Changes in the membrane properties of the oocyte of the mollusk, Patella vulgata, were analyzed following the induction of meiosis reinitiation by paleopedial ganglia extract or by the weak base ammonia. During maturation it was possible to distinguish between an early phase characterized by an initial hyperpolarization and a late phase consisting of a depolarization which triggers an action potential with a long-term overshoot (20 minutes) of the membrane potential. Major changes in individual ionic permeabilities were studied using both current and voltage clamp conditions. The depolarizing phase appears to depend on decreases in K+ membrane permeability. Finally we observed that the overshoot did not appear to be directly related to germinal vesicle breakdown (GVBD) since it was absent in Na-deprived artificial sea water and could be elicited in the presence of TEA bromide, which did not induce maturation. This last observation suggests that it may result from a change in specific K+ ion permeability due to the possible activation of stretch channels.

The dependence of meiosis reinitiation upon pH and calcium has been investigated in oocytes of a prosobranch mollusk, the limpet Patella vulgata. In this species, 10 to 80% of the population of prophase-blocked oocytes with an intact germinal vesicle (GV) eventually reach the metaphase 1 stage (spontaneous maturation) and they do so even in the absence of external calcium and sodium. Release of the metaphase block was obtained by fertilization or treatment with ionophore A 23187 or excess KCl. Ionophore application is effective in the absence of calcium. These treatments are ineffective when applied to GV prophase-blocked oocytes. In contrast, such oocytes resumed maturation up to metaphase 1 when incubated in alkaline complete or calcium and sodium-free seawater in the presence of millimolar concentrations of weak bases (ammonia, procaine, or nicotine) or after microinjection with alkaline Hepes-KOH buffers. Furthermore, when insemination, ionophore or KCl treatments preceded the application of weak bases, full activation was obtained. These results suggest that activation of P. vulgata oocytes normally proceeds in two steps involving, respectively, a change in intracellular pH followed by a rise in intracellular free calcium.

Oocytes of the sea urchin Sphaerechinus granularis and the startish Marthasterias glacialis have been submitted to U.V. irradiation before fertilization. This treatment significantly increased the incidence and severity of polyspermy in the sea urchin and was also found effective on starfish oocytes. These were found more resistant to damage than sea urchin eggs and U.V. irradiation did not affect either their response to the hormone l-methyladenine or the rate of elevation of the fertilization envelope, which assures the late and definitive block to polyspermy. Electrophysiological measurements performed on M. glacialis oocytes definitively demonstrate that U.V. irradiation completely inactivates voltage-dependent sodium channels, without altering the other main conductances, Cl- , K+ or Ca2+ . After such a treatment, the relative permeability of the membrane to Na+ as compared to K+ shifted from 0.019±0.003 to 0.003±0.002 and only the calcium component of the action potentials could be observed. However, a fertilization potential, preceded by small sperm induced steps, is still present in these conditions, although its peak and plateau level are greatly reduced. These new findings are discussed, which confirm the electrical nature of the fast block to polyspermy but question about the specificity of those sperm-gated channels which are supposed to trigger the fertilization potential.