Compartmentalization of cardiac β-adrenergic inotropy modulation by phosphodiesterase type 5

Background - Recent cell-based studies have found that cGMP synthesis and hydrolysis by phosphodiesterase ( PDE) appear compartmentalized, with nitric oxide synthase-derived and/or PDE type 5 ( PDE-5)-hydrolyzable cGMP undetected at the sarcolemmal membrane in contrast to cGMP stimulated by natriuretic peptide. In the present study, we determine the functional significance of such compartments with a comparison of beta-adrenergic modulation by PDE-5 inhibition to that of natriuretic peptide stimulation in both cardiomyocytes and intact hearts. The potential role of differential cGMP and protein kinase G stimulation by these 2 modulators was also studied. Methods and Results - Intact C57/BL6 mouse hearts were studied with pressure-volume analysis, and adult isolated myocytes were studied with fluorescence microscopy. PDE-5 inhibition with 0.1 to 1 mu mol/L sildenafil ( SIL) suppressed isoproterenol ( ISO)-stimulated contractility, whereas 10 mu mol/L atrial natriuretic peptide ( ANP) had no effect. ISO suppression by SIL was prevented in cells pretreated with a protein kinase G inhibitor. Surprisingly, myocardial cGMP changed little with SIL + ISO yet rose nearly 5-fold with ANP, whereas protein kinase G activation ( vasodilator-stimulated protein phosphorylation; ELISA assay) displayed the opposite: increased with SIL + ISO but unaltered by ANP + ISO. PDE-5 and ANP compartments were functionally separated, as inhibition of nitric oxide synthase by N-w-nitro-L-arginine methyl ester eliminated antiadrenergic effects of SIL, yet this was not restorable by co-stimulation with ANP. Conclusions - Regulation of cardiac beta-adrenergic response by cGMP is specifically linked to a nitric oxide-synthesis/PDE-5-hydrolyzed pool signaling via protein kinase G. Natriuretic peptide stimulation achieves greater detectable increases in cGMP but not protein kinase G activity and does not modulate beta-adrenergic response. Such disparities likely contribute to differential cardiac regulation by drugs that modulate cGMP synthesis and hydrolysis.