ENDOTHELIUM-DEPENDENT NORADRENALINE-INDUCED RELAXATION OF RAT ISOLATED CEREBRAL-ARTERIES - PHARMACOLOGICAL CHARACTERIZATION OF RECEPTOR SUBTYPES INVOLVED

1 The endothelium-dependence of catecholamine-induced relaxation of ra t cerebral arteries was investigated in vitro. 2 In the basilar artery (BA), the maximal relaxant response was most pronounced with noradren aline (NA), less with isoprenaline (Iso), and only very little with te rbutaline. Methoxamine and the alpha2-adrenoceptor selective agonists BHT 933 and clonidine, had no relaxant effect. 3 In BA, the relaxation by NA or Iso was markedly attenuated by N(omega)-nitro-L-arginine (L- NOARG) 10(-4) m. Short term perfusion of the vessels by Triton X 100 ( 1:1 000) suppressed the NA-induced relaxation. 4 The relaxation induce d by NA or Iso was markedly reduced in presence of L-NOARG in the post erior, medial and anterior cerebral artery. 5 In BA, NA-induced relaxa tion was non-competitively inhibited by propranolol, atenolol, and the beta1- and beta2-adrenoceptor selective antagonists, CGP 20712 A and ICI 118551. 6 The relaxant NA-effect was not affected by prazosin but was non-competitively blocked by phentolamine. 7 The Iso-induced relax ation was competitively blocked by propranolol, whereas atenolol, CGP 20712 A and ICI 118551 caused a non-competitive inhibition. 8 The expe riments indicate that the catecholamine-induced relaxation in rat isol ated cerebral arteries depends upon the endothelium. They suggest that the NA-induced relaxation of BA is mediated by different alpha- and b eta-adrenoceptors and that the Iso-induced relaxation is mediated by d ifferent beta-receptors. The findings would also be compatible with th e idea of a receptor type which cannot be characterized by the pharmac ological tools that we have used.