Eric T. Whalley

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.

New synthetic antagonists of bradykinin

1 A new synthetic bradykinin analogue was found to be an antagonist of bradykinin‐induced vascular permeability in rabbit skin. It was effective in equimolar concentrations. 2 These analogues also antagonized the action of bradykinin in contracting the guinea‐pig isolated ileum. The mean pA2 values of five different antagonists ranged from 5.3–6.4 respectively, on this preparation. 3 Our observations, together with those of others suggest that these antagonists act on the same receptor types, viz., B2, in rabbit blood vessels and in smooth muscle of guinea‐pig ileum. 4 Our results support the view that the way is now promising for the synthesis of potent specific antagonists of bradykinin for experimental and therapeutic use.

Rationale for the use of 5-HT1-like agonists in the treatment of migraine

SummaryMigraine headache is thought to be associated with a dilatation of cranial blood vessels, particularly those in the dura mater, and an accompanying localized sterile inflammatory response. Sumatriptan is a highly selective 5-HT1-like receptor agonist which selectively constricts cranial blood vessels (including those in the dura mater). It also inhibits neurogenically-mediated plasma protein extravasation in the dura mater. Haemodynamic studies in anaesthetized animals have shown that sumatriptan selectively constricts the carotid arterial circulation and this effect appears to be restricted to an effect on carotid arteriovenous anastomoses. Sumatriptan has a much more selective pharmacological profile than ergot preparations which are also used in the acute treatment of migraine. The development of sumatriptan has been based on a vascular theory of migraine and its high degree of efficacy in the treatment of migraine strengthens the argument that dilatation of cranial blood vessels is the cause of vascular headache.

Evidence for the presence of 5-HT1-like receptors in rabbit isolated basilar arteries

The 5-hydroxytryptamine (5-HT) receptor mediating contraction of rabbit isolated endothelium denuded basilar artery has been investigated. 5-HT and a variety of 5-HT receptor agonists contracted rabbit isolated basilar artery with a rank order of agonist potency: 5-carboxamidotryptamine (5-CT) greater than 5-HT greater than GR43175. None of these agonists relaxed rabbit isolated basilar artery when tone was elevated with prostaglandin F2alpha. The contractile response to both 5-HT and GR43175 was resistant to antagonism by GR38032, phentolamine, (+/-)-cyanopindolol and yohimbine. Ketanserin (100 nM) and mesulergine (100 nM) produced small significant rightward shifts of C-E curves to 5-HT with respective concentration-ratio shifts of 5.7 (1.5-21.0 95% confidence interval and 2.89 (1.1-7.6 95% confidence interval). GR43175-induced contraction was resistant to antagonism by ketanserin however the maximum response to GR43175 was significantly reduced in the presence of mesulergine, with no change in EC50. Methiothepin was a potent antagonist of the contractile actions of both 5-HT and GR43175, with respective pA2 values against each agonist of 10.3 and 9.9. The slope of the Schild regression for methiothepin against 5-HT-induced contraction was significantly less than unity. Methiothepin (100 nM) had no effect on the contractile response to the thromboxane A2 mimetic U-46619. It is concluded that 5-HT and GR43175 contract rabbit isolated basilar artery by activating a 5-HT1-like receptor. In addition 5-HT may activate a population of 5-HT2 receptors producing a further contraction of rabbit isolated basilar artery.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptors in human basilar arteries

After phenoxybenzamine (10(-5) M), pretreatment, and in the presence of propranolol (10(-6) M) and indomethacin (2.8 X 10(-6) M), dopamine caused a marked concentration-dependent relaxation of isolated strips of human basilar artery contracted with PGF2 alpha. This effect was mimicked by apomorphine, 6,7-ADTN and SK&F 38393, but N,N-diethyl dopamine, N,N-di-n-propyl-dopamine and 5,6-ADTN caused only slight relaxation. (+)-Butaclamol, cis-alpha-flupenthixol, fluphenazine and haloperidol competitively antagonised the relaxant effects of dopamine, but sulpiride was ineffective in concentrations as high as 1.3 X 10(-4) M. These findings show that the dopamine receptors in the human basilar artery closely resemble those in the smooth muscle of the rabbit isolated mesenteric and splenic arteries, and the dog renal and mesenteric arteries in vivo, but differ from those located presynaptically on sympathetic nerve terminals.

Attenuation of arterial blood pressure fall in endotoxin shock in the rat using the competitive bradykinin antagonist Lys-Lys-[Hyp2, Thi5,8, DPhe7]-Bk (B4148)

The selective competitive bradykinin (Bk) antagonist, B4148 (Lys‐Lys‐[Hyp2, Thi5,8, DPhe7]‐Bk) infused at 100 μg kg−1 min−1 into rats produced a significant inhibition of the hypotensive effect of Bk and had no effect against acetylcholine‐induced responses. In a rat model of endotoxin shock, the fall in mean arterial blood pressure in response to an intravenous injection of lipopolysaccharide from E. coli was significantly attenuated by the same infusion of B4148 compared to controls. These findings suggest that kinins are involved in the hypotensive response to endotoxin shock in rats. The development of potent Bk antagonists offers a new experimental approach for evaluating the role of kinins in this and other disease states and potential therapy in such disorders.

