Rem Selectively Abolishes β1-adrenergic Regulation Of CaV1.2 Channels In Heart

Abstract

β-adrenergic modulation of cardiac CaV1.2 channels is critical for sympathetic regulation of the heartbeat, and its disruption is a harmful hallmark of heart failure. RGK (Rem, Rem2, Rad, Gem/Kir) GTPases potently inhibit CaV channels by interacting with their auxiliary β subunits. Intriguingly, RGK proteins are present in heart, and their levels are elevated in heart failure. We examined the impact of the RGK GTPase, Rem, on CaV1.2 channels in heart cells and assessed whether there was crosstalk with the β-adrenergic modulation of the channel. Cultured adult guinea pig ventricular myocytes expressed robust CaV1.2 currents (ICa,L) (15.08 pA/pF) and responded to β1-adrenergic activation (1 μM isoproterenol + 1 μM ICI118,551) with a sharp, three-fold increase in current density. Isochronal cardiac cells expressing YFP-Rem, achieved through adenovirus infection, displayed a markedly lower basal current density (5.85 pA/pF). Nevertheless, the effect of Rem in heart is quantitatively smaller than seen in recombinant channels expressed in HEK 293 cells, which feature a virtual ablation of ICa,L. Surprisingly, the remaining Rem-insensitive ICa,L in guinea pig heart cells was essentially unresponsive to β1-adrenergic stimulation. This was not due to disruption of the signaling pathway because isoproterenol-mediated increase in cardiac IKs remained unchanged. Intriguingly, the Rem insensitive ICa,L remained responsive to forskolin. These results reveal an unexpected crosstalk between RGK GTPases and β-adrenergic signaling pathway at the level of cardiac ICa,L, and suggests that Rem selectively inhibits spatially distinct CaV1.2 channels in single heart cells.