Platelet-vessel wall interactions in hemostasis: implication of 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is implicated in the platelet-vessel wall interactions during hemostasis. Indeed, the tail bleeding time and the initial blood loss in rats are significantly increased by platelet amine depletion with reserpine/parachlorophenylalanine, by the specific 5-HT2 receptor antagonists ketanserin, R 55 667 and R 56 433 and by the ergot derivatives metergoline and methysergide. In the dose range affecting hemostasis, the serotonergic receptor antagonists also inhibit the 5-HT-amplified aggregation of ADP-sensitized platelets in rat whole blood. Bleeding times are also prolonged by alpha 1-adrenergic receptor antagonism with prazosin or phentolamine, by specific thromboxane A2-synthetase inhibition with R 59 655 or dazoxiben, and by anticoagulant treatment. The latter three mechanisms of action are not explanatory for the effect of the tested serotonergic antagonists, since bleeding times are prolonged by specific 5-HT2 receptor antagonists without an effect on alpha-adrenergic receptors, on platelet prostaglandin biosynthesis or on coagulation and fibrinolysis. The present study shows the interplay between the platelet-derived mediators 5-HT and TXA2 at the level of the platelet or the blood vessel to be of major importance in the platelet-vessel wall interactions during hemostasis.     

Role of alpha-adrenoceptors in the effects of buspirone and 5-carboxamidotryptamine in rabbit isolated thoracic aorta.

1. The role of alpha-adrenoceptors in the vascular effects of buspirone (BUS) and 5-carboxamidotryptamine (5-CT) was investigated in rabbit thoracic aorta. 2. Buspirone produced a concentration-dependent contraction. The non-selective 5-HT1 and 5-HT2-receptor antagonist methysergide and the 5-HT2 receptor antagonist ketanserin did not alter the contractile effect of buspirone. However, the competitive antagonist of alpha 1-adrenoceptors, prazosin, shifted the concentration-response curve of buspirone to the right without changing the maximal response. 3. Benextramine tetrahydrochloride monohydrate (BHC), a noncompetitive antagonist of alpha 1-adrenoceptors, inhibited the contraction induced by buspirone in a noncompetitive manner. After pretreatment with two different concentrations of BHC, the estimated apparent dissociation constants of buspirone were found to be identical. 4. In addition, buspirone antagonized the concentration-response curve of phenylephrine again showing a similar dissociation constant, suggesting a partial agonistic action of buspirone at the level of alpha 1-adrenoceptors. 5. The concentration-response curve of 5-HT showed two components in the thoracic aorta obtained from reserpine treated and untreated animals as verified by different pD2 values. The second component was observed with relatively higher concentrations of 5-CT and could be blocked by prazosin or BHC. Neither of these compounds altered the first component. After Pretreatment with BHC, the first component of 5-CT was competitively antagonized by methysergide and ketanserin, having pA2 values of 8.81 and 9.1 respectively. 6. These results suggest that the contraction induced by buspirone is mainly mediated by alpha 1-adrenoceptors, while the higher concentrations of 5-CT caused contraction via alpha 1-adrenoceptor stimulation in addition to its 5-HT2 agonistic effect.     

Potential vascular alpha1-adrenoceptor blocking properties of an array of 5-HT receptor ligands in the rat

