A match between binding to beta-adrenoceptors and stimulation of adenylyl cyclase parameters of (-)isoproterenol and salbutamol on rat brain.

Inhibition experiments of (-)[3H]CGP 12177 binding by (-)isoproterenol and salbutamol were performed on synaptosomes prepared from rat brain cortex and cerebellum. Adenylyl cyclase (AC) stimulation experiments on slices of these structures were also performed, with measuring [14C]cAMP obtained from [14C]adenine. Studying beta 1- and beta 2-adrenoceptors (beta 2AR) separately, dissociation constants of (-)isoproterenol for the high- (KH) and low- (KL) affinity states are 8 and 206 nM, respectively, for beta 1AR, vs 20 and 900 nM for beta 2AR. With salbutamol, KH and KL for beta 2AR are 37 and 1250 nM, respectively, vs 430 and 8500 nM for beta 1AR. In any case, the proportion of high-affinity state (RH) of beta 2AR in the cerebellum (59% and 35% for (-)isoproterenol and salbutamol, respectively) is twice that of beta 1AR (30% and 18%). Surprisingly, the RH of cortex beta 2AR with (-)isoproterenol (30%) is significantly lower than in the cerebellum, but not with salbutamol (35%). To allow meaningful comparisons of potencies in stimulating [14C]cAMP production, we define the coupling efficiency (CE), applicable to specified beta AR subtype and agonist, and expressed as the maximal production of mol cAMP.mol-1 beta AR.min-1. The order of CE was always in favor of (-)isoproterenol vs salbutamol on cerebellum beta 2AR > on cortex beta 2AR > on cortex beta 1AR. This order indicates the partial agonism of salbutamol for both beta AR subtypes, and an intrinsic better coupling of beta 2AR vs beta 1AR in rat brain. Moreover, this order corresponds roughly to that of RH. Hence, CE is directly correlated with RH at least for these two agonists. EC50 for cAMP production for each subtype and agonist is in the same order than the respective KL, and might only reflect the rapid return of receptor to low-affinity state after the activation of Gs protein. In binding experiments on the whole beta AR in both areas, the pseudo-Hill coefficient did not reach 1 with 0.3 mM guanosine 5'-(beta, gamma-imido)triphosphate (GppNHp). This dysfunction of GppNHp in rat brain structures might be caused by a major difference in the regulation of coupling in the ternary complex as compared with peripheral tissues.