Run-Down of L-Type Ca2+Channels Occurs on the α1Subunit☆

Abstract Run-down of L-type Ca 2+ channels in CHO cells stably expressing α 1c , α 1c β 1a , or α 1c β 1a α 2 δγ subunits was studied using the patch-clamp technique (single channel recording). The channel activity ( NPo ) of α 1c channels was increased 4- and 8-fold by coexpression with β 1a and β 1a α 2 δγ, respectively. When membranes containing channels composed of different subunits were excised into basic internal solution, the channel activity exhibited run-down, the time-course of which was independent of the subunit composition. The run-down was restored by the application of calpastatin (or calpastatin contained in cytoplasmic P-fraction) + H-fraction (a high molecular mass fraction of bovine cardiac cytoplasm) + 3 mM ATP, which has been shown to reverse the run-down in native Ca 2+ channels in the guinea-pig heart. The restoration level was 64.7, 63.5, and 66.4% for channels composed of α 1c , α 1c β 1a , and α 1c β 1a α 2 δγ, respectively, and was thus also independent of the subunit composition. We conclude that run-down of L-type Ca 2+ channels occurs via the α 1 subunit and that the cytoplasmic factors maintaining Ca 2+ channel activity act on the α 1 subunit.