SUBTYPE-SELECTIVE REGULATION OF COUPLING OF RAT CARDIAC BETA-ADRENOCEPTORS TO ADENYLATE-CYCLASE

There is now evidence from human studies to suggest that cardiac beta-adrenoceptor density and coupling to adenylate cyclase may be regulated in a subtype selective fashion. An animal model was used to investigate this further. Rats were infused for 6 days with the non-selective full agonist isoprenaline (n = 6) or the beta1-selective partial agonist xamoterol (n = 6) with sham operated rats (n = 6) for control. Beta-Adrenoceptor subtype density and coupling to adenylate cyclase were determined in left ventricular membranes. Isoprenaline infusion downregulated both beta1- (30%) and beta2- (63%) adrenoceptor subtypes with associated reduction in adenylate cyclase stimulation through both subtypes. Xamoterol did not downregulate either subtype, but selectively uncoupled beta1-adrenoceptors. Beta1- and total beta-adrenoceptor-mediated stimulation of adenylate cyclase was reduced less by xamoterol than isoprenaline. We conclude that coupling of rat cardiac beta-adrenoceptors can be regulated in a subtype selective fashion and that the partial agonist xamoterol does not desensitise beta-adrenoceptor mediated stimulation of adenylate cyclase to the same extent as the full agonist isoprenaline.