This study set out to analyse the potential ability of some 5-hydroxytryptamine (5-HT) receptor ligands widely used in cardiovascular experimental models to interact with vascular alpha1-adrenoceptors in the pithed rat. These ligands included: methiothepin, methysergide and metergoline (5-HT(1)/5-HT2); WAY-100635, buspirone, ipsapirone and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (5-HT(1A)); GR127935 (5-HT(1B/1D)); ketanserin, ritanserin, spiperone and pizotifen (5-HT2); granisetron and metoclopramide (5-HT3); tropisetron (5-HT3/5-HT4); ergotamine (5-HT(1B/1D), 5-ht(5A/5B)); clozapine (5-HT6/5-HT7); as well as LY215840 and mesulergine (5-HT2/5-HT7). For this purpose, the increases in diastolic blood pressure produced by the selective alpha1-adrenoceptor agonist, phenylephrine, were analysed before and after the above antagonists or saline. The adrenoceptor antagonist properties of prazosin (alpha1) and yohimbine (alpha2) were also analysed for comparison. Thus, the phenylephrine-induced vasopressor responses were dose-dependently antagonised with the following apparent rank order of potency by: prazosin > or = methiothepin > ketanserin > clozapine > or = lisuride > buspirone; this potency correlates with the affinity of these compounds for alpha1-adrenoceptor binding sites. In contrast, the other compounds were either devoid of any blocking effect on--or even potentiated (i.e. lisuride, methysergide, 8-OH-DPAT, granisetron and GR127935)--the responses to phenylephrine. These results show that methiothepin, ketanserin, clozapine, lisuride and buspirone can block alpha1-adrenoceptors in the rat systemic vasculature.     

The effects of ketanserin, methysergide and LY 53857 on sympathetic nerve activity

Preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Three 5-HT2 antagonists, ketanserin, methysergide and LY 53857, were infused separately over 1 h periods. Ketanserin caused a fall in, methysergide a rise followed by a fall and LY 53857 only a rise in preganglionic sympathetic nerve activity. The sympathoexcitation caused by both LY 53857 and methysergide was not associated with any changes in blood pressure and heart rate. The sympathoinhibition caused by ketanserin and methysergide was accompanied by a fall in blood pressure and heart rate. Both ketanserin and LY 53857 caused increases in femoral arterial conductance, while methysergide caused a transient decrease. These differences are explained on the basis that ketanserin possesses alpha 1-adrenoceptor antagonist and methysergide 5-HT receptor agonist properties. It is suggested that 5-HT2 sympathoinhibitory receptors are involved in central cardiovascular control.     

Receptors involved in the modulation of 5-hydroxytryptamine release in bovine cerebral arteries.

The uptake of [3H]5-hydroxytryptamine (5-HT) in bovine cerebral arteries was reduced by cocaine (1 microM), ouabain (100 microM), pretreatment with 6-hydroxydopamine (6-OHDA) (1.46 mM, 10 min) and metitepine (1 microM). Electrically-stimulated tritium release was decreased by tetrodotoxin (0.8 microM), Ca-free medium, denervation with 6-OHDA (1.46 mM, 10 min), 5-HT (10 microM), noradrenaline (1 microM) and the agonist of alpha 2-adrenoceptors B-HT 920 (0.1 and 1 microM), enhanced by metitepine (1 microM, antagonists of presynaptic 5-HT1 receptors) and rauwolscine (1 microM, antagonist at alpha 2-adrenoceptors, and also of 5-HT1D receptors) and not affected by ketanserin (1 microM, antagonist of 5-HT2 receptors), methysergide (0.1 microM, antagonist of 5-HT1 and 5-HT2 receptors) and phentolamine (1 and 3 microM antagonist of alpha-adrenoceptors and less potent of 5-HT1 receptors). The inhibitory action of 10 microM 5-HT was partially reversed by phentolamine (3 microM) and cocaine (1 microM) and completely reversed by both metitepine (1 microM) and rauwolscine (1 microM). Ketanserin (1 microM), methysergide (0.1 microM) or phentolamine (1 microM) had no effect. Rauwolscine (1 microM) antagonized the inhibition induced by both noradrenaline (1 microM) and B-HT 920 (0.1 and 1 microM). 5-HT induced tritium release which was inhibited by cocaine (an antagonist of 5-HT3 receptors) and denervation with 6-OHDA.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-HT1-like receptors requiring functional cyclo-oxygenase and 5-HT2 receptors independent of cyclo-oxygenase mediate contraction of the human umbilical artery.

