Forskolin activation of adenylate cyclase in rat myocardium with age: effects of guanine nucleotide analogs.

Beta-adrenergic and post-receptor activation of adenylate cyclase decreases with age in the rat myocardium. Forskolin-stimulated adenylate cyclase activity decreases with age to the same extent as NaF or isoproterenol stimulation, suggesting that a loss of catalytic unit activity accounts for the loss of activity with age. However, recent evidence suggests that there are both a guanine nucleotide-dependent and independent component of forskolin activation. We assessed the possible role of each of these components with age by measuring forskolin dose-response stimulation of adenylate cyclase activity in the presence and absence of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta S) and beta,gamma-imidoguanine 5'-triphosphate (Gpp(NH)p) in myocardial membranes from F-344 rats of 3, 12, and 24 months of age. Maximal forskolin stimulation of adenylate cyclase was least in the 24- (121 +/- 21 pmol cAMP/min/mg) as compared with either the 12- (212 +/- 29) or 3- (190 +/- 19) month-old rats. The presence of GDP-beta S had no effect on either the EC50 or maximum activity, and the age-related decline persisted. Gpp(NH)p plus isoproterenol enhanced forskolin activation but the effect was additive and not synergistic. There was no effect on the EC50, and the maximum activity was least in the 24-month-old rats. These data reconfirm the loss of catalytic unit activity with age and indicate that the reduced activity with age is independent of the presence or absence of activated G-protein.