Wasyl Feniuk

5‐HT1‐like receptors mediate 5‐hydroxytryptamine‐induced contraction of human isolated basilar artery

1 The 5‐hydroxytryptamine (5‐HT) receptor mediating contraction of endothelium denuded human basilar artery has been characterized in vitro. 2 5‐HT and a variety of 5‐HT agonists contracted human isolated basilar artery with a rank order of agonist potency, 5‐carboxamidotryptamine (5‐CT) > 5‐HT = methysergide > GR43175 > 8‐OHDPAT > 2‐methyl‐5‐HT. The maximum response produced by these agonists differed. 3 None of the agonists relaxed human basilar artery when tone was elevated with prostaglandin F2α, indeed further contraction was seen. 4 The contractile responses of human basilar artery to 5‐HT and the selective 5‐HT1‐like agonist GR43175 were highly reproducible whilst those to 5‐CT were not. 5 The contractile response to both 5‐HT and GR43175 was resistant to antagonism by ketanserin and GR38032, thus excluding activation of 5‐HT2 and 5‐HT3 receptors. The contractile action of 5‐HT and GR43175 was also not antagonized by (±)‐cyanopindolol, excluding the activation of receptors similar to 5‐HT1A and 5‐HT1B recognition sites identified in ligand binding studies. 6 In marked contrast, methiothepin was a potent antagonist of the contractile actions of both 5‐HT and GR43175, with a pA2 value of 8.8 against both agonists. Methiothepin (100 nm) had no effect on the contractile response to the thromboxane A2‐mimetic U46619. 7 We conclude that 5‐HT and GR43175 contract the human isolated basilar artery by activating the same receptor type. This receptor appears identical to the 5‐HT1‐like receptor causing contraction of the dog isolated saphenous vein and cerebral blood vessels from the dog and primate.

A comparison of 5‐hydroxytryptamine receptors mediating contraction in rabbit aorta and dog saphenous vein: evidence for different receptor types obtained by use of selective agonists and antagonists

1 Using recently available selective agonists and antagonists we have examined further our postulate (Apperley et al., 1980) that 5‐hydroxytryptamine (5‐HT) mediates contraction of dog saphenous vein via a different 5‐HT receptor type from that in the rabbit aorta. 2 In the rabbit isolated aorta, ketanserin and spiperone were potent, specific, competitively‐acting antagonists of the contractile effects of 5‐HT. 3 In contrast, in the dog isolated saphenous vein neither ketanserin nor spiperone caused any rightward displacement of concentration‐response curves to 5‐HT although the maximum response was reduced by about 10%. 4 In the rabbit aorta 5‐carboxamidotryptamine (5‐CONH2‐T) was a weak agonist whilst the 5‐N,N‐dimethyl and 5‐N‐ethyl derivatives were even weaker or inactive. The contractile effect of 5‐CONH2‐T in the rabbit aorta was potently and competitively antagonized by ketanserin. 5 In contrast, in the dog saphenous vein 5‐CONH2‐T and its 5‐N,N‐dimethyl and 5‐N‐ethyl derivatives were all potent agonists. The contractile effect of 5‐CONH2‐T was not markedly affected by ketanserin. 6 The profile of action of ketanserin and spiperone in the rabbit aorta is consistent with the view that 5‐HT2 receptors mediate contraction in this preparation. However, the 5‐HT receptor mediating contraction in the dog saphenous vein appears to be ‘5‐HT1‐like’, sharing a number of characteristics with the 5‐HT1 recognition site identified from [3H]‐5‐HT ligand binding studies in brain tissue. tissue.

5‐Carboxamidotryptamine is a selective agonist at 5‐hydroxytryptamine receptors mediating vasodilatation and tachycardia in anaesthetized cats

