PI3Kgamma is required for PDE4, not PDE3, activity in subcellular microdomains containing the sarcoplasmic reticular calcium ATPase in cardiomyocytes

We recently showed that phosphoinositide-3-kinase-gamma-deficient (PI3Kgamma(-/-)) mice have enhanced cardiac contractility attributable to cAMP-dependent increases in sarcoplasmic reticulum (SR) Ca(2+) content and release but not L-type Ca(2+) current (I(Ca,L)), demonstrating PI3Kgamma locally regulates cAMP levels in cardiomyocytes. Because phosphodiesterases (PDEs) can contribute to cAMP compartmentation, we examined whether the PDE activity was altered by PI3Kgamma ablation. Selective inhibition of PDE3 or PDE4 in wild-type (WT) cardiomyocytes elevated Ca(2+) transients, SR Ca(2+) content, and phospholamban phosphorylation (PLN-PO(4)) by similar amounts to levels observed in untreated PI3Kgamma(-/-) myocytes. Combined PDE3 and PDE4 inhibition caused no further increases in SR function. By contrast, only PDE3 inhibition affected Ca(2+) transients, SR Ca(2+) loads, and PLN-PO(4) levels in PI3Kgamma(-/-) myocytes. On the other hand, inhibition of PDE3 or PDE4 alone did not affect I(Ca,L) in either PI3Kgamma(-/-) or WT cardiomyocytes, whereas simultaneous PDE3 and PDE4 inhibition elevated I(Ca,L) in both groups. Ryanodine receptor (RyR(2)) phosphorylation levels were not different in basal conditions between PI3Kgamma(-/-) and WT myocytes and increased in both groups with PDE inhibition. Our results establish that L-type Ca(2+) channels, RyR(2), and SR Ca(2+) pumps are regulated differently in distinct subcellular compartments by PDE3 and PDE4. In addition, the loss of PI3Kgamma selectively abolishes PDE4 activity, not PDE3, in subcellular compartments containing the SR Ca(2+)-ATPase but not RyR(2) or L-type Ca(2+) channels.