This project constitutes an investigation into mechanisms whereby cells achieve the homeostasis of cytosolic free Ca2+ concentrations ((Ca2+)c), and allow perturbations in (Ca2+)c in response to hormones and neurotransmitters. Further, it addresses derangements in these control mechanisms which may occur in old-age. This year, we have asked the following questions. (1) What is the mechanism whereby the hormone glucagon leads to and increase in (Ca2+)c in hepatocytes? The efficacy of cyclic-AMP analogues in raising (Ca2+)c and the similar dose-response to glucagon od changes in (ca2+)c and changes in the activity of pyruvate kinase, which is phosphorylated by protein kinase A, have suggested to us that a mechanism involving solely protein kinase A is sufficient, though other mechanisms cannot be excluded. (2) What membrane carrier proteins are involved in mediating the entry of Ca2+ into myocytes when they are treated with veratridine? This compound potentiates Na+-channel activity and leads to a large increase in (Ca2+)c, an effect which we have found to be a useful tool in simulating a high work-load for our metabolic studies in myocytes. We have sought to distinguish between an involvement of Ca2+ channels and Na+/Ca2+ exchange by using the inhibitors verapamil, nitrendipine, Cd2+ and dichlorobenzamil, and have obtained answers indicating that the contribution of these two processes to total flux varies with the extracellular Na+, H+ and Ca2+ concentrations and the degree of depolarization. (3) Are there distinct alpha- and beta-adrenergic effects on the depolarization-induced entry of CA2+ into cardiac myocytes? We have studied cells loaded with Quin-2 and have identified a novel interaction, such that Ca2+ flux is activated more by beta- agonists alone than by alpha- and beta-agonists together. (4) Are there correlates at the level of protein phosphorylation of the previously described decreased responsiveness of CA2+ transport to catecholamines in the aging heart? We have identified a decreased phosphorylation of troponin in myocytes from senescent rats and are currently focusing on phospholamban.
