Ca2+-dependent activation of Cl−currents in Xenopus oocytes is modulated by voltage

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Abstract
Ca2+-activated Cl− currents ( I Cl,Ca) were examined using fluorescence confocal microscopy to monitor intracellular Ca2+ liberation evoked by flash photolysis of caged inositol 1,4,5-trisphosphate (Ins P 3) in voltage-clamped Xenopus oocytes. Currents at +40 mV exhibited a steep dependence on Ins P 3 concentration ([Ins P 3]), whereas currents at −140 mV exhibited a higher threshold and more graded relationship with [Ins P 3]. Ca2+ levels required to half-maximally activate I Cl,Ca were about 50% larger at −140 mV than at +40 mV, and currents evoked by small Ca2+ elevations were reduced >25-fold. The half-decay time of Ca2+ signals shortened at increasingly positive potentials, whereas the decay of I Cl,Calengthened. The steady-state current-voltage ( I-V) relationship for I Cl,Ca exhibited outward rectification with weak photolysis flashes but became more linear with stronger stimuli. Instantaneous I-V relationships were linear with both strong and weak stimuli. Current relaxations following voltage steps during activation of I Cl,Ca decayed with half-times that shortened from about 100 ms at +10 mV to 20 ms at −160 mV. We conclude that Ins P 3-mediated Ca2+liberation activates a single population of Cl−channels, which exhibit voltage-dependent Ca2+ activation and voltage-independent instantaneous conductance.

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