CALCIUM-DEPENDENT INACTIVATION OF L-TYPE CALCIUM CHANNELS IN PLANAR LIPID BILAYERS

Intracellular Ca2+ can inhibit the activity of voltage-gated Ca channe ls by modulating the rate of channel inactivation. Ca2+-dependent inac tivation of these channels may be a common negative feedback process i mportant for regulating Ca2+ entry under physiological and pathologica l conditions. This article demonstrates that the inactivation of cardi ac L-type Ca channels, reconstituted into planar lipid bilayers and st udied in the presence of a dihydropyridine agonist, is sensitive to Ca 2+. The rates and extents of inactivation, determined from ensemble av erages of unitary Ba2+ currents, decreased when the calcium concentrat ion facing the intracellular surface of the channel ([Ca2+](i)) was lo wered from similar to 10 mu M to 20 nM by the addition of Ca2+ chelato rs. The rates and extents of Ba2+ current inactivation could also be i ncreased by subsequent addition of Ca2+ raising the [Ca2+](i) to 15 mu M, thus demonstrating that the Ca2+ dependence of inactivation could be reversibly regulated by changes in [Ca2+](i). In addition, reconsti tuted Ca channels inactivated more quickly when the inward current was carried by Ca2+ than when it was carried by Ba2+, suggesting that loc al increases in [Ca2+](i) could actuate Ca2+-dependent inactivation. T hese data support models in which Ca2+ binds to the channel itself or to closely associated regulatory proteins to control the rate of chann el inactivation, and are inconsistent with purely enzymatic models for channel inactivation.