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 urchin Hemicentrotus pulcherrimus after activation with a phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA). Intracellular Ca²⁺ oscillation was observed using a fluorescent Ca²⁺ indicator dye, calcium green dextran. From about 20 to 80 min after the addition of TPA to 100 μM, 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 heparin, an inositol 1,4,5-triphosphate (IP₃) receptor antagonist. Therefore, long-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 the cytoplasm during the rise in [Ca²⁺]i. Actin filaments also formed when TPA-induced cytoplasmic alkalization was inhibited by exposure to Na⁺free sea water. These results suggest that the observed cytoplasmic formation of actin filaments may be related to change in the cytoplasmic [Ca²⁺]i, and not intracellular pH, induced by TPA. These phenomena may be similar to the changes in actin construction that occur during cell cycle events.