1. The interactions between 5-hydroxytryptamine (5-HT) and the antagonists ketanserin, methysergide and phentolamine were studied in isolated preparations of human umbilical artery (HUA) at physiological oxygen tension (Po2 approximately 15 mmHg) and at high PO2 (approximately 120 mmHg). 2. At physiological Po2 ketanserin, methysergide and phentolamine behaved as silent competitive antagonists of the 5-HT-induced contraction of HUA. pA2 values calculated by Schild analysis were 8.92, 8.52 and 6.37, respectively. 3. At high Po2, 5-HT-induced contractions were antagonised in a biphasic manner by ketanserin (0.1 microM); the response to low but not to high concentrations of 5-HT was resistant to blockade by ketanserin. The ketanserin-resistant component was abolished following cyclo-oxygenase inhibition by indomethacin (1 microM). 4. At high Po2, methysergide behaved as a partial agonist. Methysergide-induced contractions were inhibited but not abolished by indomethacin, and resistant to 5-HT2 receptor and alpha 1-adrenoceptor blockade. 5. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was mimicked by the selective 5-HT1-like receptor agonist 5-carboxamidotryptamine (5-CT): 5-CT was 7 fold more potent than 5-HT. 6. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was antagonised by phentolamine and the selective alpha 2-adrenoceptor antagonist Wy 26703. 7. These results suggest that (i) at physiological Po2 5-HT2 receptors almost exclusively mediate contractions induced by 5-HT, and (ii) at high Po2 the agonist potency order of 5-CT greater than 5-HT greater than methysergide suggests that ketanserin-resistant responses are mediated by 5-HT1-like receptors which require functional cyclo-oxygenase.     

In vivo Schild regression analyses using nonselective 5-HT2C receptor antagonists in a rat operant behavioral assay

RATIONALE: Although competitive antagonism experiments are critical tools in the classification of potential pharmacotherapies, no studies have quantitatively compared the potencies of 5-HT(2C) receptor antagonists using the Schild regression analysis in vivo. OBJECTIVES: To evaluate the behavioral effects of 5-HT(2C) receptor agonists and antagonists, a series of nonselective 5-HT(2C) receptor antagonists, the 5-HT(2A/2C) receptor antagonist ketanserin, the 5-HT(2B) receptor antagonist SB 204,741, the 5-HT(2B/2C) receptor antagonist SB 200,646, and the peripherally acting 5-HT(2C) receptor antagonist RS102,221 were evaluated for their capacity to competitively antagonize the agonists MK212, mCPP, or BW723C86 in rats. MATERIALS AND METHODS: Male Sprague-Dawley rats (N = 28) were trained to respond under a fixed ratio 10 schedule of food reinforcement. A multiple-trial, cumulative-dosing procedure was used to evaluate the capacity of the compounds to suppress response rates. RESULTS: MK212, mCPP, and the 5-HT(2B) receptor agonist BW723C86 dose-dependently decreased response rates. Only metergoline, mianserin, and methysergide produced a dose-dependent antagonism of the rate-decreasing effects of both mCPP and MK212. Apparent pA(2) analysis indicated that metergoline, mianserin, and methysergide were approximately equipotent as antagonists overall. Metergoline and mianserin failed to block the rate-decreasing effects of BW723C86. Ketanserin, SB 200,646, SB 204,741, and RS102,221 failed to block either mCPP or MK212, suggesting that 5-HT(2A), 5-HT(2B), or peripheral 5-HT(2C) receptors do not play a primary role in the rate-decreasing effects of these two agonists. CONCLUSIONS: Taken together, these antagonism profiles suggest that the agonists MK212 and mCPP produce their rate-decreasing effects through a combination of 5-HT receptors with the 5-HT(2C) receptor playing a prominent but not exclusive role.     

