Abstract P1076: Phosphodiesterase 1 (pde1) Modulates CAMP Signaling At The Sarcolemmal Membrane

Abstract

Phosphodiesterases (PDEs) regulate cyclic nucleotide-mediated cell signaling and offer a therapeutic means to leverage cardiac function. FDA-approved PDE3 inhibitors increase myocyte contractility by enhancing L-type calcium channel activity and sarcoplasmic reticulum (SR) calcium (Ca 2+ ) load, but cause fatal arrhythmias in patients with heart failure. The PDE1 inhibitor ITI-214 is a novel inotrope that increases cardiac contractility in animal models and human patients with heart failure. Mechanistically, PDE1 inhibition leads to activation of the L-type Ca 2+ channel without altering the SR Ca 2+ load or myofilament-Ca 2+ sensitivity. The net result is enhanced myocyte contractility with comparatively less Ca 2+ increase and arrhythmias. PDE inhibition increases cAMP levels to activate protein kinase A (PKA), a pathway that regulates Ca 2+ handling and cycling. These results led us to hypothesize that PDEs 1 and 3 differ in localized domains where each controls cAMP-mediated signaling. Live-cell imaging experiments were performed in acutely isolated guinea pig myocytes expressing Förster resonance energy transfer (FRET) sensors to monitor changes in cAMP levels. These sensors were targeted to specific sarcolemmal domains or more generally to the cytosol. Our results showed that PDE1 inhibition is the predominant regulator of cAMP at the non T-tubular sarcolemmal membrane compared to other cAMP-hydrolyzing PDEs (PDE3 and 4). In contrast to PDE1, PDEs 3 and 4 were more indiscriminate and modulated cAMP in the cytosolic as well as in membrane domains. Upon beta-adrenergic stimulation, however, PDE4 but not PDEs 1 or 3 became the predominant cAMP modulator at the T-tubular membrane domain. These results provide evidence that PDE1 is a major regulator of sarcolemmal membrane cAMP, suggesting that PDE1 inhibitory effects upon the L-type Ca 2+ channel is localized to this domain. In future studies, we will examine if PDE1 inhibition selectively activates PKA in this region, and seek to identify the interacting proteins that support PDE1 modulation of sarcolemmal cAMP/PKA signaling. Resolving this cell signaling difference would provide mechanistic insight into a novel inotrope that has already completed a Phase II clinical evaluation in patients.