Changes in intracellular cations following fertilization of sea urchin eggs. Na+/H+ and Na+/K+ exchanges

Sodium and potassium contents, as well as H+ excretion were measured at various times after fertilization in eggs of the sea urchin Paracentrotus, using a rapid filtration technique (Payan, P, Girard, U P, Christen, R & Sardet, C, Exp cell res 134 (1981) 339 [1]). Sodium content changes with a biphasic time course due to the appearance of two exchange mechanisms: 1. A transient Na+/H+ exchange with a l/l stoichiometry causes a rapid rise in intracellular sodium. This exchange lasts about 3 min. and is not inhibited by poisoning the eggs with NaCN. 2. An Na+/K+ exchange, detectable within minutes of fertilization causes sodium content of the egg to fall and settle below unfertilized level. This exchange is energy-dependent, reduced in low K+ (2 mM), totally inhibited by harmaline (10(-4) M) and only partially inhibited by ouabain (10(-2) M). Potassium content rises rapidly after fertilization by an undefined mechanism and is further increased via the Na+/K+ exchange. The Na+/Ka+ exchange increases the cytoplasmic K+/Na+ ratio of the egg from 6 to 12 in the first hour after fertilization. When fertilization occurs in low K+ SW, where the Na+/K+ exchange is reduced, the ratio Na+/K+ decreases and the embryos develop with a preponderance of ectodermal structures (animalization).