Effect of the thromboxane A2-mimetic U46619 on 5-HT1-like and 5-HT2 receptor-mediated contraction of the rabbit isolated femoral 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.

Investigation of the pronounced synergism between prostaglandin E2 and other constrictor agents on rat femoral artery

This study investigates the pronounced synergism between the weak contractile action of prostaglandin E(2) (PGE(2)) and strong actions of phenylephrine, U-46619 and K(+) on rat isolated femoral artery. The potency ranking for synergism was SC-46275 (prostanoid receptor agonist selectivity: EP(3)>>EP(1))=sulprostone (EP(3)>EP(1))>17-phenyl PGE(2) (EP(1)>EP(3)). The novel EP(3) antagonist L-798106 (0.2-1microM) blocked the enhanced action of sulprostone (pA(2)=7.35-8.10), while the EP(1) antagonist SC-51322 (1microM) did not (pA(2)<6.0). Matching responses to priming agent and priming agent/sulprostone were similarly suppressed by nifedipine (300nM) and the selective Rho-kinase inhibitors H-1152 (0.1-1microM) and Y-27632 (1-10microM). Our findings implicate an EP(3) receptor in the prostanoid component of contractile synergism. While the synergism predominantly operates through a Ca(2+) influx-Rho-kinase pathway, the EP(3) receptor does not necessarily transduce via Rho-kinase.

Low-density lipoprotein inhibits receptor-mediated prostaglandin synthesis without affecting calcium and arachidonic acid mobilization in human endothelial cells

Vascular serotonin 5-HT1 receptors have quiescent constrictor activity that is activated by other vasoactive agents such as histamine. Previously, we observed that the 5-HT1-selective agonist 5-carboxamidotryptamine (5-CT) potentiated histamine-stimulated arachidonic acid (AA) mobilization and prostaglandin production in human aortic endothelial cells (HAEC). In the present study, 5-CT was found to potentiate histamine-stimulated calcium mobilization but had no effect on intracellular calcium when added alone. Treatment of HAEC with human low-density lipoprotein (LDL) for 20 hours inhibited the histamine- plus 5-CT-stimulated production of prostaglandin F2alpha (PGF2alpha) and the prostacyclin metabolite 6-keto-PGF1alpha. However, the effects of histamine and histamine potentiation by 5-CT on intracellular Ca mobilization and AA release were resistant to LDL treatment. Conversely, the subsequent receptor-independent conversion of AA to prostaglandins was inhibited by LDL. These results demonstrate that histamine and serotonin receptor activity, measured as the stimulation of Ca and AA mobilization, is resistant to LDL exposure under mild oxidizing conditions, whereas the receptor-independent synthesis of prostaglandins is inhibited by LDL. The results also suggest that the LDL-stimulated mobilization of cellular AA is responsible for the LDL-mediated inhibition of prostaglandin synthesis. These findings suggest a mechanism by which LDL and/or atherosclerosis could promote the vascular liberation of AA that is not converted to endothelium-derived prostaglandins and is therefore available as substrate for the production of other eicosanoids.

Pharmacologic characterization of contractile serotonergic receptors in human isolated mesenteric artery

The 5-hydroxytryptamine (5-HT) receptors mediating contraction in human isolated mesenteric arteries were characterized. Endothelium-denuded human isolated mesenteric arteries were used. 5-HT induced concentration-dependent contractions in mesenteric arteries (Emax, 127.37 +/- 7.61% of 80 mM KCl maximal contraction; pD2, 6.73 +/- 0.09 [-logEC50]). Sumatriptan, a selective 5-HT1B/1D receptor agonist, induced concentration-dependent contractions in some of the arteries (Emax, 61.82 +/- 10.04%; pD2, 6.56 +/- 0.21, n = 9) but not in the others (Emax < 5%, n = 13), suggesting that functional 5-HT1B/1D receptors exist in some but not in all mesenteric arteries. GR127935 (a selective 5-HT1B/1D receptor antagonist, 3 nM) inhibited sumatriptan-induced contractions in arteries in which sumatriptan responses were strong in an insurmountable manner. GR127935 (10 nM) also inhibited 5-HT responses and shifted the concentration-response curve of 5-HT to the right significantly (p < 0.05; pD2s were 6.54 +/- 0.18 and 5.93 +/- 0.11 in the presence of vehicle and GR127935, respectively). Ketanserin (0.01-1 microM) competitively antagonized 5-HT responses in human mesenteric arteries: pA2 value was 8.40 +/- 0.25 (slope of Schild regression, 1.43 +/- 0.18; r2, 0.98). Tropisetron (5-HT3 receptor antagonist) and prazosin (alpha1-adrenoceptor antagonist) did not affect the contractions induced by 5-HT. These results suggest that 5-HT2A and 5-HT1B/1D receptors, but not 5-HT3 and alpha1-adrenoceptors, are involved in the 5-HT-induced contractions in human isolated mesenteric arteries. Sumatriptan-induced and 5-HT1B/1D receptor-mediated responses vary greatly among patients.

Increased contractile response to 5-hydroxytryptamine1-receptor stimulation in pulmonary arteries from chronic hypoxic rats: role of pharmacological synergy

1. 5-Hydroxytryptamine (5-HT)(1)-receptor-induced contraction is enhanced, or uncovered, by elevated vascular tone in many arteries including pulmonary arteries. In hypoxia-induced pulmonary hypertension, the endogenous tone of pulmonary arteries is elevated and this may contribute to increased 5-HT(1)-receptor-induced contraction. Here we investigate the influence of vascular tone induced by endothelin-1 (ET-1), neuropeptide Y (NPY), KCl, 4-aminopyridine (inactivator of K(v) channels, 4-AP) or the calcium ionophore A23187 on contractile responses to the 5-HT(1)-receptor agonist 5-carboxamidotryptamine (5-CT) in small muscular pulmonary arteries from control rats and rats exposed to chronic hypoxia. The influence of the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM) was also studied. These conditions were chosen to mimic those that influence pulmonary vascular tone in hypoxia-induced pulmonary hypertension. 2. In control rat small pulmonary arteries, only high concentrations of 5-CT (>1 microM) induced vasoconstriction. Tone induced by NPY, 4-AP and A23187 had no effect on responses to 5-CT whilst responses to 5-CT were increased by ET-1- and KCl-induced tone. In the presence of L-NAME these responses to 5-CT were enhanced further. 3. Responses to 5-CT were enhanced 3 - 4 fold in small pulmonary arteries from hypoxia-exposed, pulmonary hypertensive rats and neither L-NAME nor increasing tone with NPY, 4-AP, A23187, ET-1 or KCl had any further effect on responses to 5-CT. 4. The results suggest that inhibition of nitric oxide synthase combined with KCl- or ET-1-induced vascular tone potentiates responses to 5-HT(1)-receptor-induced contraction in pulmonary arteries in a synergistic fashion and this mimics the effects of chronic hypoxic exposure.

