Phosphatidylinositol 3-Kinase Offsets cAMP-Mediated Positive Inotropic Effect via Inhibiting Ca Influx in Cardiomyocytes

Phosphoinositide 3-kinase (PI3K) has been implicated in 2-adrenergic receptor ( 2-AR)/Gi-mediated compartmentation of the concurrent Gs-cAMP signaling, negating 2-AR–induced phospholamban phosphorylation and the positive inotropic and lusitropic responses in cardiomyocytes. However, it is unclear whether PI3K crosstalks with the 1-AR signal transduction, and even more generally, with the cAMP/PKA pathway. In this study, we show that selective 1-AR stimulation markedly increases PI3K activity in adult rat cardiomyocytes. Inhibition of PI3K by LY294002 significantly enhances 1-AR–induced increases in L-type Ca 2 currents, intracellular Ca transients, and myocyte contractility, without altering the receptor-mediated phosphorylation of phospholamban. The LY294002 potentiating effects are completely prevented by ARK-ct, a peptide inhibitor of -adrenergic receptor kinase-1 ( ARK1) as well as G signaling, but not by disrupting Gi function with pertussis toxin. Moreover, forskolin, an adenylyl cyclase activator, also elevates PI3K activity and inhibition of PI3K enhances forskolin-induced contractile response in a ARK-ct sensitive manner. In contrast, PI3K inhibition affects neither the basal contractility nor high extracellular Ca -induced increase in myocyte contraction. These results suggest that 1-AR stimulation activates PI3K via a PKA-dependent mechanism, and that G and the subsequent activation of ARK1 are critically involved in the PKA-induced PI3K signaling which, in turn, negates cAMP-induced positive inotropic effect via inhibiting sarcolemmal Ca influx and the subsequent increase in intracellular Ca transients, without altering the receptor-mediated phospholamban phosphorylation, in intact cardiomyocytes. (Circ Res. 2004;95:1183-1190.)