Alterations of cardiac beta-adrenoceptor mechanisms due to calcium depletion and repletion.

In order to understand the modification of beta-adrenoceptor linked signal transduction by changes in the intracellular Ca2+, we examined the status of beta-adrenoceptors (beta-ARs), G-proteins and adenylyl cyclase (AC) in Ca2+-deficiency and Ca2+-overload by perfusing the isolated rat heart with Ca2+-free medium for 5 min and Ca2+-containing medium for 5 min following Ca2+-free perfusion, respectively. Ca2+-depletion caused not only an increase in basal, isoproterenol-, Gpp(NH)p-, NaF- and forskolin-stimulated AC activities but also produced an increase in the beta1-AR affinity and density as well as up-regulation of G(s)-protein function and uncoupling of G(i)-protein to AC. Ca2+-repletion for 5 min following 5 min Ca2+-free perfusion reversed the increased AC activities to varying degrees. The beta1-AR affinity was further increased upon Ca2+-repletion whereas its density was decreased. Ca2+-repletion also decreased protein content for AC and beta-AR kinase but augmented the changes in G(s)- and G(i)-protein functions. Although low Na+ medium perfusion during Ca2+-depletion prevented the changes in G-proteins during both Ca2+-depletion and Ca2+-repletion periods, the increased beta1-AR affinity and density as well as changes in AC activities due to Ca2+-depletion were not affected while alterations due to Ca2+-repletion were fully prevented. These results suggest that changes in Ca2+-homeostasis may represent a mechanism for alterations in the beta-adrenergic signal transduction pathway in the heart under pathological conditions.