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 (I _Ba,l) in reconstituted channel complexes, yet the contribution of DCT to LTCC Ca²⁺ current (Ic_a,l) in cardiomy-ocyte systems is unexplored. This study tests the hypothesis that DCT attenuates cardiomyocyte I _Ca,L. We measured LTCC current and Ca²⁺ transients with DCT coexpressed in murine cardiomyocytes. We also heterologously coexpressed DCT and Ca_V1.2 constructs with truncations corresponding to the predicted proteolytic cleavage site, Ca_V1.2A1801, and a shorter deletion corresponding to well-studied construct, Ca_V1.2A1733. DCT inhibited I _Ba,l in cardiomyocytes, and in human embryonic kidney (HEK) 293 cells expressing Ca_V1.2A1801 and Ca_V1.2A1733. Ca²⁺-CaM relieved DCT block in cardiomyocytes and HEK cells. The selective block of I _Ba,l combined with Ca²⁺-CaM effects suggested that DCT-mediated blockade may be relieved under conditions of elevated Ca^{2 +}. 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.