N. Callamaras et I. Parker, Ca2+-dependent activation of Cl- currents in Xenopus oocytes is modulated by voltage, AM J P-CELL, 278(4), 2000, pp. C667-C675
Ca2+-activated Cl- currents (I-Cl,I-Ca) were examined using fluorescence co
nfocal microscopy to monitor intracellular Ca2+ liberation evoked by flash
photolysis of caged inositol 1,4,5-trisphosphate (InsP(3)) in voltage-clamp
ed Xenopus oocytes. Currents at +40 mV exhibited a steep dependence on InsP
(3) concentration ([InsP(3)]), whereas currents at -140 mV exhibited a high
er threshold and more graded relationship with [InsP(3)]. Ca2+ levels requi
red to half-maximally activate I-cl,I-Ca were about 50% larger at -140 mV t
han at +40 mV, and currents evoked by small Ca2+ elevations were reduced >2
5-fold. The half-decay time of Ca2+ signals shortened at increasingly posit
ive 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 rectif
ication with weak photolysis flashes but became more linear with stronger s
timuli. Instantaneous I-V relationships were linear with both strong and we
ak stimuli. Current relaxations following voltage steps during activation o
f I-Cl,I-Ca decayed with half-times that shortened from about 100 ms at + 1
0 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-depen
dent Ca2+ activation and voltage-independent instantaneous conductance.