Characterization of an endothelial 5-hydroxytryptamine (5-HT) receptor mediating relaxation of the porcine coronary artery

Response of Rabbit Ear and Femoral Arteries to 5-Hydroxytryptamine During Cooling

The effects of cooling on the response of cutaneous and non-cutaneous arteries to 5-hydroxytryptamine (5-HT) were analysed.

Segments 2-mm long from rabbit central ear (cutaneous) and femoral (non-cutaneous) arteries were prepared for isometric tension recording in an organ bath at 37 and 24°C (cooling). 5-HT (10−9-3 times 10−4 M) induced concentration-dependent contraction of the arteries. The sensitivity and maximal contraction of ear arteries and only the maximal contraction of femoral arteries to this amine were reduced at 24°C.

Endothelium removal or pretreatment with the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester (l-NAME, 10−5  m) did not affect the response at 37°C but reversed the decreased sensitivity at 24°C in ear arteries, and neither procedure modified the reactivity at 24 or 37°C in femoral arteries to 5-HT. At both temperatures, the response of ear arteries to 5-HT was shifted to the right by phentolamine (10−6M) more than by the 5-HT antagonist, ketanserin (3 times 10−7M), and that of femoral arteries was shifted to the right by ketanserin or the 5-HT1/5-HT2 antagonist methysergide (3 times 10−7 M) more than by phentolamine, in arteries with and without endothelium.

These data concur with the proposition that the contraction to 5-HT is mediated mainly by α-adrenergic receptors in ear arteries and mainly by 5-HT-ergic receptors in femoral arteries, and suggest that cooling reduces the sensitivity of cutaneous, but not of deep arteries to 5-HT, probably by endothelium-nitric oxide-dependent mechanisms.

Functions of 5-HT2A receptor and its antagonists in the cardiovascular system

The serotonin (5-hydroxytryptamine, 5-HT) receptors have conventionally been divided into seven subfamilies, most of which have several subtypes. Among them, 5-HT(2A) receptor is associated with the contraction of vascular smooth muscle, platelet aggregation and thrombus formation and coronary artery spasms. Accordingly, selective 5-HT(2A) antagonists may have potential in the treatment of cardiovascular diseases. Sarpogrelate, a selective 5-HT(2A) antagonist, has been introduced clinically as a therapeutic agent for the treatment of ischemic diseases associated with thrombosis. Molecular modeling studies also suggest that sarpogrelate is a 5-HT(2A) selective antagonist and is likely to have pharmacological effects beneficial in the treatment of cardiovascular diseases. This review describes the above findings as well as the signaling linkages of the 5-HT(2A) receptors and the mode of agonist binding to 5-HT(2A) receptor using data derived from molecular modeling and site-directed mutagenesis.

Insurmountable antagonism of AT-1015, a 5-HT2 antagonist, on serotonin-induced endothelium-dependent relaxation in porcine coronary artery

The purpose of this study was to examine the inhibitory effects of AT-1015, a newly synthesized 5-HT(2) receptor antagonist, on serotonin-induced endothelium-dependent relaxation in U 46619 (5 x 10(-9)M)-precontracted porcine coronary artery pre-incubated with ketanserin (3 x 10(-6)M), and then compare its effects with another potent 5-HT(2) antagonist, ritanserin. The investigation showed that AT-1015 (10(-8)-10(-6)M) caused rightward shift with significant inhibition of maximum relaxation response induced by serotonin in porcine coronary artery with endothelium. Ritanserin caused a rightward shift of serotonin-induced relaxation without decreasing maximum response at 10(-9) and 10(-8)M, but it inhibited the maximum relaxation response at 10(-7)M. The study showed that AT-1015 and ritanserin had no inhibitory effect on bradykinin-induced relaxation in porcine coronary artery with endothelium. Thus, these findings suggested that AT-1015 at concentrations of 10(-8)-10(-6)M caused noncompetitive blockade of serotonin-induced endothelium-dependent relaxation in porcine coronary artery. The antagonistic effects of AT-1015 on serotonin-induced relaxation were different from that of ritanserin, except at 10(-7)M ritanserin. The variation of inhibitory effects between these two 5-HT(2) antagonists may be due to the different chemical structure and/or interaction sites at the receptor.

