Agonist-mediated conformational changes of beta-adrenoceptors could occur independent of functional coupling to Ns

Agonist and antagonist binding characteristics of beta-adrenoceptors in turkey erythrocyte ghosts were determined at different temperatures ranging between 7 degrees C and 42 degrees C. [3H]-DHA saturation binding experiments revealed that the antagonist-receptor interaction is entropy-driven with a small enthalpic contribution. Isoproterenol/[3H]-DHA competition binding followed the law of mass action at all the investigated temperatures. The agonist-receptor interaction is enthalpy driven with a small unfavorable decrease in entropy. This is consistent with the agonist's ability to favor an endoenergetic transconformation of the receptors. Only part of the agonist-bound receptors can undergo functional coupling to the stimulatory component of the adenylate cyclase system (Ns). This coupling process is associated with "locking-in" of the agonist and becomes persistent in the presence of the alkylating reagent N-ethylmaleimide. The number of agonist/N-ethylmaleimide-sensitive sites (i.e. coupling-prone receptors) increases with the temperature until it reaches a plateau value of 50% between 27-32 degrees C. Qualitatively similar data were obtained for rat lung and turkey erythrocyte membranes. These observations suggest that the whole receptor population can undergo agonist-mediated conformational changes but that only part of them can couple to Ns.