Collar-induced elevation of mRNA and functional activity of 5-HT(1B) receptor in the rabbit carotid artery

Hypersensitivity to serotonin (5-HT) develops in rabbit collared carotid arteries. Previous data demonstrated the involvement of 5-HT(1)-like receptors which are not active in normal carotid arteries. This study investigated the interaction in the rabbit carotid artery between 5-HT and a moderate tone as this can uncover functional 5-HT(1)-like receptors. Furthermore, the expression of messenger RNA (mRNA) and protein of 5-HT(1B), 5-HT(1D) and 5-HT(2A) receptors was addressed. Silicone collars were placed around the carotid arteries of male New Zealand White rabbits for 1 week. Rings from inside (=collar) and outside (=sham) the collar were either mounted in isolated organ baths for isometric force measurements or frozen in liquid nitrogen to isolate total RNA or proteins which were subsequently analysed by respectively reverse transcriptase-polymerase chain reaction and Western blot analysis. In sham and collared rings concentration-response curves (CRC's) to 5-HT were monophasic. Only in collared segments the presence of a 5-HT(2A) antagonist (spiperone or ketanserin, 0.1 microM) revealed a biphasic CRC which was even more pronounced when a moderate tone was induced by KCl pointing to functional 5-HT(1)-like receptors. The rabbit carotid artery constitutively expressed 5-HT(1B) and 5-HT(2A) mRNA, not 5-HT(1D) mRNA. Manipulation of the carotid artery increased the 5-HT(1B) mRNA level. Collar placement raised it even further. The 5-HT(2A) mRNA level remained unchanged. All the anti-5-HT receptor antibodies tested resulted in variable, non specific patterns with multiple bands. In conclusion, collar placement elevates mRNA expression and activity of the 5-HT(1B) receptor in the rabbit carotid artery.

Contractile responses to sumatriptan and ergotamine in the rabbit saphenous vein: effect of selective 5-HT(1F) receptor agonists and PGF(2alpha)

Contractile responses to ergotamine, sumatriptan and the novel 5-HT(1F) receptor agonists, LY334370 and LY344864 were examined using the rabbit saphenous vein. Ergotamine (pEC(50)=8.7+/-0.06) was 30 fold more potent than 5-hydroxytryptamine (5-HT) (pEC(50)=7.2+/-0.13) and 300 fold more potent than sumatriptan (pEC(50)=6.0+/-0.08) in contracting the rabbit saphenous vein in vitro. The selective 5-HT(1F) receptor agonists, LY334370 or LY344864 (up to 10(-4) M), did not contract the rabbit saphenous vein. The contractile response to ergotamine in this tissue resulted from activation of both alpha(1) and 5-HT(1B/1D) receptors based on the observation that prazosin (10(-6) M), an alpha-adrenoceptor antagonist, and GR127935 (10(-8) M) a 5-HT(1B/1D) receptor antagonist, dextrally shifted the contractile response to ergotamine. In contrast, prazosin (10(-6) M) did not alter contraction to sumatriptan whereas GR127935 (10(-8) M) was a potent antagonist (-log K(B)=10.0) suggesting that sumatriptan-induced contraction of the rabbit saphenous vein was mediated only by activation of receptors similar or identical to 5-HT(1B/1D) receptors. PGF(2alpha) (3x10(-7) M) produced a modest increase (approximately 5.0 - 10.0% maximum PGF(2alpha) contraction) in saphenous vein force. Precontraction with PGF(2alpha) (3x10(-7) M) dramatically augmented the potency and maximal contractile response to sumatriptan (pEC(50)=7.1) and modestly enhanced the contractile potency of ergotamine (pEC(50)=9.0) in the rabbit saphenous vein. However, PGF(2alpha) (3x10(-7) M) only unmasked a contraction to the 5-HT(1F) receptor agonists when concentrations exceeded 10(-5) M, concentrations considerably higher than their 5-HT(1F) receptor affinities. LY334370 (10(-6) M) pretreatment did not alter contraction to either sumatriptan or ergotamine and a higher concentration (10(-5) M) of LY334370 or LY344864 inhibited contraction to sumatriptan. Thus, activation of 5-HT(1F) receptors will not induce vascular contraction (either alone or following modest tone with PGF(2alpha)) or augment contraction to other contractile agonists in the rabbit saphenous vein.

Phenylephrine precontraction increases the sensitivity of rabbit femoral artery to serotonin by enabling 5-HT1-like receptors

We used selective receptor antagonists to identify the receptors mediating the isometric contractile response to serotonin in control and phenylephrine (PHE)-precontracted rabbit femoral artery rings. Serotonin, in the absence of PHE, elicited monophasic concentration-response curves (CRCs) early, but biphasic CRCs late in the course of the study. In the monophasic curves, the threshold and maximal concentrations were 10 and 1,000 microM, respectively. In biphasic CRCs, the threshold and maximal concentrations of the first phase were 0.03 and 3 microM, respectively. The respective values for the second phase were 10 and 1,000 microM. Prazosin, 0.1 microM, a selective alpha1-adrenoceptor antagonist, inhibited the monophasic curves, but only the second phase of the biphasic curves. Ritanserin, 0.01 microM, a selective 5-HT2A-receptor antagonist, shifted the first phase of the biphasic serotonin CRCs to the right but had little effect on the second phase. PHE increased the sensitivity of rabbit femoral artery response to serotonin. This amplified response to serotonin was antagonized by 0.01 microM GR 127935T, a selective 5-HT1B-receptor antagonist. The selective 5-HT1 agonist, sumatriptan, had no effect in control femoral arteries, but caused a concentration-dependent contraction after PHE precontraction. These results suggest that 5-HT1-like receptors are normally inactive or "silent" in the absence of PHE. However, in the presence of PHE, these receptors become enabled and mediate the amplified response to serotonin. The evidence also suggests that, in the absence of PHE, alpha1-adrenoceptors mediated the contractile response to serotonin in the monophasic CRCs. In the biphasic curves observed late in the study, the first phase was mediated by 5-HT2A receptors, and the second, by the alpha1-adrenergic receptors.