1 We have attempted to characterize the 5‐hydroxytryptamine (5‐HT) receptors mediating bronchoconstriction, vasodilatation, vasodepression and tachycardia in anaesthetized cats following bilateral vagosympathectomy and β‐adrenoceptor blockade with propranolol. 2 5‐HT (1–100 μg kg−1 i.v.) caused dose‐related bronchoconstriction and tachycardia but variable and complex effects on diastolic blood pressure and carotid arterial vascular resistance. 3 In contrast, 5‐carboxamidotryptamine (5‐CT; 0.01–1 μg kg−1 i.v.) caused consistent, dose‐related decreases in diastolic blood pressure and carotid arterial vascular resistance and increases in heart rate. 5‐CT did not cause bronchoconstriction. 4 The 5‐HT‐induced bronchoconstriction was dose‐dependently antagonized by methiothepin, methysergide and ketanserin (10–100 μg kg−1 i.v.). The highest doses used of these antagonists did not antagonize bronchoconstriction induced by prostaglandin F2α. The high potency of all three antagonists indicate a 5‐HT2‐receptor mediated effect. 5 The 5‐HT‐ and 5‐CT‐induced tachycardia as well as the 5‐CT‐induced vasodepressor and carotid arterial vasodilator responses were dose‐dependently antagonized by low doses of methiothepin (10–100 μg kg−1 i.v.) and by high doses of methysergide (100–1000 μg kg−1 i.v.) but were little affected by ketanserin in doses up to 1000 μg kg−1 i.v. These selective effects of 5‐CT appear to be mediated by ‘5‐HT1‐like’ receptors.

Influence of the endothelium on contractile effects of 5-hydroxytryptamine and selective 5-HT agonists in canine basilar artery.

1 We have investigated the influence of endothelial damage on the cerebrovascular reactivity to 5‐hydroxytryptamine (5‐HT) and some selective 5‐HT agonists in canine basilar artery. 2 5‐HT, α‐methyl 5‐HT, GR 43175 (3‐[2‐dimethyl amino] ethyl‐N‐methyl‐lH‐indole‐5‐methane sulphonamide) and 5‐carboxamidotryptamine (5‐CT) produced concentration‐dependent contractions of untreated dog basilar artery with a functional endothelium. Following endothelial damage by perfusion with Triton X‐100 (0.1%), which abolished the relaxant response to substance P, the maximum contractile effect of 5‐HT, α‐methyl 5‐HT, GR 43175 and 5‐CT was markedly enhanced although there was little change in the EC50 values. Endothelial damage did not modify the vasoconstrictor effect of the thromboxane agonist, U46619, or potassium chloride. 3 Neither 5‐HT nor 5‐CT caused relaxation of untreated canine basilar arteries contracted with prostaglandin F2α, U46619, uridine triphosphate or potassium chloride. 4 These results suggest that canine basilar artery spontaneously releases endothelium‐derived relaxing factor which can attenuate the vasoconstrictor effect of 5‐HT and selective 5‐HT agonists. This effect appeared to be specific since the vasoconstrictor response to U46619 was not modified. 5 These results demonstrate that the cerebrovascular endothelium can markedly influence the reactivity of the vascular smooth muscle of canine basilar artery to 5‐HT and 5‐HT1‐like receptor agonists. However we could find no evidence that 5‐HT receptor activation stimulates endothelial cell function as it does in some other blood vessels.

Role of endothelium in haemoglobin-induced contraction of dog basilar artery

Haemoglobin (10 nM-30 microM) caused contraction of dog isolated basilar artery. In preparations which were perfused with Triton X-100 (0.1%) for one min to remove the endothelium, substance P relaxation was abolished but haemoglobin-induced contractions were unchanged. Endothelium removal or damage was confirmed histologically. These results suggest that the integrity of the endothelium is not essential for haemoglobin to produce contraction in dog basilar artery.

Neurokinin-induced changes in pial artery diameter in the anaesthetized guinea-pig.

1 The effects of selective neurokinin agents on pial artery diameter, measured with an on‐line image analyser, have been studied in anaesthetized guinea‐pigs in order to characterize the neurokinin receptors present on pial arteries. 2 Perivascular injection of either substance P (0.01–1 μm) or the selective NK1 receptor agonists, substance P methyl ester (SPOMe, 0.01–1 μm) and GR73632 (0.1 μm), increased pial artery diameter. 3 In contrast, the selective NK2 receptor agonist, GR64349 (1 μm), produced a small vasoconstriction while the NK3 receptor‐selective agonist, senktide (1 μm) was inactive. 4 Co‐administration of GR82334 (1 μm), a selective NK1 receptor antagonist, inhibited the vasodilatation produced by SPOMe (0.1 μm) but not that caused by calcitonin gene‐related peptide (CGRP, 0.01 μm). 5 The results are consistent with an involvement of NK1 receptors in the neurokinin‐induced increase in guinea‐pig pial artery diameter.