SCH 23390--a selective dopamine D-1 receptor antagonist with putative 5-HT1 receptor agonistic activity

The selective dopamine D-1 receptor antagonist SCH 23390 has been tested in vitro in the rat fundus model and in vivo in the electrically stimulated flexor reflex model. In the fundus model, SCH 23390 showed a potent agonistic activity compared to that of different 5-HT receptor agonists. Pindolol, 1-propranolol and pirenperone showed no or only weak inhibition of the SCH 23390-induced contractions in the fundus strip whereas methysergide was a potent inhibitor. The 5-HT3 receptor antagonist ICS 205-930 did not induce an inhibitory effect. In the electrically stimulated flexor reflex model in pithed rats, SCH 23390 induced a marked increase of the reflex. This increase was slightly inhibited by a mixed dopamine (DA) D-1/D-2 antagonist cis(Z)-flupentixol and by a specific DA D-2 antagonist YM 09151-2. Different reference antagonists: bicuculline (GABAergic), propranolol (beta-adrenergic), scopolamine (muscarinic), yohimbine (alpha 2-adrenergic), prazosin (alpha 1-adrenergic) were all without an antagonist effect on the SCH 23390-induced increase of the flexor reflex. Ketanserin, a selective 5-HT2 receptor antagonist, showed a weak and short-lasting inhibition of the SCH 23390 effect in high doses, whereas ritanserin showed only 35% inhibition of the SCH 23390-induced flexor reflex at a dose of 1.3 mumol/kg i.v. The mixed 5-HT1/5-HT2 antagonists methiothepin and metergoline showed a marked inhibitory effect at 2.6 mumol/kg i.v. and 3.1 mumol/kg i.v., respectively (1.3 mg/kg i.v.). These findings suggest that SCH 23390 might possess 5-HT1 receptor agonist activity.     

A paradox: the 5-HT2-receptor antagonist ketanserin restores the 5-HT-induced contraction depressed by methysergide in large coronary arteries of calf

Methysergide depresses the contractile effects of 5-hydroxytryptamine (5-HT) in bovine large coronary arteries devoid of endothelium. The IC50 of methysergide for depression of the response to 5-HT was (-log mol/l) 9.8. A low sensitivity contractile effect of 5-HT was not influenced by 1-1,000 nmol/l methysergide. The maximum force of this residual response is approximately 1/3 of the maximum force elicited by 5-HT in the absence of methysergide. Ketanserin restored the 5-HT-induced contraction depressed by methysergide. In the presence of 0.1 mumol/l ketanserin, methysergide caused depression of the 5-HT-induced effects with an IC50 (-log mol/l) of 6.5 without affecting the residual response. We propose that methysergide depresses 5-HT-induced contractions by acting on an allosteric site. The effect of binding of methysergide to the allosteric site would lead to a conformational change of the 5-HT2-receptor, thereby only allowing the production of a residual 5-HT-induced contraction. Ketanserin competes with high affinity not only with 5-HT for the 5-HT2-receptor but also with methysergide for the allosteric site, thus shifting the receptor back into its original conformation. The affinity estimate of ketanserin for the allosteric site yielded a KB (-log mol/l) of 10.3. Ketanserin (1-1,000 nmol/l) antagonized the contractile effects of 5-HT with a potency expected from its affinity for 5-HT2-receptors (-log KB, mol/l 9.4). However, micromolar concentrations of ketanserin antagonized the effects of 5-HT less than expected from its affinity for 5-HT2-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for depressant 5-HT1-like receptors on rat brainstem neurones.

1. The technique of microiontophoresis was used to evaluate the contribution of 5-HT1-like, 5-HT2- and 5-HT3-receptors to the depressant effects of 5-hydroxytryptamine (5-HT) on neurones in the midline of the medullary brainstem of the rat in vivo. 2. Depressant responses to 5-HT were resistant to antagonism by the 5-HT2-receptor antagonist ketanserin and the 5-HT3-receptor antagonist MDL 72222 applied either microiontophoretically or administered systemically. 3. Microiontophoretic or systemic administration of the 5-HT antagonist metergoline, which shows nanomolar affinity for the 5-HT1-binding site, also failed to attenuate the depressant responses to 5-HT. 4. Systemic administration of high doses of methysergide (30-40 mg kg-1) attenuated the depressant responses to 5-HT but did not block depressant responses to GABA or excitatory responses to glutamate. 5. The depressant effects of 5-HT were potently mimicked by the 5-HT1-like receptor agonists 5-carboxamidotryptamine and 8-OH-DPAT. 6. These results indicate that neither 5-HT2-receptors nor 5-HT3-receptors are involved in the depressant effects of 5-HT on midline brainstem neurones. The depressant effects of 5-carboxamidotryptamine (5-CT) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and blockade of the response to 5-HT by high doses of methysergide suggests the involvement of 5-HT1-like receptors. The lack of effect of metergoline, however, indicates that this receptor may be different from any of the 5-HT1 binding sites yet described.     