Pulmonary hypertension, anorexigens and 5-HT: pharmacological synergism in action?

In pulmonary hypertension (PHT), pulmonary vascular resistance is elevated as a result of increased pulmonary vascular tone and pulmonary vascular remodelling. Certain diet pills, such as the fenfluramines, have been associated with the development of PHT. This class of drugs act as indirect 5-HT receptor agonists and can inhibit 5-HT reuptake and cause the release of 5-HT from platelets. Many pulmonary vasoconstrictors, including 5-HT, activate both Gi- and Gq-linked receptors. Increasing evidence suggests that Gq activation might amplify Gi-linked intracellular pathways to 'uncover' or potentiate vasoconstrictor responses - a phenomenon known as pharmacological synergism, which occurs in the pulmonary circulation. In this review the evidence that 5-HT plays a role in PHT and that pharmacological synergism might contribute to its pathology is discussed.

Synthesis and serotonergic activity of substituted 2, N-benzylcarboxamido-5-(2-ethyl-1-dioxoimidazolidinyl)-N, N-dimethyltryptamine derivatives: novel antagonists for the vascular 5-HT(1B)-like receptor

The synthesis and vascular 5-HT(1B)-like receptor activity of a novel series of substituted 2, N-benzylcarboxamido-5-(2-ethyl-1-dioxoimidazolidinyl)-N, N-dimethyltryptamine derivatives are described. Modifications to the 5-ethylene-linked heterocycle and to substituents on the 2-benzylamide side chain have been explored. Several compounds were identified which exhibited affinity at the vascular 5-HT(1B)-like receptor of pK(B) > 7.0, up to 100-fold selectivity over alpha(1)-adrenoceptor affinity and 5-HT(2A) receptor affinity, and which exhibited a favorable pharmacokinetic profile. N-Benzyl-3-[2-(dimethylamino)ethyl]-5-[2-(4,4-dimethyl-2, 5-dioxo-1-imidazolidinyl)ethyl]-1H-indole-2-carboxamide (23) was identified as a highly potent, silent (as judged by the inability of angiotensin II to unmask 5-HT(1B)-like receptor-mediated agonist activity in the rabbit femoral artery), and competitive vascular 5-HT(1B)-like receptor antagonist with a plasma elimination half-life of approximately 4 h in dog plasma and with good oral bioavailability. The selectivity of compounds from this series for the vascular 5-HT(1B)-like receptors over other receptor subtypes is discussed as well as a proposed mode of binding to the receptor pharmacophore. It has been proposed that the aromatic ring of the 2, N-benzylcarboxamide group can occupy an aromatic binding site rather than the indole ring. The resulting conformation allows an amine-binding site to be occupied by the ethylamine nitrogen and a hydrogen-bonding site to be occupied by one of the hydantoin carbonyls. The electronic nature of the 2,N-benzylcarboxamide aromatic group as well as the size of substituents on this aromatic group is crucial for producing potent and selective antagonists. The structural requirement on the 3-ethylamine side chain incorporating the protonatable nitrogen is achieved by the bulky 2, N-benzylcarboxamide group and its close proximity to the 3-side chain.

Involvement of 5-HT1B receptors in collar-induced hypersensitivity to 5-hydroxytryptamine of the rabbit carotid artery

In humans intimal thickening is aprerequisite of atherosclerosis. Application of a silicone collar around the rabbit carotid artery induces an intimal thickening but in addition it increases the sensitivity to the vasoconstrictor action of serotonin (5-hydroxytryptamine, 5-HT). The 5-HT receptors involved in collar-induced hypersensitivity to 5-HT were investigated using several agonists and antagonists. One week after placement of collars around both carotid arteries of anaesthetized rabbits, rings (2 mm width) from inside (=collar) and outside (=sham) the collars were mounted in organ baths (10 ml) for isometric force measurements at 6 g loading tension. Collared rings were more sensitive to the contractile effect of 5-HT (7.6 fold) and 5-carboxamidotryptamine (31 fold, 5-CT, 5-HT1 agonist) in cumulative concentration response curves. Sumatriptan (5-HT1B/1D agonist) caused concentration-dependent constrictions in collared rings only. Collar placement did not significantly alter pA2 values (Schild regression) or apparent pKb values (non-linear regression) of spiperone and methysergide (mixed 5-HT2A/5-HT1 antagonists) or ketanserin and ritanserin (5-HT2A antagonists), indicating unchanged binding characteristics of the 5-HT2A receptor. However, the reduced slope of the Schild regression pointed to a heterogeneous receptor population in collared rings. In contrast, the apparent pKb value of methiothepin (5-HT1B antagonist) was significantly reduced by collar placement, and its antagonism shifted from non-surmountable in sham rings to surmountable in collared segments. Taken together, this study demonstrates that the serotonergic receptor involved in the hypersensitivity to 5-HT of rabbit collared carotid artery is a 5-HT1B receptor subtype.

Functional characterization and m-RNA expression of 5-HT receptors mediating contraction in human umbilical artery

