Regulation of the L-type calcium channel alpha-1 subunit by chronic depolarization in the neuron-like PC12 and aortic smooth muscle A7r5 cell lines.

The regulation of L-type voltage-dependent Ca2+ channels by chronic depolarization was studied in the aortic smooth muscle A7r5 and neuron-type PC12 cell lines, by probing the expression and the functional state of their constitutive alpha-1 subunits. PC12 cells showed, after prolonged exposure to a high-K+ depolarizing solution, a 25% reduction of the functional Ca2+ channel density which was accompanied by a decrease of the alpha-1 subunit mRNA expression. In A7r5 cells submitted to a similar protocol of depolarization, 45Ca2+ uptake measurements revealed a fall in the functional activity of L-type Ca2+ channels which was not related to a modulation of their mRNA expression, but arose from a long-term voltage-dependent channel inactivation. Accordingly, the lag time and the mechanisms of recovery were different in the two cell types. In PC12 cells, when restoring physiological culture conditions, de novo synthesis of alpha-1 subunits allowed the recovery of the original density of L-type Ca2+ channels at the membrane surface. As for the A7r5 cells, we showed that after chronic depolarization, the complete restoration of the resting membrane potential and the related Ca2+ channel activity required a 2-day incubation in physiological medium and could probably be related to a normalization of the increased intracellular Ca2+ concentration. In contrast, it is noteworthy that, in PC12 cells, the only transient increase of intracellular Ca2+ content in the first hours of depolarization could account for the long-term down-regulation of L-type Ca2+ channels.