Pharmacological characterisation of the decrease in 5-HT synthesis in the mouse brain evoked by the selective serotonin re-uptake inhibitor citalopram

The selective serotonin re-uptake inhibitor (SSRI) citalopram decreases the synthesis of 5-hydroxytryptamine (5-HT) in the mouse brain in vivo. The underlying mechanism was studied by recording the accumulation of 5-hydroxytryptophan (5-HTP) in hypothalamus and hippocampus after inhibition of the aromatic amino acid decarboxylase activity with m-hydroxybenzylhydrazine (NSD 1015). Depletion of 5-HT with reserpine markedly reduced the citalopram-induced decrease of 5-HTP but not that evoked by the 5-HT1A receptor agonist 8-OH-DPAT, which indicates that the presence of endogenous 5-HT is necessary for full effect of citalopram. In contrast to the almost complete antagonism of the decrease in 5-HT synthesis induced by 8-OH-DPAT, the 5-HT1A receptor antagonist WAY-100,635 only slightly affected the citalopram-evoked decrease in 5-HT synthesis. Likewise, the 5-HT1B receptor antagonists NAS-181 and GR127935 only slightly antagonised the citalopram effect although they strongly inhibited the decrease in 5-HT synthesis induced by the 5-HT1B receptor agonist anpirtoline. Combined treatment with 5-HT1A and 5-HT1B receptor antagonists did not produce any additive antagonistic effect on the citalopram-induced decrease in 5-HT synthesis. The 5-HT2A/2C receptor antagonist ketanserin, the 5-HT3 receptor antagonist ondansetron and the 5-HT4 receptor antagonist RS-39604 had no effect on the citalopram-induced decrease in 5-HT synthesis. The same was found for several other non-selective 5-HT receptor antagonists, e.g. cyproheptadine, dihydroergotamine, methiothepin, methysergide, metergoline and mianserin. It is concluded that the citalopram-induced decrease in 5-HT synthesis differs in sensitivity from that mediated by 5-HT1A or 5-HT1B receptor agonists and citalopram also seems to require endogenous 5-HT for its full effect.     

Renal hemodynamic responses to 5-hydroxytryptamine (5-HT): involvement of the 5-HT receptor subtypes in the canine kidney

The study was designed to define the serotonin (5-HT) receptor subtypes in the canine kidney. An intrarenal infusion of 5-HT at a dose of 5 micrograms/min in anesthetized dogs resulted in a biphasic response of renal blood flow which decreased transiently then increased above the control level during prolonged infusion. The decrease of renal blood flow was abolished by infusion of methysergide but not by ketanserin, and the subsequent increase was abolished by infusion of either ketanserin or methysergide. Terazosin, an alpha1-adrenoceptor antagonist, did not modify the renal action of 5-HT. These findings suggest that the renal blood flow response induced by 5-HT did not depend on an indirect effect via the sympathetic nervous system, the initial vasoconstriction was mediated via a 5-HT1-like receptor, and that the latter vasodilatation was mediated via a 5-HT2 receptor. The infusion of 5-HT also increased urine flow and urinary excretion of sodium. These increases were reversed by pretreatment with ketanserin and abolished by methysergide. We propose that 5-HT may exert its antidiuretic action via a 5-HT1-like receptor in the tubules but that the renal hemodynamic changes induced by 5-HT may overcome its antidiuretic action. The present results suggest the existence of a 5-HT1-like and 5-HT2 receptor in the renal vasculature and a 5-HT1-like receptor in the renal tubules.     

Involvement of 5-HT1A receptors in the antidepressant-like activity of gepirone in the forced swimming test in rats.