5-HT1-like and 5-HT2 receptors have both been described to mediate contractions to 5-HT in the human umbilical artery (HUA). However, the nature of the 5-HT receptor subtypes is unknown. 2 In isometric force studies with ring preparations of HUA alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT) and 5-hydroxytryptamine (5-HT) contracted HUA with pED50 values of 8.04 and 7.74, respectively. In the presence of a subthreshold concentration of another vasoconstrictor sumatriptan and 5-nonyloxytryptamine elicited concentration-dependent contractions with pEC50 values of 7.21 and 7.67, respectively. In the presence of the selective 5-HT1B/D receptor antagonist GR127935, contractile responses elicited by sumatriptan and 5-nonyloxytryptamine were competitively antagonized (pKB 9.01 and 9.02, respectively). In the experiments with 5-HT, GR127935 appeared to be non-competitive with shallow Schild plot slopes. The data were fitted with two linear regression lines and the calculated pKB of the high affinity component (8.90) was comparable to that expected for GR127935 at the 5-HT1B/1D receptor. Several 5-HT2 selective receptor antagonists (spiperone, cyproheptadine, pirenperone) competitively inhibited responses to 5-HT. The selective 5-HT2A antagonist ketanserin against sumatriptan and 5-nonyloxytryptamine behaved as a weak antagonist while against 5-HT demonstrated a competitive antagonism (pKB 8.56). Using specific primers for human 5-HT1B, 5-HT1D and 5-HT2A receptor genes, the reverse transcriptase-polymerase chain reaction revealed mRNA expression of 5-HT1B and 5-HT2A receptors in the HUA. The results suggest that the HUA has a functional population of 5-HT1B and 5-HT2A receptor subtypes which are involved in the contractile response to 5-HT. Contractions mediated by 5-HT1B receptors can be 'uncovered' by exposure to other vasoactive agents.

Serotonin and vasoconstrictor synergism

Contractile synergism between serotonin (5-hydroxytryptamine, 5-HT) and other vasoconstrictor substances has been observed in a number of peripheral and cerebrovascular blood vessels. This phenomenon may play an important role in certain pathological states such as hypertension, peripheral vascular disease, and coronary spasm. In the present review, we summarize studies on the synergism between serotonin and other vasoconstrictor agents and focus on a recently described type of vasoconstrictor synergism in which precontraction with a non-5-HT receptor agonist yields an enhanced contractile response to serotonin which is mediated by previously inactive or "silent" 5-HT receptor subtypes.

Plasma 5-hydroxytryptamine constricts equine digital blood vessels in vitro: implications for pathogenesis of acute laminitis

Cumulative concentration response curves to 5-hydroxytryptamine (5-HT; 10(-10)-10(-4) mol/l) were constructed using isolated rings of equine digital, facial, tail and coronary arteries (endothelium intact). 5-HT was 17.7 and 41 times more potent as a vasoconstrictor of digital arteries than facial and tail arteries respectively. Removal of the endothelium increased the vasoconstrictor potency of 5-HT in the facial artery by 3.7-fold (P<0.05) but did not alter the sensitivity of digital arteries to 5-HT. Coronary arteries failed to contract to 5-HT. Coronary arteries pre-contracted with U44069 showed concentration dependent relaxation to 5-HT, a response which was partially dependent on the presence of the endothelium. No vasorelaxant effects were found in the digital or facial arteries. The concentration of 5-HT in platelet poor and platelet rich equine plasma was found to be 6.70+/-1.1 x 10(-8) mol/l and 1.77+/-0.36 x 10(-6) mol/l (mean +/-s.e.) respectively by high performance liquid chromatography (HPLC). Plasma which contained no detectable platelets had a 5-HT concentration of 1.12+/-0.48 x 10(-8) mol/l. Isolated digital arteries constricted when exposed to dilutions of platelet poor and platelet depleted equine plasma. These plasma induced contractions were almost completely inhibited by 5-HT receptor antagonists, ketanserin and methiothepin. The change in isometric tension in rings of equine digital artery in vitro was therefore used as a bioassay for plasma 5-HT and the results obtained by this method showed an excellent correlation (r2 = 97.2%, P<0.001) with the concentration estimated by HPLC. Circulating free concentrations of 5-HT in normal horses may be sufficient to constrict digital blood vessels partially in vivo but are well below the threshold for contraction of other peripheral blood vessels examined.

5-Hydroxytryptamine-induced contraction of the marmoset aorta is mediated by a 5-HT1-like receptor

1. 5-Hydroxytryptamine (5-HT) exerts both contractile and relaxant effects in the marmoset isolated aorta, actions that are unaffected by the 5-HT2 antagonist ketanserin. The aim of the present study was to define the receptors mediating the contractile activity of 5-HT in the marmoset aorta. 2. Contractile responses were elicited in aortic rings that were either: (i) precontracted submaximally with the thromboxane A2 agonist U44069 in order to amplify the responses; or (ii) exposed to N(omega)-nitro-L-arginine (100 micromol/L) plus LY 53857 (0.1 micromol/L; a 5-HT2 receptor antagonist shown previously to inhibit relaxation). The effect of 5-HT on adenosine 3',5'-cyclic monophosphate (cAMP) formation was also investigated. 3. The effects of agonists and antagonists comprised: (i) agonist potencies in the order 5-carboxamidotryptamine > 5-HT > sumatriptan > 8-hydroxy-2-(di-n-propylamino)tetralin; (ii) inhibition of contractile action of 5-HT by the 5-HT1D antagonist GR 127935; (iii) a contractile response to methysergide; (iv) a lack of effect of tropisetron, an antagonist of 5-HT3 and 5-HT4 receptors; and (v) inhibition of forskolin-stimulated cAMP formation by 5-HT (in the presence of LY 53857), indicative of negative coupling to adenylate cyclase. 4. The above effects fulfill the criteria for a 5-HT1-like receptor. In view of the previous finding that this contractile response is insensitive to ketanserin, it is concluded that the contractile effects of 5-HT in the marmoset aorta are mediated exclusively by a 5-HT1-like receptor.

Hypoxia and response of human umbilical artery strips to 5-hydroxytryptamine: role of prostaglandin F2 alpha

1. This article examines the effects of hypoxia on the contractile response of isolated human umbilical artery strips to 5-hydroxytryptamine (5-HT). 2. Hypoxic conditions produce a large increase in the contractile response to 5-HT without a significant alteration of the sensitivity evaluated at the level of the pD2 value. Indomethacin (10 microM) reduced hypoxia-induced potentiation of the response to 5-HT and decreased the response to the monoamine under oxygenated conditions. 1-NAME (100 microM) did not further increase the effect of hypoxia on the vessel response to 5-HT and increased the response to 5-HT under oxygenated conditions. 3. Taken together, these results suggest that, at least partially, the response of human umbilical artery strips to 5-HT depends on 5-HT release of a contracting prostanoid which is a product of the cyclooxygenase pathway. Furthermore, during hypoxia in human umbilical artery strips, there appears to be impairment of the basal production and/or release of EDRF/NO. 4. A subthreshold concentration of prostaglandin F2 alpha (1 nM) potentiates the response to 5-HT in indomethacin-pretreated umbilical artery strips. The data raise the possibility that prostaglandin F2 alpha might be the prostanoid released during hypoxia, which in turn potentiates the response of the human umbilical artery to 5-HT.

