Temperature dependence of [ 3H]dihydroalprenolol binding in rat myocardium

Abstract

The β-adrenergic antagonist, [ 3H]dihydroalprenolol, was used to label binding sites in crude rat myocardial plasma membranes. The specificity of binding was dependent on the temperature of the assay. Specific binding at 22 and 37°C and at concentrations of radioligand less than 5 nM was consistent with binding to the myocardial β-receptor. Binding sites labeled at 4°C possessed quite different properties. Binding was non-stereoselective and of lower affinity. Agonist compounds were much less effective at competing for the labeled myocardial sites at 4°C than at 22°C. Those β-antagonists which additionally possess pharmacological ‘quinidine-like’ activity (e.g. propranolol, alprenolol) were potent competitors at 4°C, but competition was non-stereoselective. In contrast atenolol, a β-antagonist devoid of ‘quinidine-like’ activity was ineffective at 4°C. Furthermore, procaine, and quinidine itself were potent competitors of [ 3H]dihydroalprenolol binding at 4°C. Thus the specificity of [ 3H]dihydroalprenolol binding to rat heart membranes at 4°C appears to be directed non-stereoselectively at that portion of the competing molecule which recognized ‘quinidine-like’ as opposed to adrenergic activity.