Cytoplasmic calcium fluctuations in calcium overloaded Xenopus laevis oocytes

Cytoplasmic calcium fluctuations were studied in calcium overloaded Xenopus oocytes. Calcium sensitive chloride currents were recorded using the two-electrode voltage clamp technique. Fluctuations of chloride currents measured under the voltage clamp were elicited by injection of calcium into the cytoplasm. Contrary to infrequent injections of small calcium amounts which evoke smooth transient responses, the fluctuating chloride currents are due to overloading of intracellular calcium stores which then release calcium repeatedly. Chloride current fluctuations in calcium overloaded oocytes can be reversibly suppressed by caffeine. This effect is concentration dependent, and the amplitude decrease of fluctuations is already apparent at 2 mmol/l caffeine. Power spectra density of fluctuations have been analyzed; they exhibited this pronounced effect of caffeine. Other effective inhibitors were tetracaine and heparin. The results of the present work suggest that at least a part of the endoplasmic reticulum in Xenopus oocytes is a calcium releasing calcium store which can be activated by calcium at the resting inositol trisphosphate concentration.