Analysis of the 5-HT receptors mediating contractions in the rabbit isolated renal artery.

1 Using a number of agonist and antagonist compounds, we have attempted to characterize the responses and receptors involved in the effects of 5‐hydroxytryptamine (5‐HT) in the rabbit isolated renal artery. 2 In vessel segments precontracted with the thromboxane‐mimetic agent, U46619 (100 nm), neither 5‐HT (10−8 to 10−4 m) nor 5‐carboxamidotryptamine (5‐CT; 10−8 to 3 × 10−4 m) caused relaxations like those observed with methacholine. Both 5‐HT and 5‐CT further increased the tone of the vessels, with pD2 values of 7.1 and 7.9, respectively. 3 In the absence of U46619, both 5‐HT (10−7 to 3 × 10−3 m) and 5‐CT (10−7 to 10−3 m) contracted the rabbit renal artery, but with reduced potencies. The contractions to 5‐HT were reproducible and the rank order of potency (pD2) of the agonists was: α‐methyl‐5‐HT (5.7), sumatriptan (5.3), 5‐HT (5.1), 8‐hydroxy‐2(di‐n‐propylamino)tetralin (5.0), 5‐CT (4.7) and 5‐methoxytryptamine (4.3). 1‐(2,5‐Dimethoxy‐4‐iodophenyl)‐2‐aminopropane, flesinoxan and RU 24969 elicited either only small contractions or none at all. 4 The contractile effect of 5‐HT was unaffected by MDL 72222 (10−6 m) and metergoline (10−8 and 10−7 m), was weakly antagonized by ketanserin and phentolamine (pKB: 6.6 and 6.8, respectively), but was effectively antagonized by methiothepin (pKB: 8.6). Responses to 5‐CT and sumatriptan were affected by ketanserin, phentolamine and methiothepin similarly to 5‐HT‐induced responses. 5 Ketanserin was ineffective against noradrenaline‐induced contractions, which were antagonized by phentolamine with a pKB of 7.3. The pKB values of phentolamine against 5‐HT, 5‐CT or sumatriptan were about half a log unit lower than against noradrenaline. 6 In vascular preparations treated with cocaine (3 × 10−5 m), the potency (pKB) of phentolamine as an antagonist of the responses to noradrenaline (7.6) and 5‐HT (6.7) did not differ significantly from the values in untreated preparations. However, the difference between the pKB values of phentolamine against the two agonists was now about one log unit. 7 Pretreatment of the vascular strips with 6‐hydroxydopamine (1.5 × 10−3 m) did not significantly affect responses to 5‐HT or 5‐CT, but almost eliminated those to tyramine. Cocaine (3 × 10−5 m) slightly potentiated noradrenaline‐induced contractions, but did not significantly affect those induced by 5‐HT. 8 These data suggest that: (i) 5‐HT receptors mediating vasodilatation are not present in the rabbit renal artery smooth muscle or endothelium; (ii) the contractile effect of 5‐HT does not involve the release of noradrenaline from sympathetic nerve stores; (iii) the 5‐HT receptor in the rabbit renal artery is not of the 5‐HT2, 5‐HT3 or 5‐HT4 type. The pharmacological properties of this receptor most closely resemble those described for the heterogeneous 5‐HT1‐like category.

Characterization of the receptor mediating relaxation to substance P in canine middle cerebral artery: no evidence for involvement of substance P in neurogenically mediated relaxation

