Abstract

The kinetic and pharmacological properties of β-adrenergic receptors in turkey erythrocyte membranes have been characterized by measuring adenylate cyclase activity and specific binding of ¹²⁵I-iodohythoxybenzylpindolol (IHYP). Receptor properties have been compared to those of β₁, and β₂ receptors in a number of mammalian tissues. The affinity (K_D) of IHYP for the turkey erythrocyte β-adrenergic receptor (42 pM) was similar to the K_D for IHYP binding to either β₁ or β₂ receptors. However, the rates of both association (k₁) and dissociation (k_-1) of IHYP were 6-10 times faster when measured with β-adrenergic receptors on turkey erythrocytes than were observed for either β₁ or β₂ receptors in various mammalian tissues. Thermodynamic analysis of the k₁ for IHYP binding in the turkey erythrocyte and rat heart showed similar enthalpies of activation (ΔH‡), suggesting that the different k₁ values arise mainly from different entropies of activation (ΔH‡) in the two tissues. The order of potency of drugs for activation or inhibition of adenylate cyclase activity correlated well with that for inhibition of IHYP binding in the turkey erythrocyte. However, both the K_i and K_act values for adenylate cyclase were generally two to three times higher than the corresponding K_D value determined from studies of the inhibition of IHYP binding. The pharmacological effects of a variety of drugs with similar or different affinities for β₁- and β₂-adrenergic receptors were determined on membranes prepared from turkey erythrocytes. The K_D values for nonselective drugs in the turkey erythrocyte were identical to their K_D values for β₁, and β₂ receptors, suggesting that this receptor should be classified as a β-adrenergic receptor. However, the K_D values in the turkey erythrocyte for selective drugs did not correlate with the K_D values for these drugs at either β₁ or β₂ receptors. Furthermore, the efficacies of partial agonists at turkey erythrocyte β-adrenergic receptors did not correlate with their efficacies for either β₁ or β₂ receptors. The results demonstrate that the β-adrenergic receptor in the turkey erythrocyte has kinetic and pharmacological properties distinct from either mammalian β₁ or β₂ receptors.