Augmented contraction of the human isolated coronary artery by sumatriptan: a possible role for endogenous thromboxane

1. The antimigraine drug, sumatriptan, contracts the human coronary artery and, in some patients, elicits chest symptoms (e.g. pressure and pain), particularly after subcutaneous administration. We studied the effects of the thromboxane A2 (TxA2) analogue, U46619 and endothelin-1 on contractile responses to sumatriptan in the human isolated coronary artery as well as the role of endogenously produced TxA2 and endothelin-1 in contractions evoked by sumatriptan. 2. In the presence of U46619 (1 and 3 nM), mean concentration-response curves to sumatriptan in the human coronary artery were shifted vertically due to the initial contraction by U46619, but when this initial contraction was subtracted from the response to sumatriptan, no significant augmentation was observed. However, analysis of the degree of augmentation in individual arterial segments revealed that the augmentation was variable and related inversely to the Emax of sumatriptan in the absence of U46619 (r = 0.78 and 0.81 for 1 and 3 nM, respectively; P < 0.05). 3. Treatment with the TxA2 receptor antagonist, SQ30741 (100 nM), or incubation of vessel segments with aspirin (10 microM), significantly reduced responses to sumatriptan; in aspirin-treated vessel segments, SQ30741 failed to decrease further the contractions to sumatriptan. The decrease in Emax of sumatriptan by both SQ30741 and aspirin correlated significantly with the Emax of sumatriptan without SQ30741 (r = 0.74; P < 0.01) or aspirin (r = 0.94; P < 0.01). In aspirin-treated vessel segments, responses to sumatriptan were significantly augmented in the presence of U46619 (3 nM; P < 0.05). 4. The specificity of SQ30741 was demonstrated by its ability to antagonize coronary artery contractions to U46619 (pA2: 7.54 +/- 0.30), but not endothelin-1. Similarly, incubation with aspirin (10 microM) did not affect contractile responses to endothelin-1, but significantly reduced TxA2 production in coronary artery segments as judged by a decrease in thromboxane B2 (TxB2) from 4.77 +/- 0.98 to 1.38 +/- 0.36 ng g-1 2 h-1. 5. Endothelin-1 (1 nM) did not significantly augment contractions to sumatriptan; there was also no relationship between the degree of augmentation and the control Emax of sumatriptan in the absence of endothelin-1. Furthermore, unlike SQ30741 or aspirin, a high concentration (100 nM) of the non-selective ETA/ETB receptor antagonist, SB 209670, failed to affect contractile responses to sumatriptan. However, SB 209670 potently antagonized coronary artery contractions induced by endothelin-1 with a pA2 of 8.84 +/- 0.32. 6. Compared to control vascular segments, endothelial denudation did not reduce TxA2 production (with endothelium = 2.56 +/- 1.38 vs. without endothelium = 12.32 +/- 4.94 ng TxB2 g-1 2 h-1), suggesting that the production of TxA2 is not confined to the endothelium. The sumatriptan-induced contractions were also unaffected by endothelial denudation. 7. The results of the present study suggest that endogenously produced TxA2 enhances contractions to sumatriptan in the human isolated coronary artery. Such a mechanism may play a role in causing chest symptoms after sumatriptan by potentiating coronary vascular contraction by sumatriptan in vivo.

Headache induced by a nitric oxide donor (nitroglycerin) responds to sumatriptan. A human model for development of migraine drugs

Experimental "vascular" headache in humans may be used in characterizing new migraine drugs. The effects of sumatriptan on nitroglycerin-(NTG)-induced headache and arterial responses were therefore studied. Following a double-blind randomized crossover design, 10 healthy volunteers received sumatriptan 6 mg s.c. or placebo succeeded by 20 min NTG (0.12 microgram/kg/min) infusion. Headache was rated on a 10 points scale. Temporal and radial artery diameters and velocity in the middle cerebral artery (MCA) were measured with ultrasound. Sumatriptan reduced the NTG-induced headache, median score 1.5 versus 4 after placebo (p < 0.01) and decreased temporal and radial artery diameters 75 +/- 3 and 86 +/- 3% of baseline respectively (p < 0.05). Blood velocity in the MCA was unaffected. The NTG model may prove to be a valuable tool in the development of future migraine drugs. The results suggest that NTG headache in non-migraineurs may share mechanisms with migraine headache.

The effect of forskolin on 5-HT1-like and angiotensin II-induced vasoconstriction and cyclic AMP content of the rabbit isolated femoral artery

1. A characteristic feature of vasoconstrictor 5-HT1-like receptors in vitro is that responses mediated by these receptors are enhanced by other vasoconstrictor agents. In the present study, we have examined the influence of cellular cyclic AMP on vasoconstrictor responses to activation of 5-HT1-like receptors in isolated ring segments of the rabbit femoral artery (RbFA), and determined whether modulation of this second messenger underlies the ability of angiotensin II, an endogenous vasoconstrictor, to enhance 5-HT1-like responses. 2. In the presence of 0.1 microM ketanserin (to antagonize 5-HT2-receptors) and 0.3 microM prazosin (to antagonize alpha 1-adrenoceptors), 5-HT produced a concentration-related contraction, which was significantly augmented by pre-contraction of the vessel with 0.1-0.45 nM ([A30]) angiotensin II. Responses to 5-HT in the presence of angiotensin II were inhibited by the 5-HT1-like/5-HT2 antagonist, metergoline (1 microM). 3. The directly-acting adenylyl cyclase activator, forskolin (1 microM), abolished responses to angiotensin II and caused a rightward shift and concomitant depression of the 5-HT concentration-effect (E/[A]) curve. Higher concentrations of forskolin (> 10 microM) abolished responses to 5-HT and 1 microM sodium nitroprusside abolished responses to 5-HT and angiotensin II (n = 7). 4. In the presence of angiotensin II (0.1-0.45 nM), however, 1 microM forskolin failed to inhibit 5-HT-induced contractions; the E/A curve for 5-HT (in the presence of forskolin and angiotensin II) was not significantly different from that produced in the presence of angiotensin II alone. Similarly, the presence of angiotensin II (0.1-0.45 nM) was also able to overcome partially the inhibitory effect of 1 microM sodium nitroprusside against 5-HT-induced contractions (n = 7). In marked contrast, 5-HT failed to elicit a contraction in the presence of angiotensin II and 10 microM forskolin (n = 5). 5. 5-HT (1 microM) significantly reduced basal cyclic AMP accumulation by 35%, whereas angiotensin II (0.45 nM) was without effect. The combination of angiotensin II and 5-HT failed to alter significantly the reduction in cyclic AMP produced by 5-HT alone. Forskolin (1 microM) increased cyclic AMP levels 7 fold above basal, but neither 1 microM 5-HT nor a combination of 1 microM 5-HT and 0.45 nM angiotensin II produced a significant decrease in cyclic AMP content. 6. Whilst moderate concentrations of forskolin can inhibit the responses to either agent, simultaneous activation of angiotensin II and 5-HT1-like receptors can overcome the inhibitory effect of elevated levels of cyclic AMP. Since the potentiating effect of angiotensin II, in either the presence or absence of forskolin, occurs without significant alteration of cellular cyclic AMP, it seems likely that a cyclic AMP-independent pathway is implicated in the synergistic interaction between angiotensin II and vasoconstrictor 5-HT1-like receptors.

