The cardiac L-type calcium channel distal carboxy terminus autoinhibition is regulated by calcium

The L-type calcium channel (LTCC) provides trigger Ca2+ for sarcoplasmic reticulum Ca-release, and LTCC function is influenced by interacting proteins including the LTCC distal COOH terminus (DCT) and calmodulin. DCT is proteolytically cleaved and reassociates with the LTCC complex to regulate calcium channel function. DCT reduces LTCC barium current (IBa,L) in reconstituted channel complexes, yet the contribution of DCT to LTCC Ca2+ current (ICa,L) in cardiomyocyte systems is unexplored. This study tests the hypothesis that DCT attenuates cardiomyocyte ICa,L. We measured LTCC current and Ca2+ transients with DCT coexpressed in murine cardiomyocytes. We also heterologously coexpressed DCT and CaV1.2 constructs with truncations corresponding to the predicted proteolytic cleavage site, CaV1.2Δ1801, and a shorter deletion corresponding to well-studied construct, CaV1.2Δ1733. DCT inhibited IBa,L in cardiomyocytes, and in human embryonic kidney (HEK) 293 cells expressing CaV1.2Δ1801 and CaV1.2Δ1733. Ca2+-CaM relieved DCT block in cardiomyocytes and HEK cells. The selective block of IBa,L combined with Ca2+-CaM effects suggested that DCT-mediated blockade may be relieved under conditions of elevated Ca2+. We therefore tested the hypothesis that DCT block is dynamic, increasing under relatively low Ca2+, and show that DCT reduced diastolic Ca2+ at low stimulation frequencies but spared high frequency Ca2+ entry. DCT reduction of diastolic Ca2+ and relief of block at high pacing frequencies and under conditions of supraphysiological bath Ca2+ suggests that a physiological function of DCT is to increase the dynamic range of Ca2+ transients in response to elevated pacing frequencies. Our data motivate the new hypothesis that DCT is a native reverse use-dependent inhibitor of LTCC current.