AJP - Cell Physiology, Vol 263, Issue 4 C818-C824, Copyright © 1992 by American Physiological Society
Permeation and inactivation by calcium and manganese of bovine adrenal chromaffin cell calcium channels
R. I. Fonteriz, J. Garcia-Sancho, L. Gandia, M. G. Lopez and A. G. Garcia
Departamento de Bioquimica y Biologia Molecular y Fisiologia, Facultad de Medicina, Universidad de Valladolid, Spain.
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.
