Induction of membrane excitability in Xenopus oocytes

Immature oocytes from the African toad Xenopus laevis are not known to be excitable cells, which means that they do not generate an action potential in response to small depolarizations. However, a regenerative response is produced if successive depolarizing currents of large magnitude are applied to the oocyte membrane. This response is characterized by the occurrence of a positive transmembrane potential that can last for several minutes. The opening of voltage-dependent channels, highly selective for sodium ions, underlies the depolarization thus obtained. These channels exhibit unconventional electrophysiological and pharmacological properties, which set them apart from other types of voltage-dependent sodium channels found in excitable tissues. The opening of the oocyte sodium channels is a complex process, which includes an induction phase. During this phase, the channels change from an electrical state of inexcitability into an excitable voltage-dependent state. The induction mechanism is modulated by the temperature of the bathing medium, by the activation of enzymes (namely a phospholipase C and a protein kinase C) and by the release of calcium ions from intracellular phosphatidyl-inositol trisphosphate stores. The results summarized in this review point out the possible role that these sodium channels may play in the physiology of the oocyte.