MATCH BETWEEN BINDING TO BETA-ADRENOCEPTORS AND STIMULATION OF ADENYLYL-CYCLASE PARAMETERS OF (-)ISOPROTERENOL AND SALBUTAMOL ON RAT-BRAIN

Inhibition experiments of(-)[H-3]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 [C-1 4]cAMP obtained from [C-14]adenine. Studying beta(1)- and beta(2)-adre noceptors (beta(2)AR) separately, dissociation constants of (-)isoprot erenol for the high- (K-H) and low- (K-L) affinity states are 8 and 20 6 nM, respectively, for beta(1)AR, vs 20 and 900 nM for beta(2)AR. Wit h salbutamol, K-H and K-L for beta(2)AR are 37 and 1250 nM, respective ly, vs 430 and 8500 nM for beta(1)AR. In any case, the proportion of h igh-affinity state (R-H) of beta(2)AR in the cerebellum (59% and 35% f or (-)isoproterenol and salbutamol, respectively) is twice that of bet a(1)AR (30% and 18%). Surprisingly, the R-H of cortex beta(2)AR with ( -)isoproterenol (30%) is significantly lower than in the cerebellum, b ut not with salbutamol (35%). To allow meaningful comparisons of poten cies in stimulating [C-14]cAMP production, we define the coupling effi ciency (CE), applicable to specified PAR subtype and agonist, and expr essed as the maximal production of mol cAMP.mol(-1) beta AR.min(-1). T he order of CE was always in favor of (-)isoproterenol vs salbutamol o n cerebellum beta(2)AR> on cortex beta(2)AR> on cortex beta 1(A)R. Thi s order indicates the partial agonism of salbutamol for both beta AR s ubtypes, and an intrinsic better coupling of beta(2)AR vs beta(1)AR in rat brain. Moreover, this order corresponds roughly to that of RH Hen ce, 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 K-L, and might only reflect the rapid return of receptor to low-affinity state after the activation of G(s) protei n. In binding experiments on the whole PAR in both areas, the pseudo-H ill coefficient did not reach 1 with 0.3 mM guanosine 5'-(beta,gamma-i mido)triphosphate (GppNHp). This dysfunction of GppNHp in rat brain st ructures might be caused by a major difference in the regulation of co upling in the ternary complex as compared with peripheral tissues. (C) 1997 The Italian Pharmacological Society.