THE ACTION OF BRADYKININ AND OXYTOCIN ON THE ISOLATED WHOLE UTERUS AND MYOMETRIUM OF THE RAT IN OESTRUS

1 A technique is described for obtaining a myometrial preparation devoid of endometrium, from the uterus of the rat in oestrus. 2 Acetylcholine and prostaglandin F2α (PGF2α) produced concentration‐effect curves with the same maximal tensions and slope on the whole uterus and myometrial preparations. Concentration‐effect curves to bradykinin and oxytocin on the myometrial preparation were altered, resulting in a shift to the right and a decreased maximum response compared with those produced by the whole uterus. 3 Indomethacin produced greater antagonism of the responses of the whole uterus to bradykinin and oxytocin than to acetylcholine and PGF2at, whereas responses of the myometrium to all four agonists were similarly depressed. 4 Responses of the myometrial preparation to a range of concentrations of bradykinin and oxytocin were significantly enhanced by prior sensitization of the myometrium to PGF2flt. This significant enhancing effect of PGF2flt was only seen with the threshold dose of acetylcholine. 5 It appears that the mechanism of action of bradykinin and oxytocin on the rat uterus involves both a direct action and an indirect action. The indirect action possibly involves release of prostaglandins) from the endometrium.

Angiotensin converting enzyme inhibitors and expression of des-Arg9-BK (kinin B1) receptors in vivo

The effect of the selective kinin B1 receptor agonist des-Arg9-BK was studied on blood pressure and on the in vitro aorta of rabbits pretreated 18 h earlier with lipopolysaccharide from E. coli, an infusion of bradykinin or with one of three angiotensin converting enzyme inhibitors captopril, enalapril or teprotide. The hypotensive response in vivo and contractile response seen on the in vitro aorta was selectively increased to des-Arg9-BK in all pretreated groups compared to controls, effects which were blocked by the selective competitive kinin B1 receptor antagonist des-Arg9-[Leu8]BK. Dexamethasone given to lipopolysaccharide pretreated rabbits had no effect on the increased hypotensive response seen with des-Arg9-BK. The skin vascular permeability response to des-Arg9-BK, bradykinin and histamine remained unchanged in the groups pretreated with lipopolysaccharide or captopril compared to controls. The possible mechanism(s) whereby angiotensin converting enzyme inhibitors produce this effect and the possible relevance to the inflammatory side-effects seen with this group of drugs is discussed.

Effects of prostanoids on human and rabbit basilar arteries precontracted in vitro

The effect of a range of prostanoids on human and rabbit basilar arteries precontracted in vitro in the presence of the thromboxane receptor-blocking drug AH23848B was investigated. On the rabbit basilar artery and in the presence of AH23848B the thromboxane A2 mimetic U-46619 produced further concentration-related contractions of the tissue. All other prostaglandins (except ICI81008 and PGF 2a which had no effect) produced concentration-related relaxations with the rank order of relaxant potency being PGE2 > Iloprost > PGI2 = PGE1 = 16,16–dimethyl PGE 2 = PGD2. On the human basilar artery PGI2 and iloprost produced concentration-related relaxations with iloprost being more potent than PGI2. At high concentrations both these compounds produced reduced relaxant responses. All other prostanoids (except ICI81008 and PGD2 which had no effect) contracted the tissue, the rank order of contractile potency being 16, 16–dimethyl PGE2 > PGE2 > PGF2a = PGE2 > U46619 > ICI81008 and PGD2. It is concluded that the human basilar artery possesses two contractile prostanoid receptors, a TP receptor and one which may be of the EP-type in addition to a prostanoid receptor mediating relaxation which may be of the IP-type. The prostanoid receptor(s) mediating relaxation of the rabbit in vitro basilar artery is difficult to determine. The relevance of the observations to cerebrovascular disorders such as migraine and vasospasm is discussed.

The Vasoconstrictor Action of Sumatriptan on Human Isolated Dura Mater

Sumatriptan is a selective agonist for a 5-HT1 -like receptor sub-type which mediates contraction of some cranial blood vessels in vitro. It has been shown to be a very effective novel agent for the acute treatment of migraine and this has stimulated much interest in its clinical mode of action. It has been suggested that the pain of migraine results from a sterile neurogenic inflammatory response of intracranial blood vessels innervated by the trigeminal nerve. Furthermore, sumatriptan has been shown to inhibit plasma protein extravasation from blood vessels of the dura mater, induced by antidromic stimulation of the trigeminal nerve in the anaesthetised rat and guinea-pig. This effect could be explained by a localised vasoconstriction of the meningeal vessels. We now provide evidence that sumatriptan will potently constrict the blood vessels within the human isolated perfused dura mater. Preliminary work on characterising the 5-HT receptor involved indicates that it is a 5-HT1- like receptor. These studies add weight to the view that the anti-migraine action of sumatriptan results from selective cranial vasoconstriction.