Beta 2-adrenergic receptors contribute to catecholamine-stimulated shortening of action potential duration in dog atrial muscle.

beta 1- and beta 2-adrenergic receptors coexist in atria of humans and a variety of other species. The significance of these receptors and the quantitative extent to which beta 2-adrenergic receptors contribute to the electrophysiologic effects of catecholamines acting at beta-adrenergic receptors have not been determined. In the present study, the beta 1-selective antagonist ICI 89,406 and the beta 2-selective antagonist ICI 118,551 were used to determine the relative densities of beta 1- and beta 2-adrenergic receptors on membranes prepared from dog atria and the contribution that each subtype makes to isoproterenol-induced shortening of action potential duration measured at 75% repolarization (APD75). Computer-aided nonlinear regression analysis of the inhibition of the specific binding of [125I]iodopindolol and the inhibition of isoproterenol-stimulated APD75 shortening by these antagonists showed that a two-site model fits the data better than a one-site model (P less than 0.0001). The affinity constants for each selective antagonist determined by inhibition of APD75 shortening were similar to those determined in studies of the inhibition of the specific binding of [125I]iodopindolol to beta 1- and beta 2-adrenergic receptors. beta 2-Adrenergic receptors made up approximately equal to 25% of the total number of beta-adrenergic receptors but mediated approximately equal to 50% of the electrophysiologic effect of isoproterenol. The inhibition of the binding of [125I]iodopindolol and of the APD75 shortening by propranolol, a nonselective antagonist, was best fit by a one-site model. In other experiments, dogs were treated with reserpine and atropine to eliminate complications caused by the presence of endogenous norepinephrine and acetylcholine. Results obtained with atria from these animals were similar to those obtained in studies with control atria. These data suggest that beta 2-adrenergic receptors as well as beta 1-adrenergic receptors are involved in mediating the electrophysiologic effects of catecholamines in dog atrial muscle and that both beta 1- and beta 2-adrenergic receptors are present on atrial muscle cells.