Cytoplasmic Ca2+ oscillation coordinates the formation of actin filaments in the sea urchin eggs activated with phorbol ester

Changes in the intracellular Ca2+ concentration ([Ca2+]i) and the formation of actin filaments were investigated in unfertilized eggs of the sea urchi n Hemicentrotus pulcherrimus after activation with a phorbol ester, 12-O-te tradecanoyl phorbol-13-acetate (TPA). Intracellular Ca2+ oscillation was ob served using a fluorescent Ca2+ indicator dye, calcium green dextran. From about 20 to 80 min after the addition of TPA to 100 mu M, there was a rise in [Ca2+]i, which was followed by Ca2+ oscillation. A change in [Ca2+]i in response to TPA was not observed in eggs that had been injected with hepari n, an inositol 1,4,5-triphosphate (IP3) receptor antagonist. Therefore, lon g-term exposure to a high concentration of TPA seems to induce Ca2+ release via the IP3 pathway, as well as causing the release of diacylglycerol from membrane lipids. Moreover, the elongation of actin filaments occurred in t he cytoplasm during the rise in [Ca2+]i. Actin filaments also formed when T PA-induced cytoplasmic alkalization was inhibited by exposure to Na+-free s ea water. These results suggest that the observed cytoplasmic formation of actin filaments may be related to change in the cytoplasmic [Ca2+]i, and no t intracellular pH, induced by TPA. These phenomena may be similar to the c hanges in actin construction that occur during cell cycle events. Cell Moti l. Cytoskeleton 42:27-35, 1999. (C) 1999 Wiley-Liss, Inc.