Calcium-dependent inactivation of the dihydropyridine-sensitive calcium channels in GH3 cells.
The inactivation of calcium channels in mammalian pituitary tumor cells (GH3) 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 -40 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 approximately 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 less than 10(-8) M; (b) replacing extracellular calcium with barium; (c) increasing the prepulse voltage from +10 to +60 mV; or (d) increasing the intracellular concentration of cAMP, either 'directly' with dibutyryl-cAMP or indirectly by activating adenylate cyclase with forskolin or vasoactive intestinal peptide. Thus, inactivation of the dihydropyridine-sensitive calcium channels in GH3 cells only occurs when membrane depolarization leads to calcium ion entry and intracellular accumulation.
Kalman
D
D
Department of Biology, University of California, Los Angeles 90024.
O'Lague
P H
PH
Erxleben
C
C
Armstrong
D L
DL
D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects*; D002460:Cell Line; D004594:Electrophysiology; D010911:Pituitary Neoplasms / Q000378:metabolism; D051381:Rats
D015220:Calcium Channels; D003994:Bucladesine; D002118:Calcium