Compartmentalization of Cardiac (cid:1) -Adrenergic Inotropy Modulation by Phosphodiesterase Type 5 Molecular Cardiology

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 (cid:1) -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 (cid:2) mol/L sildenafil (SIL) suppressed isoproterenol (ISO)-stimulated contractility, whereas 10 (cid:2) 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 (cid:1) 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 (cid:1) ISO but unaltered by ANP (cid:1) 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 (cid:1) -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 (cid:1) -adrenergic response. Such disparities likely contribute to differential cardiac regulation by drugs that modulate cGMP synthesis and hydrolysis. ( Circulation . 2007;115:2159-2167.) these interactions is important because drugs that target synthesis and catabolism are of increasing interest in the treatment of cardiovascular disease. In the present study, we determine the physiological significance of such compartments in adult mouse myocytes and intact hearts. PDE-5 inhibition with sildenafil suppressed isoproterenol-stimulated contractility, whereas atrial NP had no effect. Yet in the presence of isoproterenol, myocardial cGMP levels rose little with concomitant PDE-5 inhibition but increased markedly with atrial NP. In contrast, protein kinase G activation showed the opposite pattern, yet it was linked to the antiadrenergic effect. Preexposure to the nitric oxide synthase inhibitor N w -nitro-L-arginine methyl ester blocked the ability of sildenafil to suppress isoproterenol stimulation, and this was not restored by co-stimulation with atrial NP. These data reveal striking functional compartmentation of cGMP/protein kinase G regulation of (cid:1) -stimulation linked to nitric oxide–stimulated/PDE-5–hydrolyzed but not NP-stimulated cGMP pools. A routine measure of myocardial cGMP more likely reflects NP-stimulated pools, but this does not necessarily translate to measurable protein kinase G activation. These disparities may underlie differential regulation of heart function and structure by NP versus nitric oxide–dependent cGMP simulation or by PDE-5 inhibitors.