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 plasma membrane of Xenopus oocytes, was examined in excised inside-out macropatches using a rapid perfusion system. 2. Application of Ca2+-containing Ringer solution resulted in the activation of a current whose reversal 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 generated when Ca2+ was omitted from the solution. The current is therefore a Ca2+-activated Cl- one. 3. Following exposure to Ca2+, the half-time for activation of I-Cl,I-Ca was not voltage dependent, whereas deactivation was strongly so. 4. I-Cl,I-Ca was stable in the continuous presence of Ca2+ and showed no sign of inactivation or adaptation. 5. Comparison of the size of the currents (normalized to pipette resistance) from the animal and vegetal poles revealed that I-Cl,I-Ca had a highly polarized distribution. The current density was almost 10 times higher in the animal pole. 6. The results suggest that Cl- channels provide a continuous and reliable indication of submembranous Ca2+, at least in an excised patch, and the clustering of the Cl- channels renders it necessary to exert caution in interpreting results involving the kinetics of Ca(2+)signalling, when I-Cl,I-Ca is used as the sole monitor of calcium.