Run-down of L-type Ca2+ channels occurs on the α1 subunit

Run-down of L-type Ca2+ channels in CHO cells stably expressing alpha(1c), alpha(1c)beta(1a), or alpha(1c)beta(1a)alpha(2)delta gamma subunits was studied using the patch-clamp technique (single channel recording). The channel activity (NPo) of alpha(1c), channels was increased 4- and 8-fold by coexpression with beta(1a) and beta(1a)alpha(2)delta gamma, 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 rundown 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 Ca2+ channels in the guinea-pig heart. The restoration level was 64.7, 63.5, and 66.4% for channels composed of alpha(1c), alpha(1c)beta(1a), and alpha(1c)beta(1a)alpha(2)delta gamma, respectively, and was thus also independent of the subunit composition. We conclude that run-down of L-type Ca2+ channels occurs via the al subunit and that the cytoplasmic factors maintaining Ca2+ channel activity act on the alpha(1) subunit. (C) 1998 Academic Press.