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

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Abstract
Electrically excitable, Na-selective channels are induced in the membrane of the oocytes of X. laevis when it is submitted to prolonged positive potentials. Under a long positive voltage-clamp step, the membrane current, initially outward, becomes inward with a sigmoidal time course. The mean time of half-maximal inward current (t1/2) is about 18 s at 16.degree. C when stepping the membrane potential to +55 mV. The rate of channel induction was very temperature-dependent (Q10 .apprx. 5). In an Arrhenius plot, the t1/2 for induction at temperatures between 5.degree. and 22.degree. 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 membrne was maintained at resting potential after induction, the ability to produce inward currents with short depolarizing steps slowly disappeared with a t1/2 of 4 min at 16.degree. C. The temperature dependence for disappearance was larger than that found for induction (Q10 .apprx. 7). The rate of disappearance was not dependent on holding the membrane potential in the range -30 to -100 mV. Induction proceeded in Ca-free medium. Cycloheximide, a potent protein synthesis inhibitor had no effect (100 .mu.g/ml) on the induction rate. Isobutylmethylxanthine (IBMX) or theophylline (phosphodiesterase inhibitors) applied externally (10-4 M) did not affect the induction or disappearance rates. Mechanisms such as protein synthesis or 2nd messenger (such as Ca or cAMP) 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 0 and -10 mV. It was very much reduced at temperatures below about 16.degree. C.