Characterization of the contraction to 5-HT in the canine colon longitudinal muscle

1. Although conscious dogs have often been used for colonic motility studies with 5-hydroxytryptamine (5-HT), the effects of 5-HT on the isolated colon have not been thoroughly characterized yet. The current study was undertaken to characterize the response to 5-HT of the canine isolated colon longitudinal muscle.
2. Longitudinal strips of canine midcolon deprived of (sub)mucosa were prepared for isotonic measurement. 5-HT induced contractions from 3 nM onwards, which were not affected by selective inhibition of 5-HT re-uptake, monoamine oxidase or blockade of α-adrenoceptors. Tetrodotoxin (0.3 μM) did not affect the responses to 5-HT, suggesting that smooth muscle 5-HT receptors are involved. The selective 5-HT₄ receptor antagonist SB 204070 (10 nM) slightly enhanced contractions to 5-HT and therefore it was included in the organ bath solution in all further experiments. The 5-HT₁ and 5-HT₂ receptor antagonist methysergide (0.1 μM) depressed the curve to 5-HT, but the selective 5-HT₃ receptor antagonist granisetron (0.3 μM) had no effect.
3. Besides 5-HT, α-methyl-5-HT (α-Me-5-HT), 5-methoxytryptamine (5-MeOT), 2-methyl-5-HT (2-Me-5-HT) and 5-carboxamidotryptamine (5-CT) also induced contractions, with the following rank order of potency (pEC₅₀ values in parentheses): 5-HT (6.9)=α-methyl-5-HT (6.9)>2-Me-5-HT (5.8)=5-MeOT (5.7)=5-CT (5.6), indicative of 5-HT₂ receptor involvement. α-Me-5-HT produced a bell-shaped curve, which was not affected by α-adrenoceptor blockade. 5-HT, 5-MeOT, 2-Me-5-HT and 5-CT produced a monophasic concentration-response curve, consistent with an interaction with a single receptor site. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and tryptamine only induced contractions at a concentration exceeding 1 μM.
4. The selective 5-HT_2B receptor antagonist SB 204741 (0.3 μM) did not affect the curve to 5-HT. Ketanserin, cisapride and spiroxatrine behaved as competitive antagonists with pK_b values of, respectively, 8.4, 8.1 and 6.7. Spiroxatrine (1 μM) shifted the curve to 5-MeOT rightward yielding an apparent pA₂ of 7.1. Other antagonists at 5-HT_2A receptors also surmountably inhibited the contractions to 5-HT (apparent pA₂ value in parentheses): mesulergine (8.2), cinanserin (8.2), yohimbine (6.2) and mianserin (8.6). However, as well as a rightward shift, methiothepin (8.3), pizotifen (8.6) and spiperone (8.8) also caused a depression of the curve, indicative of ‘pseudo-irreversible'' antagonism. Taken together, the above mentioned affinity estimates most closely corresponded to literature affinity values for 5-HT_2A receptors.
5. It was concluded that 5-HT induces contractions of the canine midcolon longitudinal muscle primarily by stimulation of smooth muscle 5-HT_2A receptors. The presence of inhibitory 5-HT₄ receptors cannot be ruled out.

     

Functional roles of cyclic guanosine 3′,5′-monophosphate analogue in cerebral vasodilation

• 1.1. Vasodilating effects of cyclic nucleotides in cerebral vasculature were examined using membrane permeable cyclic nucleotide analogues, 8-bromoguanosine 3′,5′-cyclic monophosphate (8-Br-cGMP) and 8-bromoadenosine 3′,5′-cyclic monophosphate (8-Br-cAMP).
• 2.2. In isolated canine basilar artery (CBA), 8-Br-cGMP but not 8-Br-cAMP, significantly inhibited Ca²⁺-induced and agonist [serotonin(5-HT), prostaglandin(PG)F_2α or endothelin]-induced contraction, in a concentration-dependent manner.
• 3.3. When Ca²⁺ was depleted from intracellular store sites by pretreatment with A23187, 8-Br-cGMP but not 8-Br-cAMP strongly attenuated contractions induced by Ca²⁺-influx.
• 4.4. Neither 8-Br-cGMP nor 8-Br-cAMP modified contraction induced by caffeine which elicits Ca²⁺ release from intracellular Ca²⁺ store.
• 5.5. 8-Br-cGMP lowered the high K⁺-induced sustained [Ca²⁺] elevation.
• 6.6. These results suggest that, at least in CBA, cGMP exerts its inhibitory effect on the contraction induced by influx of Ca²⁺, by reducing the level of [Ca²⁺]_i and reducing [Ca²⁺]_i sensitivity of the contractile machinery.

     

Multidirectional contraction of human hypertrophied prostate

• 1.1. The purpose of the present investigation is to evaluate muscle contraction in two directions (longitudinal and circumferential to urethra) physiologically and morphologically for α-adrenoceptor agonists.
• 2.2. Norepinephrine (10⁻⁷−10⁻⁴ M), phenylephrine (10⁻⁷−10⁻⁴ M) and clonidine (10⁻⁷−10⁻⁴ M) induced contractions in a dose-dependent manner on human prostate from patients with benign prostatic hypertrophy (BPH).
• 3.3. No significant differences were observed between longitudinal and circumferential directions of human prostate in 50% of the maximal muscle contraction (EC₅₀ values) and the maximal muscle contractions caused by any agents used.
• 4.4. Morphometric analysis for muscle in prostates was performed using formalin-fixed, paraffin-embedded sections stained by the Mallory-Azan method.
• 5.5. There was no significant difference in the density of the muscle area between longitudinal and circumferential directions of prostatic strips.
• 6.6. These results suggest that there are no significant differences in responsiveness of α-adrenoceptor agonists and the smooth muscle contents in longitudinal and circumferential directions to urethra, for human hypertrophied prostate.

     

Comparison of Angiotensin II Type-1 and Type-2 Receptor Antagonists on Angiotensin II– Induced IP3 Generation in Cardiomyocytes

• 1.Angiotensin II (ang II) produced significant (P<0.01) increases of inositol 1,4,5-mono-, -di- and -triphosphates (IP₁, IP₂ and IP₃) within 1 min of treatment of cardiomyocytes prepared as primary culture from 7-day-old chick embryo hearts.
• 2.The ang II receptor type 1 (AT₁-R) antagonist losartan blocked ang II–stimulated production of IP₃; however, the inhibition was not complete even at 10⁻⁵ M.
• 3.The ang II receptor type 2 (AT₂-R) antagonist PD123319 blocked ang II–induced IP₃ production but to a lesser extent than losartan. At 10⁻⁵ M, losartan reduced ang II–induced formation of IP₃ by 71%, whereas PD123319 reduced IP₃ formation by ang II by 40%.
• 4.Neither losartan nor PD123319, 10⁻⁵ M, affected IP₃ formation in cardiomyocytes that were not treated by ang II.
• 5.The combination of both antagonists, at concentrations that each partly reduced IP₃, completely inhibited IP₃ formation. Thus AT₁ and AT₂ receptor blockade may be necessary to completely block the effects of ang II mediated by the IP₃ signal transduction pathway.