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

Ca2+-activated Cl- currents (I-Cl,I-Ca) were examined using fluorescence confocal microscopy to monitor intracellular Ca2+ liberation evoked by flash photolysis of caged inositol 1,4,5-trisphosphate (InsP(3)) in voltage-clamped Xenopus oocytes. Currents at +40 mV exhibited a steep dependence on InsP(3) concentration ([InsP(3)]), whereas currents at -140 mV exhibited a higher threshold and more graded relationship with [InsP(3)]. Ca2+ levels required to half-maximally activate I-cl,I-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,I-Ca lengthened. The steady-state current-voltage (I-V) relationship for I-Cl,I-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,I-Ca decayed with half-times that shortened from about 100 ms at + 10 mV to 20 ms at -160 mV. We conclude that InsP(3)-mediated Ca2+ liberation activates a single population of Cl- channels, which exhibit voltage-dependent Ca2+ activation and voltage-independent instantaneous conductance.