AN ENDOGENOUS INACTIVATING INWARD-RECTIFYING POTASSIUM CURRENT IN OOCYTES OF XENOPUS-LAEVIS

An endogenous inward-rectifying K+ current is described, which is pres ent in native oocytes of some Xenopus laevis donors. Experiments were performed using defolliculated oocytes from donor frogs obtained from two different suppliers. In all oocytes from animals from one source, an inward-rectifying K+ current could be elicited with negative pulses from a holding potential of -20 mV in external solutions with a high K+ concentration. Increasing external K+ concentrations increased the amplitude of this current and shifted the reversal potential towards m ore positive potentials. In 118 mM KCl, the inward-rectifying K+ curre nt partially inactivated between -20 and -80 mV and completely inactiv ated at more negative membrane potentials; 50% steady-state inactivati on occurred near -50 mV. The time course of inactivation of the inward -rectifying current could be well fitted with two exponentials. The sl ow time constant had values of about 500 ms and was voltage independen t. In contrast, the fast time constant and the time to reach the peak inward current decreased with more negative membrane potentials. Ba2+, Cs+, quinine (all 5 mM) and 50 mM tetraethylammonium partially blocke d the inward-rectifying K+ current, whereas 10 mM 4-aminopyridine was without blocking effect. The oxidant chloramine-T blocked the inward-r ectifying K+ current without slowing its inactivation.