Thursday, November 21, 2024
11/21/2024
Project Summary The body has innate mechanisms to increase cardiac function under varying physiologic conditions. Inotropes, a class of heart failure drugs, utilize these mechanisms to therapeutically improve cardiac function. Some inotropes, however, are safe while others increase the risk of side effects like arrhythmias. This is an intriguing question when considering two drugs that improve heart function by inhibiting phosphodiesterases (PDEs). PDE inhibition mechanistically increases cyclic nucleotide cAMP levels and thus activate protein kinase A (PKA) cell signaling pathways in the heart. Pharmacologic inhibition of PDEs 1 or 3 augments cardiac function. Chronic PDE3 inhibition (PDE3i), however, increases fatal arrhythmias in patients. While the arrhythmogenic effects of chronic PDE1i remains unknown, acute inhibition is safe. We do not yet fully understand how PDE 1 or 3 inhibition mechanisms differ to explain these differences. The goal of this study is to uncover fundamental cardiac contractility mechanisms that distinguish the actions of the two inotropes. We test the central hypothesis that appropriate compartmentalized control of cAMP/PKA signaling will accomplish specified improvement in cardiac contractility. This is supported by our prior findings that PDE3i activated PKA signaling to increase L-type calcium channel (LTCC) and the sarcoplasmic reticulum (SR) calcium load, but that PDE1i selectively augmented LTCC activity. And while β-blockers abrogated the inotropic effects of PDE3i, the effects for PDE1i persisted. In preliminary data, we show that sarcolemmal cAMP is predominantly hydrolyzed by PDE1 in living myocytes. Furthermore, PDE4, but not PDE1, inhibition synergistically increase cAMP that is produced upon β-adrenergic receptor stimulation. The investigation of compartmentalized myocardial contractility mechanisms in health and disease will be divided into three aims. In Aim 1, we will delineate cyclic nucleotides-activated pathways downstream of PDE1 inhibition that increase LTCC function. This will be done using biosensors of cAMP, cGMP, and PKA in isolated myocytes using Forster resonance energy transfer (FRET) imaging. LTCC function will be tested using patch clamping. Healthy, heart failure guinea pigs, and genetically modified mice will be used. Aim 2 will investigate the role of cell excitation and calcium cycling as mechanisms for PDE1i inotropy. We will test ion fluxes across the sarcolemmal membrane and SR combining patch clamping and confocal imaging using calcium indicators in guinea pig myocytes. Finally, in Aim 3 we will evaluate the arrhythomogenic effects of long-term PDE1i and expose any potential mechanisms in comparison to chronic PDE3i. Healthy or heart failure guinea pigs treated with PDE1 or PDE3 inhibitor will undergo ex vivo optical mapping mechanocoupling study or serial, conscious electrocardiogram study using telemetry device. PDE1i has completed clinical evaluation in heart failure patients. Thus, understanding how these PKA signaling pathways differ will critically guide future trial designs. Moreover, using rational design from the knowledge this study produces, we can refine the next generation of inotropes and other heart failure drugs.
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