Transregulation of adenylyl-cyclase-coupled inhibitory receptors in heart failure enhances anti-adrenergic effects on adult rat cardiomyocytes

OBJECTIVE: The aim was to study the L-type calcium current (ICa,L) in cardiac myocytes as a possible target of insulin in the regulation of cardiac function. METHOD: Using the whole-cell configuration of the patch-clamp technique, we investigated the stimulation of ICa,L by insulin in isolated rat ventricular myocytes. RESULTS: The stimulation of ICa,L by insulin was dose-dependent (EC50 = 33 nM) and reversible. Maximum stimulation of ICa,L over basal ICa,L was 86 +/- 11% (n = 25) at 1 microM insulin. Insulin (1 microM) shifted the current-voltage relationship and potential-dependent availability of ICa,L to more negative potentials by about 3.5 and 1.5 mV, respectively. The maximum conductance of ICa,L was increased by 1 microM insulin, from 26 +/- 4 to 39 +/- 5 nS (n = 11). Isoproterenol (100 nM), which stimulated ICa,L by 156 +/- 23% (n = 10) over basal ICa,L, acted faster than insulin. The half-maximum stimulation of ICa,L by isoproterenol and insulin was reached after 44 +/- 5 and 80 +/- 9 s, respectively. Insulin and isoproterenol responses were not additive. Insulin (1 microM) and isoproterenol (100 nM) stimulation of ICa,L was inhibited by Rp-cAMPS (1 mM) to 12 +/- 3 and 32 +/- 4%, respectively. Insulin (1 microM) increased cAMP content in rat cardiomyocytes by about two-fold. Insulin-like growth factor-1 (IGF-1; 5 microM) increased ICa,L by only 5.9 +/- 0.9% (n = 6). CONCLUSIONS: Our data show that insulin stimulates the L-type calcium current in isolated rat ventricular myocytes in a dose-dependent and reversible manner and suggest that this effect is mediated by insulin receptors and the cAMP-dependent protein kinase.