Inhibited by the Ca2+-Calmodulin Complex in Cardiac Sarcoplasmic Reticulum

Cardiac sarcoplasmic reticulum (SR) has several chloride (Cl-) channels, which may neutralize the charge across the SR membrane generated by Ca 2+ movement. We recently reported a novel 116picosiemen Clchannel that is activated by protein kinase A-dependent phosphorylation in cardiac SR. This Clchannel may serve as a target protein in the receptor-dependent regulation of cardiac excitation-contraction coupling. To understand further regulatory mechanisms, the effects of Ca2+ on the Clchannel were studied using the planar lipid bilayer-vesicle fusion technique. In the presence of calmodulin (CaM, 0.1 ,imol/L per microgram SR vesicles), Ca 21 (3 ,tmol/L to 1 mmol/L) added to the cis solution reduced the channel openings in a concentration-dependent fashion, whereas Ca2+ (1 nmol/L to 1 mmol/L) alone or CaM (0.1 to 1 gmol/L per microgram SR vesicles) with 1 nmol/L Ca2+ did not affect the channel activity. This inhibitory effect of Ca2+ in the presence of CaM was prevented by CaM inhibitors N-(6 aminohexyl)-5-chloro-1-naphthalenesulfonamide and calmidazolium but not by CaM kinase II inhibitor KN62. These results suggest that the Ca2'-CaM complex itself, but not CaM kinase II, is involved in this channel inhibition. Thus, the cardiac SR 116-picosiemen Clchannel is regulated not only by protein kinase A-dependent phosphorylation but also by the cytosolic Ca2+-CaM complex. This is a novel second messenger-mediated regulation of Clchannels in cardiac SR membrane. (Circ Res. 1993;73:751-757.)