β-Adrenergic receptors: Evidence for negative cooperativity

Abstract

The specific binding of [ 3H] (−)alprenolol to sites in frog erythrocyte membranes provides a tool for directly assessing ligand binding to adenylatecyclase coupled β-adrenergic receptors. Hill Plots of such binding data yield slopes (n H=“Hill Coefficients”) less than 1.0, suggesting that negatively cooperative interactions among the β-adrenergic receptors may occur. The existence of such negative cooperativity was confirmed by a direct kinetic method. The dissociation of receptor bound [ 3H] (−)alprenolol was studied under two conditions: 1) with dilution of the ligand-receptor complex sufficient to prevent rebinding of the dissociated tracer and 2) with this same dilution in the presence of excess unlabeled (−)alprenolol. If the sites are independent, the dissociation rates must be the same in both cases. However, the presence of (−)alprenolol increases the rate of [ 3H] (−)alprenolol dissociation, indicating that negatively cooperative interactions among the β-adrenergic receptor binding sites do occur.