CALCIUM-DEPENDENCE AND DISTRIBUTION OF CALCIUM-ACTIVATED CHLORIDE CHANNELS IN XENOPUS OOCYTES

1. The Ca2+-dependent Cl(-)current (I-Cl,I-Ca), expressed in the plasm a membrane of Xenopus oocytes, was examined in excised inside-out macr opatches using a rapid perfusion system. 2. Application of Ca2+-contai ning Ringer solution resulted in the activation of a current whose rev ersal potential shifted to the right by 51 +/- 5.2 mV when Cl- in the pipette solution was lowered from 119.3 to 10 mM. No currents were gen erated when Ca2+ was omitted from the solution. The current is therefo re a Ca2+-activated Cl- one. 3. Following exposure to Ca2+, the half-t ime for activation of I-Cl,I-Ca was not voltage dependent, whereas dea ctivation was strongly so. 4. I-Cl,I-Ca was stable in the continuous p resence of Ca2+ and showed no sign of inactivation or adaptation. 5. C omparison of the size of the currents (normalized to pipette resistanc e) from the animal and vegetal poles revealed that I-Cl,I-Ca had a hig hly polarized distribution. The current density was almost 10 times hi gher in the animal pole. 6. The results suggest that Cl- channels prov ide a continuous and reliable indication of submembranous Ca2+, at lea st in an excised patch, and the clustering of the Cl- channels renders it necessary to exert caution in interpreting results involving the k inetics of Ca(2+)signalling, when I-Cl,I-Ca is used as the sole monito r of calcium.