Rem-GTPase regulates cardiac myocyte L-type calcium current

Rationale: The L-type calcium channels (LTCC) are critical for maintaining Ca2+-homeostasis. In heterologous expression studies, the RGK-class of Ras-related G-proteins regulates LTCC function; however, the physiological relevance of RGK-LTCC interactions is untested. Objective: In this report we test the hypothesis that the RGK protein, Rem, modulates native Ca2+ current (I-Ca,I-L) via LTCC in murine cardiomyocytes. Methods and Results: Rem knockout mice (Rem(-/-)) were engineered, and I-Ca,I-L and Ca2+-handling properties were assessed. Rem(-/-) ventricular cardiomyocytes displayed increased I-Ca,I-L density. I-Ca,I-L activation was shifted positive on the voltage axis, and beta-adrenergic stimulation normalized this shift compared with wild-type I-Ca,I-L. Current kinetics, steady-state inactivation and facilitation was unaffected by Rem(-/-). Cell shortening was not significantly different. Increased I-Ca,I-L density in the absence of frank phenotypic differences motivated us to explore putative compensatory mechanisms. Despite the larger I-Ca,I-L density, Rem(-/-) cardiomyocyte Ca2+ twitch transient amplitude was significantly less than that compared with wild type. Computer simulations and immunoblot analysis suggests that relative dephosphorylation of Rem(-/-) LTCC can account for the paradoxical decrease of Ca2+ transients. Conclusions: This is the first demonstration that loss of an RGK protein influences I-Ca,I-L in vivo in cardiac myocytes.