Abstract Body: Modulators of cardiac cyclic adenosine 3’5’-monophosphate (cAMP) and/or its effector protein kinase A (PKA) are used to treat heart disease. Elucidating the mechanisms of endogenous and pharmacological modulators of cAMP, including phosphodiesterase (PDE) inhibitors, is key to predicating alternative drug therapies for heart failure (HF). PDEs regulate confined cAMP/PKA signaling pathways, depending on PDE isoform and localization. PDE3 regulates the SR and sarcolemmal membrane cAMP pools, controlling calcium influx and efflux at these domains. PDE1 regulates sarcolemmal cAMP/PKA signaling. Despite these differences, inhibitors of either increase heart contractility. However, inhibition of PDE3 is pro-arrhythmic, unlike that of PDE1. The regulation of localized cAMP signaling is key to controlled regulation of cell function, with ramifications for HF, and which GPCR lies upstream of PDE1 remains unknown. We hypothesized that the three cAMP-hydrolyzing cardiac PDEs (1, 3, 4) that are found in the human heart would regulate distinct cAMP signaling pathways.
Using the cAMP biosensor EPAC-SH187, we performed fluorescence resonance energy transfer experiments in healthy guinea pig adult ventricular myocytes. Cells were treated with inhibitors of PDEs 1, 3, and 4 along with GPCR agonists isoproterenol, glucagon (gcg), and amthamine (amt), respectively for β-adrenergic, gcg and histamine H2 receptors at sub-maximal doses. Cell function and calcium data were obtained using IonOptix.
We first asked if PDEi would synergistically regulate pools of cAMP, and found that PDE4i does at pools of cAMP generated by beta-agonism. PDE3i regulates that generated by amt at the H2R, with comparable effects observed in simultaneous or prior H2R stimulation. The combination of these sub-saturating doses exerted strong inotropic and lusitropic effects in paced myocytes. In neither of these domains, PDE1i changed cAMP levels. However, PDE1i intriguingly attenuated the level of gcg-stimulated cAMP production, suggesting a feedback loop mechanism.
We conclude that PDEs 1, 3, and 4 regulate distinct pools of cAMP. Importantly, we demonstrate this in a species where the myocardial expression of these PDEs match that of humans as opposed to smaller rodent models.