Sildenafil: A Beta-2 Adrenergic Receptor-, Protein Kinase G-, and Protein Kinase A-Dependent Treatment for Adverse Cardiac Function and Excitation-Contraction Coupling Associated with Diabetes

Abstract

INTRODUCTION: In humans with type 2 diabetes and heart hypertrophy, the inhibition of phosphodiesterase 5 (PDE5) by sildenafil improves cardiac outcomes. However, the mechanism by which sildenafil improves cardiac function is unclear.We have observed a relationship between PDE5 and beta-2 adrenergic receptor (B2AR), an important transducer of sympathetic stimulation.HYPOTHESIS: Inhibition of PDE5 with sildenafil will restore cyclic guanosine monophosphate (cGMP) produced by B2AR, as well as cyclic adenosine monophosphate (cAMP) due to an increase in PDE5/PDE3 interaction. Sildenafil will lead to improvement in BAR-dependent cardiac excitation-contraction coupling (ECC) and overall heart function via increased PKG and PKA activity. METHODS: To explore this mechanism, we fed wild type and B2AR knockout mice high fat diet until wild type mice had reduced ejection fraction. At this point, mice were acutely injected with or treated for one month with sildenafil, and echocardiogram analysis with isoproterenol injection was performed. Cardiomyocytes were isolated and used for ECC analysis; fluorescent resonant energy transfer (FRET) analysis of cGMP, cAMP and PKA; and Western blot analysis of ECC proteins. RESULTS: Both acute and sustained treatment with sildenafil improved in vivo response to sympathetic stimulation in a B2AR-dependent manner. Similarly, when cells were treated acutely ex vivo and when mice were sustainably treated with sildenafil, sildenafil improved isolated myocyte ECC response to isoproterenol. Inhibition of PKG with DT2, and PKA with myrPKI confirmed improvements were PKG- and PKA-dependent.FRET analysis indicated sildenafil improved BAR-stimulated cAMP and PKA activity in adult diabetic mouse cardiomyocytes. Through this study, we have begun to elucidate mechanisms by which PDE5 inhibition is an effective treatment for diabetics with heart hypertrophy.