Calcium dependent variance of chloride current fluctuations in Xenopus laevis oocytes.

Calcium induced chloride currents were studied in Xenopus oocytes using the two-electrode voltage clamp technique. Fluctuations of chloride currents measured under voltage clamp were elicited by acetylcholine. Fluctuations were superimposed on a larger chloride current. These chloride currents are due to an increase of intracellular calcium. However, an injection of calcium into the cytoplasm evoked a current which was smooth. The presence of fluctuations in Xenopus oocytes, when only InsP3 is involved in the calcium release, suggests that fluctuations are primarily due to calcium release from InsP3-sensitive calcium stores. Power spectra density of fluctuations have been analyzed. Variances of parts of records are well correlated with the corresponding average chloride currents, and thus with the calcium concentration in the cytoplasm beneath the membrane. These results characterize the mechanism of oscillatory calcium release from internal sources into the cytoplasm.