Permeation and inactivation by calcium and manganese of bovine adrenal chromaffin cell calcium channels.
Stimulation of fura-2-loaded bovine chromaffin cells with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP; 10 microM) or depolarization with high [K+] (50 mM) accelerated the entry of both Ca2+ and Mn2+, used here as a Ca2+ surrogate for Ca2+ channels. Removal of extracellular Na+ prevented the effects of DMPP but did not modify the effects of K+, indicating that Na+ is necessary for coupling of Ca2+ entry to the nicotinic receptor activation and that the ionophore associated with it is functionally impermeable to divalent cations. DMPP- as well as K(+)-evoked Ca2+ and Mn2+ influx were blocked completely by Ni2+ but only partially by dihydropyridines, suggesting that, in addition to L-type Ca2+ channels, other Ca2+ entry pathways may be present. Inactivation of Ca2+ channels, followed by comparing the rates of Mn2+ uptake at different time periods after the addition of DMPP or high K+, did not happen in the absence of extracellular Ca2+. When 1 mM Ca2+ was present, a delayed inhibition (half time, 10-20 s) was observed, suggesting that it is not due to the entry of Ca2+ itself but to the increase of the cytoplasmic Ca2+ concentration ([Ca2+]i) that takes a few seconds to develop. The influx of Ca2+, estimated from the increase of [Ca2+]i, was also impaired in a time-dependent fashion by previous entry of Mn2+. Inactivation of Ca2+ entry was achieved at estimated mean intracellular Mn2+ concentrations as low as 10(-9) M.
Fonteriz
R I
RI
Departamento de Bioquímica y Biología Molecular y Fisiología, Facultad de Medicina, Universidad de Valladolid, Spain.
Garcia-Sancho
J
J
Gandia
L
L
Lopez
M G
MG
Garcia
A G
AG
D000311:Adrenal Glands / Q000166:cytology / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism*; D002417:Cattle; D002478:Cells, Cultured; D002838:Chromaffin System / Q000166:cytology / Q000378:metabolism*; D004095:Dihydropyridines / Q000494:pharmacology; D004246:Dimethylphenylpiperazinium Iodide / Q000494:pharmacology; D016257:Fura-2; D008345:Manganese / Q000378:metabolism* / Q000494:pharmacology; D009532:Nickel / Q000494:pharmacology; D010539:Permeability; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology
D015220:Calcium Channels; D004095:Dihydropyridines; D008345:Manganese; D004246:Dimethylphenylpiperazinium Iodide; D009532:Nickel; D012964:Sodium; D011188:Potassium; D002118:Calcium; D016257:Fura-2