Intracellular pH and Ca2+ were measured with BCECF- and Calcium Green-dextran during maturation and fertilization of oocytes of the limpet Lottia gigantea. Maturation of oocytes from prophase to metaphase I of meiosis was induced in seawater adjusted to pH 9 with NH4OH. Intracellular pH rose during maturation induction, and maturation was also induced by microinjecting pH 8, but not pH 7, HEPES buffer. Intracellular Ca2+ rose during NH4OH-induced maturation, but maturation was not inhibited when the increase was blocked by microinjection of BAPTA. When the metaphase I oocytes were fertilized, there was an abrupt increase in intracellular Ca2+, and activation (polar body formation) failed to occur in BAPTA-injected oocytes. Intracellular pH did not rise during fertilization. These observations show that maturation from prophase to metaphase I of meiosis is pH-dependent and activation of the metaphase I oocytes is Ca2+-dependent. A Ca2+ action potential was present in both immature and mature oocytes but was more prominent in mature oocytes whose input resistance was higher. Fertilization produced a long-lasting (17-20 min) Na+-dependent fertilization potential with superimposed oscillations resembling Ca2+ action potentials. (C) 2001 Wiley-Liss, Inc.
