Fertilization of amphibian eggs: a comparison of electrical responses between anurans and urodeles

In Pleurodeles waltl and Ambystoma mexicanum, which exhibit physiological polyspermy, the membrane potential in most eggs did not change in any consistent pattern during 45 min after fertilization; in some cases, a slow hyperpolarization began 5 to 15 min after insemination and continued for 10-15 min. These eggs then slowly depolarized, reaching a stable value of -10 to +10 mV, about 45 min after fertilization. Membranes of eggs activated by A23187 or by electrical stimulus showed a similar behavior. The diversity of responses does not correlate with the number of sperm fusing with the egg. Holding the membrane potential at a constant value between -40 and +40 mV during insemination did not prevent fertilization nor delay sperm-egg interactions. The fertilization or activation potential of Rana temporaria eggs consists of a rapid (1 sec) depolarization accompanied by a sudden decrease in membrane resistance. The activation potential can be triggered by A23187 and by calcium iontophoresis; its amplitude depends on the (Cl-)0 and to a lesser extent on the (Na+)0. Fertilization was prevented when the membrane potential was clamped above +15 mV. However, slowing the rise time (5 to 8 sec instead of 1 sec) and reducing the amplitude (10-20 mV instead of 40-60 mV) of the fertilization potential, both by injecting negative current, never induced polyspermy.