Defolliculated Xenopus oocytes were voltage clamped in bathing solutions containing 115 mM KCl and 1.8 mM CaCl2. External application of H2O2 transiently elicited voltage-dependent outward rectifying currents within several seconds. Upon depolarization to +50 mV these currents had an activation time constant of 370 ms and reached amplitudes of up to 70 microA. This current was also observed in oocytes without the vitelline membrane. 2. The current was abolished by 500 microM niflumic acid, by the replacement of external Cl- by methanesulphonate, or when extracellular Ca2+ was removed indicating the involvement of Ca2+-activated Cl- channels, which are very abundant in Xenopus oocytes. 3. While the current could be recorded in bathing solutions containing Li+, K+, Rb+, Cs+ and NH4+, extracellular Na+ abolished the current completely (IC50 = 6 mM Na+). 4. The H2O2-induced Cl- current was half-maximally blocked by approximately 25 microM 2'4'-dichlorobenzamil, 250 microM MgCl2, 100 microM CdCl2 and 100 microM NiCl2. These substances have been shown to block Na+-Ca2+ exchangers in various tissues. 5. The data are consistent with the existence of an endogenous Na+-Ca2+ exchanger in the plasma membrane of Xenopus oocytes, which runs in reverse mode in the absence of high external Na+ and the presence of external Ca2+. This endogenous component has to be considered when Xenopus oocytes are used for heterologous expression studies.
