Induction and disappearance of excitability in the oocyte of Xenopus laevis: a voltage-clamp study

Abstract

Electrically excitable, sodium-selective channels are induced in the membrane of the oocytes of Xenopus laevis when it is submitted to prolonged positive potentials (Kado, Marcher & Ozon, 1979; Baud, Kado & Marcher, 1982). Under a long positive voltage-clamp step, the membrane current, initially outward, becomes inward with a sigmoidal time course. The mean time to half-maximal inward current (t 1/2) is about 18 s at 16 degrees C when stepping the membrane potential to +55 mV. The rate of channel induction was very temperature dependent (Q10 about 5). In an Arrhenius plot, the t 1/2 for induction at temperatures between 5 and 22 degrees C showed a single slope. The rate of induction was dependent on the membrane potential, increasing exponentially with positive membrane potential (e-fold for a 20 mV change). When the membrane was maintained at resting potential after induction, the ability to produce inward currents with short depolarizing steps slowly disappeared with a t 1/2 of 4 min at 16 degrees C. The temperature dependence for disappearance was larger than that found for induction (Q10 about 7). The rate of disappearance was not dependent on holding the membrane potential in the range -30 to -100 mV. Induction proceeded in calcium-free medium. Cycloheximide, a potent protein synthesis inhibitor had no effect (100 micrograms/ml) on the induction rate. Isobutylmethylxanthine (IBMX) or theophyllin (phosphodiesterase inhibitors) applied externally (10(-4) M) did not affect the induction or disappearance rates. From the present results, mechanisms such as protein synthesis or a second messenger (such as calcium or cyclic AMP) do not appear to be involved. During the depolarization necessary to obtain induction, another conductance was also activated. It was more slowly established, appeared to be non-saturable and had a reversal potential between zero and -10 mV. It was found to be very much reduced at temperatures below about 16 degrees C.