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Ca(2+)-dependent activation of Cl(-) currents in Xenopus oocytes is modulated by voltage
Authors:Callamaras N  Parker I
Affiliation:Laboratory of Cellular and Molecular Neurobiology, Department of Neurobiology and Behavior, University of California Irvine, Irvine, California 92697-4550, USA.
Abstract:Ca2+-activatedCl- currents (ICl,Ca) wereexamined using fluorescence confocal microscopy to monitorintracellular Ca2+ liberation evoked by flash photolysis ofcaged inositol 1,4,5-trisphosphate (InsP3) involtage-clamped Xenopus oocytes. Currents at +40 mV exhibited asteep dependence on InsP3 concentration([InsP3]), whereas currents at-140 mV exhibited a higher threshold and more graded relationshipwith [InsP3]. Ca2+ levelsrequired to half-maximally activate ICl,Ca wereabout 50% larger at -140 mV than at +40 mV, and currents evokedby small Ca2+ elevations were reduced >25-fold. Thehalf-decay time of Ca2+ signals shortened at increasinglypositive potentials, whereas the decay of ICl,Calengthened. The steady-state current-voltage (I-V) relationshipfor ICl,Ca exhibited outward rectification withweak photolysis flashes but became more linear with stronger stimuli.Instantaneous I-V relationships were linear with both strongand weak stimuli. Current relaxations following voltage steps duringactivation of ICl,Ca decayed with half-times that shortened from about 100 ms at +10 mV to 20 ms at -160 mV. We conclude that InsP3-mediated Ca2+liberation activates a single population of Cl-channels, which exhibit voltage-dependent Ca2+ activationand voltage-independent instantaneous conductance.

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