Characteristics of Adrenoceptors and [3H]Nitrendipine Receptors of Porcine Vascular Smooth Muscle: Differences Between Coronary Artery and Aorta

Characteristics of the bindings of [H](−)dihydroalprenolol, [I](−) iodocyanopindolol, [H]prazosin, [H]yohimbine, and [H]nitrendipine to porcine coronary membranes were investigated and the results compared with studies of porcine aortic membranes. In the equilibrium binding study carried out in sarcolemma-enriched fractions, there were no major differences in the Kd values of these radioligands between coronary artery and aorta. However, the densities of/β-, α1, and α2-adrenocetors and [H]nitrendipine receptors of coronary artery were 258, 12, 12, and 561 fmol/mg protein, respectively, while those of aorta were 37,525,1,000, and 215 fmol/mg protein. β-Adrenergic agonists competed with [H](−)dihydroalprenolol binding sites in coronary artery, the order of potency being (−)isoproterenol > (−)norepinephrine > (−)epinephrine > (+)isoproterenol. In case of aorta, the order was (−)isoproterenol > (−)epinephrine > (−)norepinephrine. The competition by (±)bisoprolol β,-selective antagonist) and ICI 118,5510β2-selective antagonist) for [I](–)iodocyanopindolol binding sites in coronary artery resulted in nonlinear Hofstee plots (β1β2 = 90%10%). In case of aorta, linear Hofstee plots were obtained. From these results, we conclude that (1) coronary β- receptors in pigs are predominately of β1-type, while those of aorta are of %bT2-type; (2) regarding the relative population of adrenoceptors, coronary artery is β-dominant (β/α = 11), while aorta is α- dominant (β/α = 0.02); (3) compared withα-adrenoceptors, coronary artery has a greater number of [H]nitrendipine binding sites (nitrendipine/α-adrenoceptor = 23) than aorta (nitrendipine/α-adrenoceptor = 0.14).