Effects of sarpogrelate, a novel 5-HT2 antagonist, on 5-HT-induced endothelium-dependent relaxations in porcine coronary artery

The aim of the present study was to examine the effects of sarpogrelate, a 5-HT2 antagonist, on 5-HT-induced endothelium-dependent relaxation in isolated porcine coronary artery preincubated with ketanserin (3 x 10(-6) M) and precontracted by U 46619 (5 x 10(-9) M) and compare its effects with other 5-HT2 antagonists such as ritanserin and cyproheptadine. The investigation showed that sarpogrelate (10(-7)-10(-5) M) had a weak antagonistic effect on 5-HT-induced relaxation and its effect was weaker than that of ritanserin (10(-9)-10(-7) M) and cyproheptadine (10(-8)-10(-6) M). The rank order of the antagonistic effects was: ritanserin > cyproheptadine > sarpogrelate. The study also showed that both sarpogrelate and ritanserin had no inhibitory effect on bradykinin-induced relaxation. In our previous study, we investigated the binding affinity of sarpogrelate, ritanserin and cyproheptadine to the 5-HT2A-receptor in rabbit cerebral cortex membranes and the pKi values found were 7.22, 8.98 and 7.54, respectively (M. Rashid et al., Jpn J Pharmacol 87, 189-194, 2001). Rank order of the calculated ratio of concentration of pA2 or pD'2 vs Ki was: sarpogrelate > ritanserin > cyproheptadine. Thus, these findings suggest that sarpogrelate has the lowest antagonistic effect on 5-HT-induced endothelium-dependent relaxation and the highest selectivity towards 5-HT2A receptor and might also be the safest drug with respect to its clinical implications in comparison with ritanserin and cyproheptadine.

Sumatriptan elicits both constriction and dilation in human and bovine brain intracortical arterioles

1. Little is known about serotonin (5-HT) receptors present on brain microvessels that are innervated by brainstem serotonergic neurons. Using 5-HT, sumatriptan and subtype selective 5-HT(1) receptor agonists and/or the 5-HT(1) receptor antagonist GR127935, we characterized the 5-HT receptors involved in regulating microvascular tone of pressurized intracortical arterioles (approximately 40--50 microm) isolated from human and bovine cerebral cortex. The role of nitric oxide (NO) on these responses was assessed with the N(omega)-nitro-L-arginine (L-NNA, 10(-5) M), an inhibitor of NO synthesis. Bovine pial arteries were studied for comparative purposes. 2. At spontaneous tone, 5-HT induced a dose-dependent constriction of human and bovine microarteries (respective pD(2) values of 7.3+/-0.2 and 6.9+/-0.1); a response potently inhibited by GR127935 (pIC(50) value of 8.5+/-0.1) in bovine microvessels. 3. In both species, the 5-HT(1) receptor agonist sumatriptan induced a biphasic response consisting of a small but significant dilation at low concentrations (1 and/or 10 nM) followed by a constriction at higher doses (pD(2) for contraction of 6.9+/-0.1 and 6.6+/-0.2 in human and bovine vessels, respectively). Pre-incubation with L-NNA abolished the sumatriptan-induced dilation and significantly shifted the dose-response of the constriction curve to the left. In contrast, the selective 5-HT(1D) (PNU-109291) and 5-HT(1F) (LY344864) receptor agonists were devoid of any vasomotor effect. 4. In bovine pial vessels, 5-HT and sumatriptan elicited potent constrictions (respective pD(2) of 7.2+/-0.1 and 6.6+/-0.1), a weak dilation being occasionally observed at low sumatriptan concentrations. 5. A significant negative correlation was observed between pial and intracortical vessels diameter and the extent of the dilatory response to 10(-9) M sumatriptan. Together, these results indicate that sumatriptan, most likely via activation of distinctly localized microvascular 5-HT(1B) receptors, can induce a constriction and/or a dilation which is sensitive to inhibition of NO synthesis and dependent on the size and, possibly, the existing tone of the vessels.