Influence of vascular tone on vasoconstrictor responses to the 5-HT 1-like receptor agonist sumatriptan in anaesthetised rabbits

The cardiovascular profile of the 5-HT1-like receptor agonist sumatriptan has been studied in anaesthetised rabbits pretreated with chlorisondamine (0.5 mg kg-1 i.v.) and enalapril (0.3 mg kg-1 i.v.) to eliminate autonomic reflexes and minimise endogenous vasoconstrictor tone. Under these conditions sumatriptan (2-100 micrograms kg-1 i.v.) produced modest increases in carotid vascular resistance but had no significant influence on heart rate, blood pressure or mesenteric vascular resistance. In a similarly pretreated group of animals in which vasoconstrictor tone was elevated by infusion of angiotensin (100 ng kg-1 min-1 i.v.) sumatriptan caused moderate increases in blood pressure (55 +/- 5 to 65 +/- 5 mm Hg after 25 micrograms kg-1 i.v.) and mesenteric vascular resistance (1.4 +/- 0.2 to 1.6 +/- 0.2 mm Hg min ml-1 after 25 micrograms kg-1 i.v.) and tended to produce a greater carotid vasoconstriction (3.6 +/- 0.5 to 4.7 +/- 0.7 mm Hg min ml-1 after 25 micrograms kg-1). These effects were antagonised by methiothepin 0.3 mg kg-1 i.v. implying the involvement of 5-HT1-like receptor stimulation. Hence, the presence of angiotensin produces a modest amplification of the vasoconstrictor effects of sumatriptan and, in particular, unmasks a constriction of the mesenteric vascular bed. The degree of synergy observed between these two vasoconstrictors was, however, less marked than might have been expected on the basis of previous isolated tissue studies.

Effects of U46619 on contractions to 5-HT, sumatriptan and methysergide in canine coronary artery and saphenous vein in vitro

1. The aim of this study was to investigate the mechanism of enhanced reactivity to 5'-hydroxytryptamine (5-HT) and sumatriptan previously observed in human isolated coronary arteries when active force was raised with the thromboxane A2-mimetic, U46619. 2. Ring segments of dog isolated coronary artery and saphenous vein were suspended in organ baths and cumulative concentration-contraction curves to 5-HT, sumatriptan and methysergide were constructed in the absence and presence of low concentrations of U46619. 3. In both endothelium-intact and endothelium-denuded rings of coronary artery, precontraction with U46619 to low (< 10% Fmax; the contraction to a maximum depolarizing 125 mM KCl Krebs solution; KPSS) levels of active force had no effect on either the maximum contraction or sensitivity (pEC50) to 5-HT, sumatriptan and methysergide. 4. Ketanserin (1 microM) had no effect on contractions to sumatriptan and methysergide in endothelium-denuded coronary artery rings, but reduced the maximum contraction to 5-HT by approximately 90% to a value (5% Fmax) similar to that for sumatriptan and methylsergide. Under these conditions, U46619 precontraction had no effect on either pEC50 or maximum for 5-HT, sumatriptan or methysergide. 5. In rings of saphenous vein with endothelium and treated with ketanserin (1 microM), 5-HT and sumatriptan caused equal maximum responses of 65% Fmax which were approximately double that of methysergide (32% Fmax). The maximum responses and sensitivity to 5-HT, sumatriptan, methysergide and noradrenaline were unaffected by precontraction with U46619. 6. Pretreatment of the saphenous vein with sodium nitroprusside (SNP; 10 microM) caused a small sustained relaxation and significantly depressed the maximal contraction to 5-HT without affecting sensitivity and abolished the contraction curve to sumatriptan and methysergide. When the relaxation response to SNP was reversed with U46619 (1-4 nM), the contraction curves to 5-HT, sumatriptan and methysergide were similar to those obtained prior to relaxation with SNP. In contrast, the same treatment with SNP had little affect on the contraction curve to noradrenaline.7 In conclusion, the pattern of U46619-enhanced reactivity of 5-HT, sumatriptan and methysergide in SNP-treated dog saphenous vein, highlights the importance of functional antagonism when assessing reactivity to contractile agonists in isolated blood vessels.

