The wave of activation current in the egg of the medaka fish

An extracellular vibrating electrode was used to measure the ring-shaped wave of inward current, the activation current, that propagates at 10 micron/sec across the egg of the medaka fish, Oryzias latipes, from the site of sperm-egg fusion at the animal pole to the vegetal pole. This activation wave is due to a localized increase in the conductance to Na+, K+, and Ca2+ and reflects the propagated opening of these ion channels. The earliest detectable current begins to enter the animal pole 20 sec after the initiation of the fertilization potential, so the first ion movements responsible for the fertilization potential are below the resolution of the vibrating probe system. These channels are present in both the animal and vegetal hemispheres, but the magnitude of the activation current is about seven times greater in the animal hemisphere. An outward current of smaller magnitude and spread out over a larger area precedes and follows the inward current except at the point of fertilization where the current is first inward. The current direction is dependent on the external Na+ concentration, and in the more physiological solution of 10% NaCl-Yamamoto's Ringer's, its direction reverses to become outward, apparently carried by K+ efflux. Raising the external Ca2+ in this same low-Na+ medium reverses the current so that it becomes inward again and increases the propagation velocity of the wave, suggesting a Ca2+ component to the inward current. Current enters a given region on the egg's surface about 16 sec before any vesicle fusion occurs in that region. Iontophoresis of inositol-1,4,5,-trisphosphate immediately triggers egg activation with a minimum activating charge of 0.6 nC.