Functional evaluation of 5-hydroxytryptamine receptor activity in rat resistance vessels

1. To characterize 5-hydroxytryptamine (5-HT) receptors in rat perfused mesenteric vascular bed (MVB), the effect of 5-HT and related compounds was investigated by functional assay. 2. In quiescent preparations, 5-HT elicited a concentration-dependent conctractile response. After addition of ketanserin, a 5-HT2 receptor antagonist, EC50 values were significantly higher than in controls. 3. In noradrenaline (NA)-precontracted preparations, under continuous infusion of ketanserin, 5-HT, 5-carboxamidotryptamine (5-CT) and sumatriptan produced relaxation. Their rank order of relaxant potency and maximum effect were sumatriptan > 5-HT > 5-CT. Methysergide (1 microM) and spiperone (20-100 nM) caused a rightward shift of the relaxation curve to sumatriptan. These data suggest that vasodilatation in rat MVB is mediated by an 'atypical' subtype of 5-HT1-like receptor, which reveals a pharmacological profile similar to that of the 5-HT1D receptor. The involvement of both 5-HT3 and 5-HT4 receptors can be ruled out, since tropisetron (up to 10 microM) was not able to antagonize the relaxant effect by sumatriptan. 4. Under granisetron infusion (3 microM), the contractile response evoked by perivascular nervous stimulation, but not exogenous NA contraction, was significantly reduced (P < 0.001). These data demonstrate the presence of 5-HT3 receptors in peripheral neurones, modulating neurotransmitters release.

Endothelial 5-HT receptors mediate relaxation of porcine pulmonary arteries in response to ergotamine and dihydroergotamine

1. The aim of the present study was to investigate whether antimigraine ergot compounds may act at endothelial 5-hydroxytryptamine (5-HT) receptors which trigger the release of endothelium-derived relaxing factor (EDRF). Changes in tone of porcine isolated pulmonary arteries were measured isometrically. The integrity of the endothelium was assessed by the bradykinin-induced relaxation of prostaglandin F2 alpha (PGF2 alpha, 3 microM)-precontracted vessels. 2. The ergot derivatives ergotamine, dihydroergotamine (DHE) and dihydroergocristine, as well as 5-HT and (+/-)-alpha-methyl-5-HT, elicited a reversible endothelium-dependent relaxation of PGF2 alpha-precontracted arterial ring segments. The relaxation to both ergotamine and 5-HT was associated with an increase in cyclic GMP. After pretreatment of the vessels with NG-nitro-L-arginine methyl ester (200 microM), or removal of endothelium by mechanical rubbing, the relaxant responses were abolished. 3. The mean pEC50 values for relaxant responses followed the order: (+/-)-alpha-methyl-5-HT (8.80) > 5-HT (8.75) > ergotamine (8.17) > DHE (7.70) > 5-carboxamidotryptamine (7.62) > dihydroergocristine (7.17). 4. The relaxant effects of both ergotamine and dihydroergotamine were resistant to block by indomethacin (3 microM), prazosin (1 microM) and ketanserin (1 microM). However, the ergotamine-induced relaxation was highly susceptible to block by pizotifen (pA2 = 8.23), norclozapine (pA2 = 8.20), methiothepin (-log IC50 = 7.26), rauwolscine (pA2 = 7.24) and mesulergine (pA2 = 6.64). Each antagonist inhibited the relaxant responses to (+/-)-alpha-methyl-5-HT in the same manner with similar potency as that determined against ergotamine. 5. Recently, mRNA transcripts for 5-HT1D beta and 5-HT2B receptors have been demonstrated in porcine pulmonary arteries. The rank order of potencies of agonists and antagonists in the present study suggests that the relaxant responses to 5-HT and ergot derivatives are mediated through activation of endothelial 5-HT receptors which are similar to the 5-HT2B receptor subtypes.

The effect of tacrolimus (FK506) and cyclosporin A (CyA) on peripheral serotonergic mechanisms in uremic rats

The aim of this study was to investigate the mechanism of hypertension and thromboembolic events induced by cyclosporin (CyA) and tacrolimus (FK506) in respect to their action on the serotonergic blood system. The study was carried out on healthy rats and those with experimental chronic renal failure. CyA injected into healthy and uremic rats caused an increase in serotonin (5-HT) concentration levels in whole blood and platelets. Concomitantly a rise in systolic blood pressure was observed. Platelet aggregation values were significantly higher in uremic rats given CyA. FK506 had no influence on 5-HT blood content, blood pressure or platelet aggregation values. It is concluded that 5-HT may play a role in the development of hypertension and thrombotic events caused by CyA. Furthermore, lower incidence of these complications during FK506 treatment could be the result of this drug's limited effect on the serotonergic blood system.

