Maturation of Xenopus oocytes: II. Observations on membrane potential

Large, progesterone-responsive oocytes within their follicles have an average resting potential of about ?25 mV. When manually dissected out of their follicles, most of these oocytes undergo a hyperpolarization over the next 30–60 min to values of about ?60 to ?80 mV. The relatively high negative membrane potentials previously recorded on dissected amphibian oocytes may thus be an artifact in the sense that such measurements do not reflect the electrical characteristics of the oocyte within the follicle. The available evidence indicates that the hyperpolarization reflects the activation of an electrogenic Na⁺,K⁺-transport process. One of the terminal events of oogenesis appears to be a suppression of the generation of the Na⁺,K⁺-transport process when oocytes are ovulated artificially (by dissection) or naturally. Continuous, long-term recordings on dissected oocytes reveal that a rather pronounced depolarization of the membrane potential together with an inflection in the recorded potential around the time of germinal vesicle breakdown takes place in the presence of progesterone. Recordings of oocytes within the follicle reveal similar changes, although reduced in absolute magnitude. In both cases, final membrane potentials of ?10 to ?15 mV are achieved. The electrophysiological changes which accompany the normal maturation process thus do not appear to be as pronounced as previously indicated.