Rationale: Baseline contractility of mouse hearts is modulated in a phosphatidylinositol 3-kinase-γ-dependent manner by type 4 phosphodiesterases (PDE4), which regulate cAMP levels within microdomains containing the sarcoplasmic reticulum (SR) calcium ATPase type 2a (SERCA2a).
Objective: The goal of this study was to determine whether PDE4D regulates basal cardiac contractility.
Methods and results: At 10 to 12 weeks of age, baseline cardiac contractility in PDE4D-deficient (PDE4D(-/-)) mice was elevated mice in vivo and in Langendorff perfused hearts, whereas isolated PDE4D(-/-) cardiomyocytes showed increased whole-cell Ca2+ transient amplitudes and SR Ca2+content but unchanged L-type calcium current, compared with littermate controls (WT). The protein kinase A inhibitor R(p)-adenosine-3',5' cyclic monophosphorothioate (R(p)-cAMP) lowered whole-cell Ca2+ transient amplitudes and SR Ca2+ content in PDE4D(-/-) cardiomyocytes to WT levels. The PDE4 inhibitor rolipram had no effect on cardiac contractility, whole-cell Ca2+ transients, or SR Ca2+ content in PDE4D(-/-) preparations but increased these parameters in WT myocardium to levels indistinguishable from those in PDE4D(-/-). The functional changes in PDE4D(-/-) myocardium were associated with increased PLN phosphorylation but not cardiac ryanodine receptor phosphorylation. Rolipram increased PLN phosphorylation in WT cardiomyocytes to levels indistinguishable from those in PDE4D(-/-) cardiomyocytes. In murine and failing human hearts, PDE4D coimmunoprecipitated with SERCA2a but not with cardiac ryanodine receptor.
Conclusions: PDE4D regulates basal cAMP levels in SR microdomains containing SERCA2a-PLN, but not L-type Ca2+ channels or ryanodine receptor. Because whole-cell Ca2+ transient amplitudes are reduced in failing human myocardium, these observations may have therapeutic implications for patients with heart failure.