Characterization of beta-adrenoceptors in pig basilar artery from functional and radioligand binding studies.

beta-Adrenoceptors in pig basilar arteries were investigated by measuring the relaxation response to norepinephrine and by a radioligand binding assay with [3H]-dihydroalprenolol (DHA). Norepinephrine induced concentration-dependent relaxations. The relaxation responses were independent of the presence of endothelial cells, and they were competitively antagonized by (+/-)-propranolol, atenolol, butoxamine and ICI 118,551. Specific [3H]-DHA binding to beta-adrenoceptors was saturable, reversible and high affinity (Kd = 1.4 nM), with a Bmax of 48.7 fmol/mg protein. Computer analysis of inhibition of [3H]-DHA binding by atenolol, butoxamine and ICI 118,551 gave a beta 1: beta 2-adrenoceptor ratio of approximately 65:35. The pA2 values of these antagonists were significantly correlated with the Ki values for beta 1-adrenoceptor determined by the radioligand binding assay. The present findings indicate that the relaxation responses to norepinephrine are predominantly mediated through the stimulation of beta 1-adrenoceptors on vascular smooth muscle cells in a pig basilar artery.