5-HT1-like and 5-HT2A receptors mediate 5-hydroxytryptamine-induced contraction of rabbit isolated mesenteric artery

The contractions induced by 5-hydroxytryptamine (5-HT) and the 5-HT1-like receptor agonist, sumatriptan, were investigated in the open ring preparations of rabbit mesenteric artery in order to characterize the 5-HT receptors. 5-HT induced concentration-dependent contractions. Sumatriptan did not induce any contraction of unstimulated rings, whereas it elicited concentration-dependent contractions in preparations given a moderate tone by a threshold concentration of prostaglandin F2 alpha (PGF2 alpha). Pargyline, cocaine or normetanephrine were without significant effect on the contractions induced by 5-HT and sumatripan. The 5-HT concentration-effect curve was clearly biphasic. Methiothepin (0.01 microM) shifted the both phases of the concentration-effect curve to the right. Ketanserin (0.1 microM) shifted the second, low affinity, phase and prazosin did not alter concentration-effect curve to 5-HT. The sumatriptan concentration-effect curve was shifted by methiothepin (0.01 microM) to the right (pKB = 9.19) but not by ketanserin (1 microM). Concentration-effect curves to 5-HT and sumatriptan were not affected by the 5-HT3 receptor antagonist tropisetron (1 microM). These results suggest that 5-HT1-like type receptors are responsible for the first phase of 5-HT-induced contraction and 5-HT2A receptor for the second phase, in rabbit mesenteric artery. Sumatriptan-induced contractions appear to be mediated by 5-HT1-like type receptors in this artery. These results also suggest that this kind of amplification may be a common feature of vascular 5-HT1-like type receptor as has been shown in other vascular segments such as rabbit femoral, iliac and renal arteries, and guinea-pig iliac artery.

Presence of vasoconstrictor 5HT1-like receptors revealed by precontraction of rabbit isolated mesenteric artery

1. A series of 5-hydroxytryptamine (5-HT) receptor agonists including 5-HT, 5-carboxamidotryptamine (5-CT) and sumatriptan produced little or no contraction of rabbit isolated mesenteric arteries under resting tone conditions, even at concentrations up to 10(-4) M. 2. When the same agonists were retested in mesenteric artery preparations pre-contracted with the thromboxane-mimetic, U46619, each demonstrated concentration-related vasoconstrictor activity. 5-CT and 5-HT were especially potent and effective in this model giving EC50 values of 4.3 x 10(-9) M and 1.6 x 10(-8) M respectively and maximum effects equivalent to those of KCl 80 mM. In preparations precontracted by U46619 (conditions retained throughout the rest of the study) the order of agonist potency was 5-CT > 5-HT > RU 24969 = sumatriptan > 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) > cisapride. 3. The vasoconstrictor effects of 5-CT were competitively antagonized by methiothepin (pA2 8.20) but resistant to antagonism by a range of other 5-HT receptor antagonists, i.e. pindolol (5-HT1A/5-HT1B), propranolol (5-HT1B), spiperone (5-HT2A), ondansetron (5-HT3), ICS 205930 (5-HT3/5-HT4) and SDZ 205557 (5-HT4). 5-CT responses were slightly antagonized by a high concentration of ritanserin (5-HT2A/5-HT2C). Responses to 5-HT and sumatriptan were also antagonized by methiothepin with similar affinity (pA2/pKB values congruent to 8.0). 4. Metergoline and rauwolscine (10(-7)-10(-6) M) antagonized the effects of 5-CT in a non-competitive fashion giving pKBapp values of 7.13 (metergoline) and 6.86 (rauwolscine). 5. Vasoconstrictor responses to 5-HT were not modified in the presence of ritanserin (3 x 10-7 M) orspiperone (3 x 10-7 M) and only modestly antagonized by ketanserin (10-6 M) suggesting that 5-HT2Areceptors do not make a significant contribution in this model.6. Hence, precontraction of rabbit mesenteric arteries reveals potent vasoconstrictor effects of 5-HT and related agonists. Based on the agonist potency order and the antagonist studies performed, the receptor subtype responsible has the characteristics of a 5-HT1-like (probably 5-HTlD) receptor. This study therefore demonstrates a particularly striking example of vasoconstrictor synergy involving 5-HT1-like receptors.

Interactions between 5-hydroxytryptamine, noradrenaline and the thromboxane-A2 mimetic U44069 in the marmoset isolated aorta

1. The effects of 5-hydroxytryptamine (5-HT) in the absence and presence of noradrenaline (NA) or the thromboxane-A2 mimetic, U44069, were investigated in ring preparations of marmoset aorta. 2. 5-HT (0.001-10 mumol/L) produced little or no contractile response in preparations at basal tone. When the tone was elevated to 50% of maximum with NA the predominant response to 5-HT was relaxation. The 5-HT2 receptor antagonist LY53857 (0.1 mumol/L) unmasked a contractile response to low concentrations of 5-HT (0.01-1.0 mumol/L) and reduced relaxation to high concentrations of 5-HT (1.0-10 mumol/L) in vessels precontracted with NA. 3. In U44069-contracted vessels, 5-HT was contractile in the range 0.01-1 mumol/L and relaxant in concentrations of 6.0-10.0 mumol/L. Ketanserin (1.0 mumol/L) had no effect on the contraction or relaxation to 5-HT. 4. The relaxant response to 5-HT was not significantly diminished in endothelium-impaired arteries. 5. In conclusion, 5-HT exerts complex inhibitory and excitatory actions on the marmoset aorta. The inhibitory actions are not endothelium-dependent and the excitatory actions do not appear to involve the 5-HT2 receptor.

Overnight storage of the porcine isolated splenic artery enhances endothelium-dependent contractions to NG-nitro-L-arginine methyl ester without impairing endothelium-dependent dilator function

This study examined the influence of overnight storage on endothelium-independent contractions to 5-hydroxytryptamine (5-HT), endothelium-dependent contractions to NG-nitro-L-arginine methyl ester (L-NAME), and endothelium-dependent relaxations to substance P (SP) and L-arginine, using the porcine isolated splenic artery. In endothelium-intact (E+) segments from fresh porcine isolated splenic arteries or segments from the same vessels stored overnight at 4 degrees C, either in Krebs-Henseleit saline or in Krebs-Henseleit saline containing 1 mM L-arginine, 5-HT caused concentration-related contractions that were similar under all three conditions. Overnight storage enhanced contractions of the splenic artery to L-NAME, an effect not observed if the vessels were co-stored with 1 mM L-arginine. L-NAME failed to contract endothelium-denuded (E-) segments from fresh tissues or tissues stored overnight, indicating that its constrictor effects were endothelium-dependent. SP caused concentration-related, endothelium-dependent relaxations of the splenic artery that were inhibited by 100 microM L-NAME, indicating that the relaxations could be attributed to the stimulated release of NO from endothelial cells. Established contractions to 100 microM L-NAME in E+ segments from fresh tissues, or segments from the same tissues stored overnight at 4 degrees C, either in Krebs-Henseleit saline or in Krebs-Henseleit saline containing 1 mM L-arginine, were all reversed by 1 mM L-arginine to similar extents, indicating that overnight storage did not affect endothelium-dependent dilator responses to L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular receptors for 5-hydroxytryptamine: distribution, function and classification

The complex actions of 5-hydroxytryptamine (5-HT) on blood vessels result from interactions with a multiplicity of specific 5-HT receptors. Dramatic advances recently have been made in defining the receptor subtypes involved in terms of their pharmacology and biochemistry, as well as their molecular biology, provoking an evolution of the criteria used for classifying and naming them. This article reviews the distribution and function of different 5-HT receptor types present on vascular smooth muscle, endothelium and perivascular neurones, and considers ways in which they can be positively defined and differentiated using traditional pharmacological approaches. The characteristics of each receptor type are also considered in terms of current biochemical and molecular perspectives on 5-HT receptor classification.