Functional serotonin 5-HT1D receptors and 5-HT1D beta receptor mRNA expression in human umbilical vein endothelial cells

Pharmacological evidence has suggested the presence of 5-hydroxytryptamine (5-HT, serotonin), 5-HT(1D) receptors on endothelial cells but these receptors have never been identified unambiguously on this type of cells. We now report that human umbilical vein endothelial cells (HUVEC) express 5-HT(1D) receptors coupled to inhibition of cyclic AMP formation. 5-HT and 5-HT(1D) receptor agonists 5-carboxamidotryptamine (5-CT) and sumatriptan were approximately equipotent at inhibiting forskolin-stimulated cyclic AMP accumulation in HUVEC (mean pEC50 7.6-8.2, maximal effect 30% inhibition). The 5-HT(1A) receptor antagonist, 8-OH-DPAT was clearly less potent (pEC50 6.2) and less efficacious. The selective 5-HT(1D) receptor agonist, GR127935 (1 nM) markedly inhibited the effect of 5-HT (apparent pK(B) 10.8). Reverse transcription-polymerase chain reaction analysis showed the mRNA for 5-HT(1D beta) receptors to be expressed in HUVEC. These results demonstrate the presence of functional 5-HT(1D) receptors and the expression of 5-HT(1D beta) receptor mRNA in HUVEC. They support the involvement of 5-HT(1D beta) receptors in endothelial-mediated responses to 5-HT.

Expression of serotonin receptor mRNAs in blood vessels

Using RT-PCR we distinguished mRNAs for all known G-protein coupled serotonin receptors expressed in various rat and porcine blood vessels. Nearly all vessels expressed 5HT1D beta, 5-HT2A, 5-HT2B, 5-HT4, and 5-HT7 receptor mRNA to different extents. New splice variants of the porcine 5-HT4 receptor were observed. Similar PCR assays were performed with endothelial and smooth muscle cells from human pulmonary artery, aorta, and with endothelial cells from human coronary artery and umbilical vein. All endothelial cells expressed 5-HT1D beta, 5-HT2B, and 5-HT4 receptor mRNA, whereas in smooth muscle cells 5-HT1D beta, 5-HT2A, 5-HT7, and in some experiments 5-HT2B receptor mRNA were found. A model for the regulation of vascular tone by different 5-HT receptors is proposed.

Effect of 5-hydroxytryptamine on the membrane potential of endothelial and smooth muscle cells in the pig coronary artery

1. Many endothelium-dependent vasodilators hyperpolarize the endothelial cells in blood vessels. It is not known whether these hyperpolarizations are linked to nitric oxide synthesis or to an endothelium-derived hyperpolarizing phenomenon, since most of the vasodilators release both factors. In this context, we first verified that the endothelium-dependent relaxations induced by 5-hydroxytryptamine (5-HT) on pig coronary arteries are due only to the activation of the nitric oxide pathway. Then we studied the effects of 5-HT on membrane potential of endothelial and smooth muscle cells. 2. In the absence of endothelium, 5-HT caused a concentration-dependent contraction of coronary artery strips. No change of the smooth muscle cell membrane potential was observed during contraction to 1 microM 5-HT. 3. In the presence of 1 microM ketanserin to suppress the contractile effect of 5-HT, 5-HT induced concentration-dependent relaxation of endothelium-intact strips precontracted by 10 microM prostaglandin F2 alpha (PGF2 alpha). These relaxations were suppressed by 1 microM NG-nitro-L-arginine, an inhibitor of nitric oxide synthesis, showing that they were produced predominantly by nitric oxide. 4. In the presence of 1 microM ketanserin, 1 microM 5-HT did not change the smooth muscle cell membrane potential of strips precontracted by either 10 microM PGF2 alpha or by 10 microM acetylcholine (ACh). In the same conditions, 1 microM 5-HT caused a weak 2.6 +/- 0.4 mV hyperpolarization, of the endothelial cells. 5. In conclusion, the fact that 5-HT did not change the membrane potential of smooth muscle cells and only weakly hyperpolarized the endothelial cells during relaxations, suggests that in both cell types no electrical events accompany activation of the nitric oxide pathway. This is in contrast to the hyperpolarizations observed in endothelial and smooth muscle cells when the endothelium-derived hyperpolarization factor (EDHF) pathway is activated.