The antidepressant-like activity of gepirone, a drug with a high and selective affinity for 5-hydroxytryptamine1A (5-HT1A) receptors, was studied in the forced swimming test in rats. The drug, administered intraperitoneally in single doses of 2.5-20 mg/kg, potently and dose-dependently shortened the immobility time. The anti-immobility effect of gepirone (10 mg/kg) was dose-dependently antagonized by the 5-HT1A receptor and alpha 1-adrenoceptor antagonist, NAN-190 (0.25 and 0.5 mg/kg), the beta-adrenoceptor blocker with the affinity for 5-HT1A and 5-HT1B receptors, pindolol (2 and 4 mg/kg), the 5-HT1A, 5-HT2 and dopamine receptor blocker spiperone (0.01 and 0.03 mg/kg) and by the dopamine receptor antagonist, haloperidol (0.125 and 0.25 mg/kg). On the other hand, the non-selective 5-HT receptor antagonist, metergoline (2 and 4 mg/kg), the selective 5-HT2 receptor antagonist, ketanserin (1 and 2 mg/kg), the selective alpha 1-adrenoceptor blocker, prazosin (0.25 and 0.5 mg/kg) and the beta-blockers with no affinity for 5-HT receptors, betaxolol (4 and 8 mg/kg) and ICI 118,551 (4 and 8 mg/kg), did not affect the anti-immobility effect of gepirone. The effect of gepirone was not modified, either, in animals with a lesion of the 5-HT system, produced by p-chloroamphetamine (PCA, 2 x 10 mg/kg) or p-chlorophenylalanine (PCPA, 3 x 300 mg/kg). The results obtained suggest that the anti-immobility effect of gepirone is mediated by activation of 5-HT1A receptors, most probably located postsynaptically and that dopamine may be involved in this action.     

Ketanserin blocks alpha 1-adrenoceptors of porcine vascular smooth muscle cells

The effect of ketanserin on alpha-adrenoceptors was studied in membrane preparations of the porcine aorta using [3H]prazosin and [3H]yohimbine binding assays to identify alpha 1- and alpha 2-adrenoceptors. Ketanserin bound to alpha-adrenoceptors and the Ki value of ketanserin for alpha 1-adrenoceptors was 8.3 nM, a value practically equal to that of phentolamine (Ki = 7.2 nM). The Ki value of ketanserin for alpha 2-adrenoceptors was 3.3 microM. Thus, at the doses prescribed clinically, ketanserin blocks alpha 1- but not alpha 2-adrenoceptors of porcine vascular smooth muscle.     

Inhibition of 5-hydroxytryptamine-induced and -amplified human platelet aggregation by ketanserin (R 41 468), a selective 5-HT2-receptor antagonist

Ketanserin, a selective 5-HT2-receptor antagonist, inhibits the reversible aggregation induced by 5-hydroxytryptamine (5-HT) in human platelet-rich plasma (PRP). In this respect, the compound is equipotent to cyproheptadine and more active than methysergide (IC50: 1.66 x 10(-8) M, 1.44 x 10(-8) M and 5.62 x 10(-8) M respectively). Ketanserin is active against 5-HT-induced platelet aggregation after both in vitro and oral administration to human volunteers. At concentrations up to 500 times in excess of the IC50 for 5-HT-induced platelet reactions, ketanserin does not affect the aggregation induced by ADP, epinephrine, collagen or Thrombofax, the prostaglandin biosynthesis of thrombin-stimulated platelets, nor the active uptake of 14C-5-HT by platelets. 5-Hydroxytryptamine amplifies the human platelet aggregation induced by threshold concentrations of ADP, collagen, epinephrine, norepinephrine and induced irreversible aggregation of platelets pre-sensitized with Thrombofax. This amplification by 5-hydroxytryptamine results in a platelet response typical for the potentiated agonist; for the combination of the monoamine with collagen, the serotonergic amplification results in enhanced aggregation, release of beta-TG and PF4 and excessive formation of TXB2. Ketanserin, after both in vitro and oral administration to man reduces the amplified response to the level of the potentiated agonist. The present evidence suggests the presence of functional 5-HT2 receptors on the human platelet, different from those involved in the uptake of the monoamine.     