5-Hydroxytryptamine (5-HT) contracts the guinea-pig isolated iliac artery via 5-HT1-like and 5-HT2 receptors

The characterization of 5-hydroxytryptamine (5-HT) receptors mediating contractions of the guinea-pig isolated iliac artery was studied when the basal tone was slightly increased by prostaglandin F2 alpha (PGF2 alpha). In the presence of ketanserin (1 mumol/l), 5-HT and several 5-HT receptor agonists induced contractile responses with the rank order of agonist potency: 5-HT = 5-carboxyamidotryptamine (5-CT) = lysergol > ergometrine = methylergometrine > RU 24969 approximately 5-methoxytryptamine (5-MeOT) > methysergide > sumatriptan > tryptamine. Concentration-effect curves to the ergot alkaloids, lysergol, ergometrine, methylergometrine and methylsergide, were biphasic. In the presence of ketanserin (1 mumol/l), contractile responses to 5-HT, 5-CT, RU 24969, 5-MeOT, sumatriptan and tryptamine were antagonized by methiothepin (30 nmol/l) and flesinoxan (3 mumol/l) with approximate pKB values of 8.5-9.0 and 6.0-6.3, respectively. The first phase of contraction produced by the ergot alkaloids, lysergol, ergometrine, methylergometrine and methysergide, were blocked by methiothepin (30 nmol/l) and flesinoxan (3 mumol/l), respectively, with approximate pKB values about 8.4-8.7 and 6.2-6.4, respectively. The mechanism underlying the second phase of contraction remains to be established. Maximum responses of the concentration-effect curves to 5-HT (1 nmol/l-1 mumol/l) were concentration-dependently depressed by ketanserin (1 nmol/l-1 mumol) and spiperone (30 nmol/l-0.3 mumol/l) and reached approximately 60% of the 5-HT maximum response in the presence of ketanserin (1 mumol/l) and spiperone (0.1 mumol/l), respectively. Agonist potency of 5-HT was not affected by the antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of contractile responses to 5-hydroxytryptamine and sumatriptan in human isolated coronary artery: synergy with the thromboxane A2-receptor agonist, U46619

1. The interaction between the thromboxane A2 receptor agonist, U46619 and two 5-hydroxytryptamine (5-HT) receptor agonists, the non-selective, naturally occurring agonist, 5-HT and the selective 5-HT1-like agonist, sumatriptan were studied in human epicardial coronary arteries in vitro. 2. Coronary artery rings (2-4 mm in diameter) were prepared from epicardial arteries from explant hearts of patients undergoing heart transplant (cardiomyopathy, n = 13; ischaemic heart disease, n = 10) and unused donor hearts (n = 5). Each ring of artery was set at optimal resting conditions to record changes in isometric force. 3. The majority of artery rings developed phasic, rhythmic contractions either spontaneously or in response to all vasoconstrictor agonists tested. Both the spontaneous and agonist-induced phasic contractions were abolished by nifedipine (0.1 microM). 4. Concentration-contraction curves to 5-HT-receptor agonists and noradrenaline (NA), were first constructed in artery rings that did not develop phasic activity. 5-HT and ergometrine were the most potent agonists with EC50 values of 6.8 +/- 0.2 and 7.7 +/- 0.2 (-log M) respectively. Potencies (EC50's) to sumatriptan, methysergide and noradrenaline could not be determined due to their poor ability to contract the coronary artery. Maximum contractions (Emax; normalized as a percentage of the contraction to a maximum-depolarizing concentration of K+ in physiological salt solution (KPSS)) for 5-HT, ergometrine, sumatriptan, methysergide and noradrenaline were 40 +/- 10, 9 +/- 3, < 5, < 5 and < 5% respectively. 5. In arteries without phasic activity, U46619 (1 nM) caused an increase in force of 3.8 +/- 1% KPSS. With U46619 present, the Emax values for 5-HT, ergometrine, sumatriptan and methysergide were all markedly increased. For 5-HT and sumatriptan, E., values were 92+/- 4% and 49 +/- 14% KPSSrespectively. The presence of U46619 did not significantly change the sensitivity (EC50) to 5-HT.6. In a separate series of arteries, nifedipine (0.1 microM) was used to block phasic, contractile activity. The synergy observed between U46619 and 5-HT or sumatriptan still occurred although the Emax values for each agonist were depressed but the EC50 values were again unaffected.7. In conclusion, these in vitro studies indicate that the normally poor contractions to sumatriptan, inhuman coronary arteries are significantly enhanced when active force is induced with a thromboxane A2-receptor agonist, U46619. The enhanced response is not specific for either sumatriptan or 5-HT,-like receptors since contractions to 5-HT, ergometrine and methysergide were also potentiated by U46619.

Pre-contraction with histamine and U46619 unmasks a 5-HT1-like receptor in rabbit renal artery

Rabbit renal arteries were virtually unresponsive to 5-hydroxytryptamine (5-HT) and 5-carboxamidotryptamine (5-CT) under control conditions (including the presence of ketanserin 10(-6) M). However, both agents produced prominent contractions over the range 10(-9)-10(-5) M in tissues pre-contracted with threshold concentrations of either histamine or U46619. These responses were considered to be mediated by activation of 5-HT1-like receptors based on the potency order 5-CT > or = 5-HT and the sensitivity of agonist responses to antagonism by methiothepin (10(-7) M). Pre-contraction with different types of vasoconstrictor can therefore unmask functional 5-HT1-like receptors in rabbit renal artery.