The effects of chronic depolarization on L-type 1,4-dihydropyridine-sensitive, voltage-dependent Ca2+ channels in chick neural retina and rat cardiac cells

Abstract

Chick neural retina cells contain functional L-type voltage-dependent Ca2+ channels sensitive to 1,4-dihydropyridines. To investigate the effects of chronic depolarization, cells were grown in medium containing elevated K+. After 4-h to 4-day treatments with elevated K+ (12–73 mM), there was a concentration-dependent decrease in high affinity [3H]PN200-110 binding. Saturation analysis of cells treated for 4 days with 40 mM K+ showed a reduction in maximum ligand binding with no change in affinity. Control and experimental Bmax values were 70.7 ± 6.4 and 42.2 ± 4.5 fmol/mg protein, respectively, and control and experimental KD values were 70.2 ± 7.4 and 68.6 ± 7.4 × 10−12 M. The effect of chronic depolarization was time-dependent, reversible, and without effect on cellular protein content. Reduction in 45Ca2+ uptake following chronic depolarization correlated well with the reduction in [3H]PN200-110 binding. The calcium ionophore A23187, 10−6 M for 24 h, also decreased the binding site density. The calcium channel antagonist D600 had no effect alone on [3H]PN200-110 binding; however, D600 blocked the down-regulation of calcium channels induced by chronic depolarization. The mechanism for Ca2+ channel down-regulation may involve calcium entry, since the effect was blocked by D600 and mimicked by the calcium ionophore A23187. Chronic depolarization with either elevated K+ or veratridine, or chronic treatment with A23187 had no effect on calcium channels in rat neonatal ventricular myocytes, although these cells express functional channels of the 1,4-dihydropyridine-sensitive class. That apparently similar channels in cardiac cells were not affected by chronic depolarization likely indicates structural or regulatory differences between cardiac and neuronal L-type Ca2+ channels.Key words: calcium channels, calcium antagonists, 1,4-dihydropyridines, channel regulation, retinal neurons.