Peripheral 5-HT2-like receptors. Can they be classified with the available antagonists?

Interactions between 5-hydroxytryptamine (5-HT) and the so-called 5-HT2 receptor antagonists ketanserin, spiperone, trazodone and methysergide were studied in isolated preparations of the rabbit aorta, rat jugular vein, and rat caudal artery. Trazodone and spiperone were apparently simple competitive antagonists since they produced antagonism that was surmountable over the concentration range studied and, in each tissue, their apparent affinity appeared to be independent of the antagonist concentration. Furthermore, concentration-ratios obtained with the two antagonists in combination suggested that antagonism was additive, implying mutual competition with a single population of 5-HT receptors. Ketanserin was a non-surmountable antagonist of 5-HT in the rat caudal artery and methysergide demonstrated surmountable, competitive antagonism only in the rabbit aorta. Antagonist dissociation constants estimated for apparently competitive interactions showed that ketanserin, spiperone and trazodone expressed affinities which differed according to the tissue used. In the case of trazodone, affinity estimates differed by as much as 12 fold. These discrepancies were independent of the 5-HT receptor agonist used and could not be attributed to an inadequate equilibration of the antagonist. These results can be interpreted in two ways: either the receptors in the different tissues are heterogeneous or the antagonists used here must be considered as unreliable probes for the classification of 5-HT2-like receptors.     

Antagonism of the antidiarrhoeal effect of clonidine and the lethal effect of noradrenaline in rats: a reliable procedure to evaluate the in-vivo α1- and α2-blocking activity of drugs?

Eight compounds with alpha-adrenergic blocking activity were tested for their ability to antagonize the antidiarrhoeal effect of clonidine (clonidine test) and the lethal effect of noradrenaline (noradrenaline test). Six of the compounds studied are alpha-adrenergic blocking agents with known alpha 2/alpha 1 selectivity. Two compounds, ketanserin (R 41 468) and butanserin (R 53 393), are 5-hydroxytryptamine S2-antagonists. The ED50-values (mg kg-1) obtained in the clonidine test were: phentolamine (0.34), RX781094 (0.34), yohimbine (0.51), piperoxan (9.36), butanserin (greater than 5.0), prazosin (greater than 10.0), phenoxybenzamine (greater than 40.0), and ketanserin (greater than 80.0). In the noradrenaline test the ED50's (mg kg-1) were: butanserin (0.014), prazosin (0.032), phentolamine (0.59), phenoxybenzamine (1.02), ketanserin (4.69), RX781094 (12.4), piperoxan (21.5), and yohimbine (25.0). The selectivity alpha 2/alpha 1-ratios (ED50 clonidine/ED50 noradrenaline were: yohimbine (0.020), RX781094 (0.027), piperoxan (0.44), phentolamine (0.58), ketanserin (greater than 39), prazosin (greater than 312), and butanserin (greater than 357). These results show that yohimbine and RX781094 are equipotent and relatively selective alpha 2-antagonists; piperoxan and phentolamine block both alpha 1- and alpha 2-receptors at closely related doses; ketanserin, prazosin and butanserin are selective blockers of alpha 1-receptors, ketanserin being very weak, prazosin and butanserin being very potent compounds in this respect. The potent and selective alpha 1-blocking activity of butanserin, combined to its 5-HT S2-antagonism makes butanserin a very interesting experimental drug in view of earlier reported data concerning the amplifying effects between 5-hydroxytryptaminergic and noradrenergic vascular mechanisms.     