Canine renovascular responses to sumatriptan and 5-carboxamidotryptamine: modulation through endothelial 5-HT1-like receptors by endogenous nitric oxide

1. In anaesthetized dogs, intra-left atrial (i.l.a.) administration of the 5-HT1-like receptor agonists, sumatriptan (1-10 micrograms kg-1) and 5-carboxamidotryptamine (0.03-0.3 micrograms kg-1) produced dose-related reductions in renal blood flow and vascular conductance, which were characterized by their rapid onset and recovery. 2. In these animals, i.v. administration of the inhibitor of nitric oxide (NO) synthase, NG-nitro-L-arginine methyl ester (L-NAME; 10 mg kg-1) significantly augmented the renal vasoconstrictor responses to i.l.a. sumatriptan and 5-carboxamidotryptamine. 3. The effects of L-NAME upon these responses to sumatriptan and 5-carboxamidotryptamine were significantly reversed by subsequent i.v. administration of L-arginine (1000 mg kg-1). 4. L-NAME significantly attenuated the systemic hypotensive responses to i.v. acetylcholine (0.3-3 micrograms kg-1) and this effect was also reversed by L-arginine. 5. L-NAME had no effect upon the renal vasoconstrictor response to i.l.a. administration of angiotensin II, nor did it affect the renal vascular conductance recovery response to brief mechanical occlusion of the renal artery. 6. These data suggest that sumatriptan and 5-carboxamidotryptamine stimulate the release of NO through the activation of a 5-HT1-like receptor located on the endothelial cells. 7. It is concluded that in canine renal vasculature, 5-HT1-like agonists (and presumably endogenous 5-hydroxytryptamine) can cause simultaneous activation of a 5-HT1-like receptor on both vascular smooth muscle and endothelial cells. The net renal vascular response to these agonists is therefore a function of both the vascular smooth muscle vasoconstriction and the concurrent vasodilator influence of NO released from the endothelium.

Endothelium-dependent relaxation of porcine pulmonary arteries via 5-HT1C-like receptors

In PGF2 alpha-precontracted pulmonary arteries with intact endothelium, 5-hydroxytryptamine (5-HT, 1.0-100 nmol/l) caused a concentration-dependent reversible relaxation, at higher concentrations the contractile response prevailed. In endothelium-denuded vessels relaxation was absent. 5-HT-induced relaxation of precontracted pulmonary arteries was probably mediated by release of an endothelium-derived relaxing factor (EDRF). Preincubation of the arteries with methylene blue or NG-nitro-L-arginine (200 mumol/l) attenuated the relaxant effect. The 5-HT-induced relaxation was accompanied by an increase in cGMP. Indomethacin (3 mumol/l) did not influence the 5-HT-induced relaxation indicating that eicosanoids are not involved in the relaxant response to 5-HT. The 5-HT1C and 5-HT2 receptor agonist alpha-methyl-5-HT was as potent as 5-HT in inducing relaxation. The rank order of relaxant potency of the agonists investigated was alpha-methyl-5-HT > 5-HT > 5-methoxytryptamine > tryptamine > omega-methyl-5-HT > 5-carboxamidotryptamine > 2-methyl-5-HT > 5,6-dihydroxytryptamine > m-chlorophenylpiperazine > sumatriptan > 8-OH-DPAT. Phentolamine, pindolol and ICS 205-930 did not interfere with the relaxant effect. The 5-HT2 receptor antagonist ketanserin (1 mumol/l) inhibited the contractile response but did not alter vasodilatation. Apart from the blockade of the contractile effects, mesulergine, cyproheptadine and mianserin (0.1-3.0 mumol/l, each) induced a parallel shift to the right of the concentration-response curve for the relaxation induced by alpha-methyl-5-HT or 5-HT. Spiperone (0.3 mumol/l) exerted weak inhibitory effects on relaxation and contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Amplifying effect of serotonin on contractile responses in rat aorta and depletion of intracellular Ca-stores

