Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

A61603-induced contractions of the porcine meningeal artery are mediated by alpha1- and alpha2-adrenoceptors

It has recently been shown that A61603 (N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydro-naphthalen-1-yl]methane sulphonamide), a potent alpha(1A)-adrenoceptor agonist, decreased carotid artery conductance in anaesthetized pigs by a novel non-adrenergic mechanism. In this study, we set out to pharmacologically characterize A61603-induced contractions of the porcine isolated meningeal artery. While the maximum contractile responses of the artery were similar, A61603 (E(max): 183 +/- 23% of 100 mM KCl; pEC(50): 7.25 +/- 0.18) was more potent than noradrenaline (E(max): 156 +/- 16%; pEC(50): 5.75 +/- 0.17) or phenylephrine (E(max): 163 +/- 20%; pEC(50): 5.63 +/- 0.02). Prazosin (pA(2): 9.36 +/- 0.23) and, to a lesser extent, rauwolscine (pK(b): 6.36 +/- 0.38) and yohimbine (pK(b): 7.30 +/- 0.15) antagonised the contractions to A61603. The 5-HT(1B) (GR127935; N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide) and 5-HT(2) (ritanserin) receptor antagonists failed to affect the responses to A61603, but methiothepin, which, in addition, has a high affinity for alpha-adrenoceptors, proved an effective antagonist. The A61603-induced responses were suppressed by the cAMP stimulator forskolin, but not by the protein kinase C inhibitor chelerythrine. Our results suggest that the contraction of porcine isolated meningeal artery by A61603 is mediated via mainly alpha(1)-(probably alpha(1A)) and, to a lesser extent, alpha(2)-adrenoceptors, involving the adenylyl cyclase, but not the diacylglycerol pathway.

Effects of current and prospective antimigraine drugs on the porcine isolated meningeal artery

Vasoconstriction to agonists at serotonin (5-hydroxytryptamine; 5-HT) receptors and alpha-adrenoceptors, as well as vasodilatation induced by alpha-CGRP, have been well described in the porcine carotid circulation in vivo. The present study sets out to investigate the effects of current and prospective antimigraine drugs on porcine meningeal artery segments in vitro. Sumatriptan, ergotamine, dihydroergotamine, isometheptene and clonidine failed to contract the meningeal artery, but 5-HT, noradrenaline and phenylephrine induced concentration-dependent contractions. The contractions to 5-HT were competitively antagonized by the 5-HT(2A) receptor antagonist ketanserin, whilst those to noradrenaline were antagonized by alpha(1)-(prazosin), alpha(2)-(rauwolscine and yohimbine) and alpha(2C/2B)-(OPC-28326) adrenoceptor antagonists. Whilst dobutamine and salbutamol were ineffective, alpha-CGRP produced concentration-dependent relaxations that were antagonized by the CGRP(1) receptor antagonist olcegepant. In agreement with their lack of contractile effect, sumatriptan and ergotamine failed to influence forskolin-stimulated cyclic AMP accumulation in the porcine meningeal artery; in contrast, both compounds decreased forskolin-stimulated cyclic AMP accumulation in the human isolated saphenous vein, where they induced contractions. Finally, using RT-PCR, we could demonstrate the presence of mRNAs encoding for several 5-HT receptors (5-HT(1B), 5-HT(1D), 5-HT(1F), 5-HT(2A) and 5-HT(7)) and adrenoceptors (alpha(1A), alpha(1B), alpha(1D), alpha(2A), alpha(2B), alpha(2C), beta(1) and beta(2)), as well as that for the calcitonin receptor like receptor, a component of the CGRP(1) receptor. These results suggest that: (i) the porcine meningeal artery may not be involved in the vasoconstriction of the carotid vascular bed elicited by antimigraine drugs in anaesthetized pigs, and (ii) the mismatch between the presence of receptor mRNA and the lack of response to sumatriptan, dobutamine and salbutamol implies that mRNAs for the 5-HT(1B) receptor and beta(1)- and beta(2)-adrenoceptors are probably unstable, or that their density is too low for being translated as receptor protein in sufficient quantities.

Role of cyclic AMP in stimulation-evoked release of 3H-noradrenaline from rabbit isolated ear artery

The possible involvement of cyclic AMP in the stimulation-evoked 3H-overflow from rabbit isolated ear artery preloaded with 3H-noradrenaline was studied. Cyclic AMP (10(-5)-3 x 10(-4)M), 8-bromo-cyclic AMP (3 x 10(-4)M) and adenosine (10(-5)-3 x 10(-4)M) enhanced stimulation-evoked 3H-overflow. Dibutyryl-cyclic AMP (10(-5)-3 x 10(-4)M) had no effect. Theophylline (3 x 10(-5)M) and the phosphodiesterase inhibitor AH 21-132 (3 x 10(-5)M) did not alter the enhancement of 3H-overflow caused by either cyclic AMP or adenosine. Forskolin (3 x 10(-6)M) and the phosphodiesterase inhibitors ICI 63 197 (10(-4)M) and AH 21-132 (3 x 10(-6)-3 x 10(-5)M) increased stimulation-evoked 3H-overflow. Forskolin (10(-6)M) enhanced the effect of ICI 63 197 (3 x 10(-5)M) but it did not alter the effect of AH 21-132. It is concluded that cyclic AMP is involved in the stimulation-evoked release of noradrenaline from postganglionic sympathetic nerves in the rabbit ear artery.