Abstract 1538: PDE5 Activation Enhances Myocyte Contractility And Relaxation By Modulating Calcium Cycling
Cyclic GMP-speficic phosphodiesterase 5 (PDE5) inhibition by its inhibitor such as sildenafil blunts acute β-adrenergic-stimulated cardiac contractility in normal hearts. In failing hearts, however, PDE5 activity is down-regulated and dyslocalized, and the anti-β-adrenergic effect from PDE5 inhibition is lost, suggesting a potential mechanism for contractile depression and β-adrenergic downregulation. To test the role of PDE5 expression/activity in cardiac myocytes in modulating cardiac function, we developed a cardiac specific inducible Tet-off PDE5 overexpressing mouse (TG). After induction of the transgene, PDE5 protein expression was increased 2 and 5 fold in transgenic whole hearts and isolated myocytes, respectively. Basal cardiac function (echocardiogram) was generally similar in adult TG mice with PDE5 over-expressed for 1 month to littermate control (WT). Overexpressed PDE5 localized to myocyte z-bands as does endogenous PDE5 (confocal immunofluorescence). Importantly, TG myocytes (n=160~220 from 3 hearts) showed increased basal and isoproterenol (ISO) stimulated sarcomere shortening and peak calcium transients, and improved relaxation (table⇓). ISO-stimulated phospholamban (PLB) phosphorylation was 2 fold higher in TG myocytes as compared to WT controls (P<0.05) in a dose dependent manner, while SERCA2a expression and TnI phosphorylation were unaltered. In vivo pressure volume loop analysis further revealed that baseline contractility was increased and afterload-dependent relaxation was shorter (better relaxation) in TG. These results provide the first genetic evidence that increased PDE5 expression in myocytes enhances myocyte contractility at rest and with acute β-adrenergic stimulation, which is coupled to enhanced calcium cycling and PLB phosphorylation, and further supports the notion that decreased PDE5 activity itself might contribute to downregu-lation of β-adrenergic signaling in failing hearts.