1. Serotonin, 1 microM, induces a contractile response in isolated rat aorta in the presence or absence of extracellular Ca. 2. In Ca-free media, the fast phasic contraction is lower in magnitude and further addition of serotonin evokes no response. 3. Recovery of the contractile response in Ca-free medium is obtained by a 40 min incubation in Ca-containing solution. 4. In Ca, Mg-free medium, the response to serotonin is significantly higher than that obtained in the presence of Mg. 5. An amplifying effect of serotonin on the contractile responses induced by serotonin itself or by noradrenaline was observed in Ca-containing but not in Ca-free solution.

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 (5-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). alpha-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, alpha-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 x 10(-6) M) suppressed renal artery contractions elicited by 5-HT, 2-methyl-5-HT, alpha-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 ct-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-HTI sites, this compound cannot be regarded as a selective 5-HT3 receptor agonist. Similarly, a-methyl-5-HT and 5-methoxytryptamine have only a limited selectivity for 5-HT2 and 5-HT4 receptors, respectively.

Histamine inhibits 5-hydroxytryptamine release from the porcine small intestine: involvement of H3 receptors

Strips of the porcine small intestine were incubated in vitro, and the release of 5-hydroxytryptamine (5-HT) was determined by high-pressure liquid chromatography with electrochemical detection. Removal of the mucosa resulted in a large reduction (95%) of tissue 5-HT, suggesting that enterochromaffin cells are the main source of 5-HT. The release of 5-HT was reduced by 70% after omission of calcium. Tetrodotoxin and hexamethonium reduced the release of 5-HT by 30%-40% in a nonadditive manner, indicating a spontaneous neuronal (nicotinic) excitatory input to the enterochromaffin cells. Histamine inhibited the release of 5-HT by about 50%. This effect was not affected by mepyramine or cimetidine but was effectively blocked by thioperamide, indicating the involvement of H3 receptors. The selective H3-receptor agonist R-alpha-methyl-histamine also inhibited 5-HT release. Because the effect of R-alpha-methyl-histamine was also observed in the presence of tetrodotoxin, an indirect, neuronally mediated action could be excluded. Therefore, the inhibitory H3 receptors may be localized directly at the enterochromaffin cells.

A subfamily of 5-HT1D receptor genes

The recent discovery and characterization of three new 5-HT1 receptor clones and the pharmacological characterization of one orphan receptor (dog RDC4) has revealed a surprising complexity within the 5-HT1D receptor subfamily. This receptor subfamily, which is believed to be the target of the anti-migraine drug sumatriptan and may regulate feeding behavior, anxiety, depression, cardiac function and movement, can now be approached on a molecular level. These cloning discoveries have also taught us an important general lesson about the molecular pharmacology of G protein-coupled receptor genes: species homologues of a gene (the equivalent gene in different species) may be highly homologous in amino acid sequence yet display very different pharmacological properties. Conversely, two different genes in the same species (intraspecies subtypes) that display only moderate degrees of transmembrane amino acid homology can display nearly indistinguishable pharmacological properties. In discussing the implications of these findings for both 5-HT receptors and G protein-linked receptors in general, Paul Hartig, Theresa Branchek and Richard Weinshank approach the question: why have so many receptor subtypes been preserved in the genome? In addition, controversy has been raging for several years over the classification of 5-HT1B receptors (found only in rat brain) and 5-HT1D receptors. Were they different subtypes or simply species homologues of the same receptor? Recent cloning studies have apparently complicated this issue, but the answer to the question is, in fact, becoming clearer.

Nicotinic and muscarinic modulation of 5-hydroxytryptamine (5-HT) release from porcine and canine small intestine