1 The aim of this study was to characterize the neurokinin receptor which mediates relaxation of dog isolated middle cerebral artery by the use of selective agonists and antagonists and to establish whether substance P is involved in the neurogenically mediated relaxant response in this vessel. 2 Substance P caused concentration‐related, endothelium‐dependent relaxations of dog isolated middle cerebral artery, contracted with prostaglandin F2α. The selective NK1 receptor agonists, GR73632 and substance P methyl ester (SPOMe), also caused relaxation with similar maximum effects to those of substance P. GR73632 and SPOMe were approximately 20 times and 6 times less potent respectively than substance P. The selective NK2 and NK3 receptor agonists, GR64349 and senktide, were only weakly active in causing relaxation being at least 425 times and 245 times less potent respectively than substance P. 3 The selective NK1 receptor antagonist, GR82334, was a potent, specific, competitive antagonist of the relaxant effects of substance P. In contrast, the selective NK2 receptor antagonist, R396 (10 μm) had no effect on the response to substance P. 4 Electrical field stimulation of dog isolated middle cerebral artery, contracted with prostaglandin F2α, caused neurogenically mediated, non‐adrenergic non‐cholinergic (NANC) relaxations. These NANC relaxations were unaffected by endothelium removal, GR82334 (10 μm) or by capsaicin (10 μm) treatment. However, the nitric oxide synthesis inhibitor, L‐NG‐monomethyl arginine methyl ester (L‐NMMA) (100 μm) markedly attenuated the response to electrical stimulation. 5 These results suggest that substance P causes relaxation of dog isolated middle cerebral artery via activation of NK1 receptors. However, substance P does not appear to be involved in NANC neurotransmission. In contrast, the marked inhibitory effect of L‐NMMA on NANC relaxations implicates nitric oxide in NANC neurotransmission in this vessel.

Rabbit isolated renal artery contractions by some tryptamine derivatives, including 2-methyl-5-HT, are mediated by a 5-HT1-like receptor.

1 Despite the fact that 5‐hydroxytryptamine (S‐HT)‐induced contractions of the rabbit isolated renal artery are mediated by a receptor belonging to the heterogeneous 5‐HT1‐like category, we observed that the so‐called selective 5‐HT3 receptor agonist, 2‐methyl‐5‐HT, caused a concentration‐dependent contraction of this vessel. This study was therefore undertaken to analyze the effects of 2‐methyl‐5‐HT in the renal artery segments, either quiescent or precontracted with U46619 (10−7 m). α‐Methyl‐5‐HT and 5‐methoxytryptamine, which have high affinities for 5‐HT2 and 5‐HT4 receptors, respectively, were used for comparison. 2 In the precontracted vessel segments, the maximum contractile responses obtained with 2‐methyl‐5‐HT, α‐methyl‐5‐HT, 5‐methoxytryptamine and 5‐HT were similar to those in the quiescent segments. However the pD2 values were higher in the precontracted segments, making them about 4–100 fold more sensitive. 3 Neither MDL 72222 (10−6 m) nor tropisetron (3 × 10−6 m) suppressed renal artery contractions elicited by 5‐HT, 2‐methyl‐5‐HT, α‐methyl‐5‐HT or 5‐methoxytryptamine, thus ruling out the involvement of 5‐HT3 as well as 5‐HT4 receptors. 4 On the other hand, both methiothepin (10−8 and 10−7 m) and ketanserin (10−7 and 10−6 m) caused a rightward shift of agonist concentration‐effect curves. The two antagonists had similar pA2 values against the different agonists tested on either quiescent or precontracted vessels, but ketanserin (apparent pA2: 6.6 to 7.0) was between 20–100 fold less potent than methiothepin (apparent pA2: 8.4 to 8.8). 5 The results of this functional study permit us to conclude that the contractile effects of 2‐methyl‐5‐HT as well as α‐methyl‐5‐HT and 5‐methoxytryptamine on the rabbit isolated renal artery are mediated by a 5‐HT1‐like receptor. Since, in addition, the reported ligand binding affinity of 2‐methyl‐5‐HT at 5‐HT3 receptors is similar to both the ligand binding affinity and the functional pD2 at 5‐HT1 sites, this compound cannot be regarded as a selective 5‐HT3 receptor agonist. Similarly, α‐methyl‐5‐HT and 5‐methoxytryptamine have only a limited selectivity for 5‐HT2 and 5‐HT4 receptors, respectively.

Serotonin receptors Therapeutic prospects in cardiovascular disease.

Serotonin is widely distributed throughout the body, can have potent and profound effects on the cardiovascular system, and may be involved in the pathophysiology of several cardiovascular disease states. Recent advances in the classification of serotonin receptors, along with the identification of selective drug tools for some of these receptor types, enables the putative role of different serotonin receptor subtypes in both the etiology and treatment of disease states to be explored. This review considers the potential therapeutic application of selective serotonin receptor agonists and antagonists in cardiovascular disease.