Demonstration of β1-adrenoceptor mediating relaxation of porcine coronary artery by radioligand binding and pharmacological methods

β-adrenoceptors in the porcine coronary artery were characterized by a radioligand binding assay using (−)-[3H]dihydroalprenolol (DHA) and also by measuring the relaxant response of isolated conorary artery to norepinephrine. Specific (−)-[3H]DHA binding in the porcine coronary artery was saturable, revirsible and of high affinity (Kd=1.6 nM) with a maximal number of binding sites of 63 fmol/mg protein, and it showed a pharmacological specificity as well as stereoselectivity which characterized β-adrenoceptors. The Hofstee analysis of inhibition of (−)-[3H]DHA binding by atenolol, practolol and ICI 118551 has shown that the averaged concentration of β1 and β2-adrenoceptors in this tissue was 68 % and 32 % respectively. The relaxant response of isolated coronary artery to norepinephrine was competitively antagonized by (−)propranolol, (+)propranolol, atenolol, practolol and ICI 118551. The pA2 values of these adrenoceptor antagonists were significantly correlated with the Ki values for β1 but not β2-adrenoceptors determined by the (−)-[3H]DHA binding assay. Thus, the present study demonstrates that the relaxant response of porcine coronary artery to norepinephrine is predominantly mediated through the stimulation of β1-adrenoceptors on vascular smooth muscles.