Strips of porcine and canine small intestine were incubated in vitro and the release of 5-hydroxytryptamine (5-HT) was determined by HPLC with electrochemical detection. The spontaneous outflow of 5-HT from the porcine and canine small intestine largely reflects calcium-dependent 5-HT secretion from enterochromaffin cells which are under a spontaneous neuronal, excitatory input as indicated by the inhibitory effect (30-40%) of tetrodotoxin. In both species, nicotine enhanced the release of 5-HT in a concentration-dependent manner by a maximum of about 50% at 100 microM. This effect was blocked by the nicotine receptor antagonist hexamethonium, but not by the subtype-selective nicotine receptor antagonist alpha-bungarotoxin. The effect of nicotine was rapidly desensitized. The presence of tetrodotoxin abolished the effect of nicotine on 5-HT release in canine tissue but not in porcine tissue. The presence of the muscarine receptor antagonist scopolamine prevented the effect of nicotine on 5-HT release from canine tissue, but significantly enhanced 5-HT release from porcine tissue. The muscarine receptor agonist oxotremorine inhibited 5-HT release from porcine tissue, but increased 5-HT release from canine tissue. However, in the presence of tetrodotoxin, oxotremorine enhanced 5-HT release in tissue from both species. In conclusion, activation of nicotine receptors induce the release of 5-HT from porcine and canine small intestine. In the dog, the effect of nicotine is mediated via the release of acetylcholine which then stimulates 5-HT release via muscarine receptors on the enterochromaffin cells. In the pig, the stimulatory effect of nicotine appears to be located directly on the enterochromaffin cells. In addition, activation of neuronal muscarine receptors in the porcine small intestine induced the release of a previously unidentified neurotransmitter which inhibited 5-HT release. Nicotine, via cholinergic interneurons, also appears to induce the release of this inhibitory neurotransmitter which opposes the direct stimulatory action of nicotine on 5-HT release.

5-Hydroxytryptamine: a chameleon in the heart

5-HT has profound effects on cardiac rate and force in a variety of animal species, including humans. The main initial response to 5-HT is a short-lasting bradycardia, mediated via a Bezold-Jarisch-like reflex, and initiated by stimulation of 5-HT3 receptors present on cardiac vagal afferents. Once this bradycardia is suppressed, 5-HT induces cardiac stimulation which, true to its chameleonic nature described here by Pramod Saxena and Carlos Villalón, is mediated by different mechanisms and receptors in different species. In several species, including humans, coronary vasodilatation is mediated by 5-HT1-like receptors, while both 5-HT1-like and 5-HT2 receptors mediate vasoconstriction. This knowledge may lead to a better assessment of the possible role of 5-HT in cardiovascular pathologies and to the development of selective 5-HT receptor agonists and antagonists for therapeutic usefulness in heart failure, coronary vasospasm and to avoid potential cardiac side-effects.

Characterization of the 5-HT receptor mediating endothelium-dependent relaxation in porcine vena cava

1. 5-Hydroxytryptamine (5-HT) relaxes rings of neonatal porcine isolated vena cava by both an endothelium-dependent and an endothelium-independent mechanism. The receptor mediating the latter response has been shown to be a 5-HT1-like receptor (positively coupled to adenylyl cyclase) located on the vascular smooth muscle. The features of the endothelium-dependent response to 5-HT in this preparation are now described. 2. In ring preparations contracted with the stable thromboxane-A2-mimetic, U-46619 (10 nM), and in the presence of the 5-HT2 receptor antagonist ketanserin (1 microM), low concentrations of 5-HT (1-100 nM) evoked an endothelium-dependent, rapid, 'spike-like' relaxation. Higher concentrations of 5-HT (0.1-10 microM) elicited a more sustained, but endothelium-independent relaxation. 3. Relaxation induced by low concentrations (1-100 nM) of 5-HT was abolished by endothelium removal, and was markedly (but not totally) inhibited by the guanylate cyclase inhibitor, methylene blue (10 microM) or by the inhibitor of endothelium-derived nitric oxide (NO) synthesis, L-NG-monomethylarginine (L-NMMA; 100-500 microM). 4. The endothelium-dependent response to 5-HT was mimicked by alpha-methyl-5-HT, 5-methoxytryptamine, tryptamine and 2-methyl-5-HT, but not by sumatriptan or 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT) at concentrations up to 10 microM. In contrast, relaxation evoked by 5-carboxamidotryptamine (5-CT) was endothelium-independent. 5. The endothelium-dependent relaxation induced by 5-HT or alpha-methyl-5-HT was antagonized by methysergide, methiothepin, cyproheptadine and metergoline, but not by ketanserin, spiperone, ondansetron, verapamil, cyanopindolol, mesulergine, ICS 205-930, or indomethacin. 6. These results suggest that the endothelium-dependent relaxation of porcine vena cava induced by 5-HT is largely mediated by the release of NO (although other endothelium-derived relaxing factors may also be involved) and that 5-HT is acting at a receptor which is not '5-HT1-like', 5-HT2, 5-HT3 or 5-HT4 and is not comparable to recognised 5-HT receptor ligand binding sites. The characteristics of this receptor are discussed in relation to the endothelial 5-HT receptor types in other blood vessels.

