H2O2-INDUCED CHLORIDE CURRENTS ARE INDICATIVE OF AN ENDOGENOUS NA-CA2+ EXCHANGE MECHANISM IN XENOPUS OOCYTES()

1. Defolliculated Xenopus oocytes were voltage clamped in bathing solu tions containing 115 mM KCl and 1.8 mM CaCl2. External application of H2O2 transiently elicited voltage dependent outward rectifying current s within several seconds. Upon depolarization to +50 mV these currents had an activation time constant of 370 ms and reached amplitudes of u p to 70 mu A. This current was also observed in oocytes without the vi telline membrane. 2. The current was abolished by 500 mu M niflumic ac id, by the replacement of external Cl- by methanesulphonate, or when e xtracellular Ca2+ was removed indicating the involvement of Ca2+-activ ated Cl- channels, which are very abundant in Xenopus oocytes. 3. Whil e the current could be recorded in bathing solutions containing Li+, K +, Rb+, Cs+ and NH4+, extracellular Na+ abolished the current complete ly (IC50 = 6 mM Na+). 4. The H2O2-induced Cl- current was half-maximal ly blocked by approximately 25 mu M 2'4'-dichlorobenzamil, 250 mu M Mg Cl2, 100 mu M CdCl2 and 100 mu M NiCl2. These substances have been sho wn to block Na+-Ca2+ exchangers in various tissues. 5. The data are co nsistent 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 use d for heterologous expression studies.