Allosteric properties of the 5-HT2 receptor system of the rat tail artery

Summary We present an analysis of the interactions of 5-hydroxytryptamine (5-HT) and antagonists (methysergide, ketanserin, ritanserin) with the 5-HT₂ receptor system of strips of rat tail artery. The mode of action of ritanserin was also studied on strips of calf coronary arteries. 1. Ketanserin competitively antagonized 5-HT-induced effects in rat tail artery with an affinity (pK_B = 9.4 nmol/l) consistent with the assumption of an interaction of 5-HT and ketanserin at 5-HT₂-receptors. 2. Methysergide reduced to 50–60% the maximum response to 5-HT in rat tail artery. Concentration-effect curves for 5-HT became biphasic in the presence of methysergide with quickly and slowly developing contractions at low and high concentrations of 5-HT, respectively. 100 nmol/l ketanserin completely restored effects of 5-HT depressed by low concentrations of methysergide (< 10 nmol/l). Higher concentrations of methysergide in the presence of 100 nmol/l ketanserin again depressed the effects of 5-HT. 3. Ritanserin resembles methysergide by causing insurmountable antagonism of 5-HT-induced contractions which can be prevented by ketanserin in both rat tail artery and calf coronary artery. These results are inconsistent with competition between ritanserin and 5-HT for the 5-HT₂ receptor. 4. The findings are consistent with the assumption of an interaction of ketanserin and methysergide or ritanserin with an allosteric site near the 5-HT₂-receptor. Both methysergide and ritanserin appear to antagonize the effects of 5-HT through an allosteric site which is distinct from the 5-HT₂ receptor. Send offprint requests to A. J. Kaumann at the above address     

Effect of ketanserin on blood pressure in rats

Ketanserin, a selective serotonergic (5-HT2) antagonist, also has affinity for alpha 1-adrenoceptors. It is not clear whether the hypotensive mechanism of ketanserin is due to its antagonistic action to 5-HT2 receptor or to its affinity for alpha 1 adrenoceptors. The hypotensive mechanism of ketanserin was studied in both stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY). Anesthetized rats were used (alpha-chloralose + urethane, i.p.). Up to 3 ml of blood was drawn from each rat for analysis. Plasma norepinephrine (NE) was determined by radioenzymatic assay. Plasma serotonin (5-HT) was determined by HPLC-ECD. Adrenal nerve discharges were counted by a digital pulse counter. Ketanserin (0.5 mg/kg, 5.0 mg/kg, i.v.) produced a dose-dependent reduction of mean arterial pressure (MAP) in both SHRSP and WKY. MAP of SHRSP decreased significantly as compared with WKY. Both plasma NE and 5-HT showed a tendency to increase during ketanserin administration (5.0 mg/kg, i.v.). Ketanserin significantly antagonized the BP response induced by exogenously injected 5-HT (30 micrograms/kg) and NE (10 micrograms/kg). Adrenal nerve activity was reduced in parallel with the decrease in BP and HR. These findings suggest that ketanserin produced a decrease in BP via both peripheral and central action in rats.     

Interaction of 5-hydroxytryptamine and ketanserin in rat vas deferens subjected to low frequency field stimulation

The interaction of 5-hydroxytryptamine (5HT) and ketanserin was investigated in isolated rat vas deferens. Ketanserin (10(-7) M) almost completely abolished the phasic and the following rhythmic contractions induced by 5HT, whereas the inhibition by prazosin (10(-6) M) or methysergide (10(-6) M) of 5HT-induced contractions were incomplete. The amplitude of twitch contractions of vas deferens subjected to low frequency (0.1 Hz) field stimulation were substantially unchanged by 5HT (10(-7)-10(-5) M) per se. After pretreatment of the tissue with ketanserin (10(-8)-10(-6) M), 5HT, in a concentration-dependent manner, attenuated the amplitude of twitch contractions. Such attenuation of the amplitude was not observed after pretreatments with methysergide (10(-8)-10(-6) M) or prazosin (10(-7)-10(-5) M). The 5HT-induced inhibition of twitch contractions in the presence of ketanserin was not antagonized by phentolamine, propranolol, methysergide, morphine, promethazine, cimetidine, atropine or indomethacin. It is suggested that 5HT has dual (excitatory and inhibitory) effects upon nerve transmission of rat vas deferens, and only the excitatory effect is suppressed by ketanserin.