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein via presynaptic 5-HT receptors similar to the 5-HT 1D subtype

The human saphenous vein preincubated with [3H]noradrenaline was used to determine the pharmacological properties of the release-inhibiting presynaptic serotonin (5-HT) receptor on the sympathetic nerves. The overflow of tritium evoked by transmural electrical stimulation (2 Hz) was concentration-dependently inhibited by drugs known to stimulate 5-HT receptors in the following rank order: oxymetazoline greater than or equal to 5-HT greater than 5-carboxamidotryptamine = 5-methoxytryptamine = sumatriptan greater than tryptamine greater than N,N(CH3)2-5-HT = yohimbine = 8-hydroxy-2-(di-n-propylamino)-tetraline. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT1B and 5-HT1D binding sites, but not with those for 5-HT1A or 5-HT1C binding sites. 5-Aminotryptamine, methysergide, ipsapirone, cyanopindolol, SDZ 21009 and metergoline dit not produce a significant inhibition. Metitepine and methysergide antagonized the inhibitory effect of 5-HT, whereas spiroxatrine, propranolol, ketanserin and ICS 205-930 did not. These data exclude the idea that the inhibitory presynaptic 5-HT receptor on the sympathetic nerves belongs to the 5-HT2 and 5-HT3 receptor class; the pattern of agonist potencies suggests that the receptor is very similar to the 5-HT1D receptor subtype.

The preservation of functional activity of smooth muscle and endothelium in pig coronary arteries after storage at -190 degrees C

Pig coronary arteries have been investigated in-vitro using fresh tissue or after storage at -190 C in foetal calf serum containing 1.8 M dimethyl sulphoxide. Attention was paid to modulation of contractile activity and endothelium-dependent relaxation. After cryopreservation of the arteries maximal contractile responses to both 5-hydroxytryptamine (5-HT) and prostaglandin F2 alpha (PGF2 alpha) were markedly reduced and the pD2 values for both agonists were slightly, but significantly, diminished. Nevertheless, 5-HT antagonism by ketanserin and pizotifen was unchanged. Endothelium-independent relaxant responses of precontracted arteries to isoprenaline, forskolin, 3-isobutyl-1-methylxanthine, nitroprusside, atriopeptin III and cromakalim were generally unchanged after storage. Mechanical removal of the endothelium by rubbing enhanced the contractile response to PGF2 alpha in both fresh and stored arteries to a similar extent. In addition, endothelium-dependent relaxant responses to both 5-HT and substance P were well maintained, suggesting release of endothelium-derived relaxing factor by the stored arteries. The evidence suggests that after cryopreservation of pig coronary arteries at -190 degrees C mechanisms of relaxation, in particular those which are endothelium-dependent, are well maintained.

Activation and pulmonary toxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids unsaturated in the 1,2 position are hepatotoxins. Certain of them, such as monocrotaline, are also pneumotoxins, producing pulmonary arterial hypertension and right ventricular hypertrophy as a delayed response two weeks after administration. Pneumotoxicity is the result of hepatic metabolism, the lung itself being unable to bioactivate pyrrolizidine alkaloids. The changes produced in the lung following exposure to pneumotoxic pyrrolizidine alkaloids are reviewed, together with the factors and interventions which modify or influence these changes. In the main, the earliest changes are seen in vascular smooth muscle and in the interactions between the smooth muscle and the endothelium. The search to identify the pneumotoxic metabolite is reviewed. It is generally accepted that pyrroles, or dehydroalkaloids, are responsible for the toxicity of pyrrolizidines. However, the primary pyrroles are intensely reactive, hydrolyzing and polymerizing within seconds in aqueous solution. Evidence for and against the pneumotoxin being a primary pyrrole or a stabilized secondary conversion product of a primary pyrrole is discussed.