Calcium-dependent Inactivation of the ... - BioMedSearch

Calcium-dependent Inactivation of the Dihydropyridine-sensitive Calcium Channels in GHa Cells DANIEL KALMAN, PAUL H . O'LAGUE, CHRISTIAN ERXLEBEN, a n d DAVID L. ARMSTRONG From the Department of Biology, University of California at Los Angeles, Los Angeles, California 90024 ABSTRACT The inactivation of calcium channels in mammalian pituitary tumor cells (GHa) was studied with patch electrodes under voltage clamp in cell-free membrane patches and in dialyzed cells. The calcium current elicited by depolarization from a holding potential of - 4 0 mV passed predominantly through one class of channels previously shown to be modulated by dihydropyridines and cAMP-dependent phosphorylation (Armstrong and Eckert, 1987). When exogenous calcium buffers were omitted from the pipette solution, the macroscopic calcium current through those channels inactivated with a half time of ~ 10 ms to a steady state level 40-75% smaller than the peak. Inactivation was also measured as the reduction in peak current during a test pulse that closely followed a prepulse. Inactivation was largely reduced or eliminated by (a) buffering free calcium in the pipette solution to