Abstract 353: Genetic GRK2 Abolishment Enhances Excitation-Contraction Coupling and Inhibits Adverse Ca2+-Handling Remodeling after Myocardial Infarction
Background: G-protein coupled receptor kinase 2 (GRK2) is the main regulator of beta-adrenergic signaling in the heart. During heart failure its level and activity are increased. Previously, GRK2 has been linked to the disease progression and GRK2 inhibition was identified as a novel molecular therapeutic strategy. Here we present for the first time the role of GRK2 in excitation-contraction coupling (ECC) and Ca2+ handling in normal and infarcted hearts using cardiac-specific, conditional GRK2 knockout (KO) mice.
Methods and results: Cardiac function (echocardiography), single myocyte contractility, Ca2+ transients, L-type Ca2+ channel currents, sarcoplasmic reticulum (SR) Ca2+ load, and NCX-activity were analyzed in normal control mice and 28 days after left ventricular myocardial infarction. Conditional GRK2KO (αMHC-Cre × GRK2 fl/fl) and wildtype mice (GRK2 fl/fl) were studied. In cardiac myocytes isolated from normal hearts single myocyte contractility and Ca2+ transients were similar at baseline. L-type Ca2+ channel currents were higher in GRK2KO mice with a lower SR Ca-load, and a higher fractional SR release. After stimulation with isoproterenol GRK2KO mice revealed significant increases in contractility and Ca2+ transients, which were not mediated through the L-type Ca2+ channel, but through the SR. Interestingly, post MI GRK2KO mice show improved survival and enhanced cardiac function, which are at least in part explained by increased single myocyte contractility and Ca2+ transients at baseline. Loss of GRK2 was associated with a significantly enhanced L-type Ca2+ channel current, which contributed to these benefits. Stimulation with isoproterenol significantly enhanced contractility and Ca2+ transients in the GRK2KO mice, pointing towards a preservation of beta-adrenergic responsiveness. However, this gain in ECC was not mediated through the L-type Ca2+ channel, arguing in favor of a differential regulation of ECC and Ca2+ handling through GRK2.
Conclusion: Loss of GRK2 in normal cardiac myocytes enhances the ECC gain. In post-MI heart failure genetic GRK2 abolishment improves survival, cardiac function, and normalizes Ca2+ handling and contractility through an apparent positive modulation of the L-type Ca2+ channel.