The Journal of General Physiology, Vol 100, 253-268, Copyright © 1992 by The Rockefeller University Press
Calcium-dependent inactivation of the calcium current activated upon hyperpolarization of Paramecium tetraurelia
RR Preston, Y Saimi and C Kung
Laboratory of Molecular Biology, University of Wisconsin-Madison 53706.
The Ca2+ current activated upon hyperpolarization of Paramecium tetraurelia
decays over a period of 150-200 ms during sustained steps under voltage
clamp. At membrane potentials between -70 and approximately -100 mV, the
time course of this inactivation is described by a single exponential
function. Steps negative to approximately -100 mV elicit currents that
decay biexponentially, however. Three lines of evidence suggest that this
current's inactivation is a function of intracellular Ca2+ concentration
rather than membrane potential: (a) Comparing currents with similar
amplitudes but elicited at widely differing membrane potentials suggests
that their time course of decay is a sole function of inward current
magnitude. (b) The extent of current inactivation is correlated with the
amount of Ca2+ entering the cell during hyperpolarization. (c) The onset
and time course of recovery from inactivation can be hastened significantly
by injecting cells with EGTA. We suggest that the decay of this current
during hyperpolarization involves a Ca(2+)-dependent pathway.
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