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.