Two phases of calcium requirement during starfish meiotic maturation

During meiosis in oocytes of the starfish, Asterina pectinifera, a Ca²⁺ transient has been observed. To clarify the role of Ca²⁺ during oocyte maturation in starfish, an intracellular Ca²⁺ blocker, TMB-8, was applied. The oocyte maturation induced by 1-methyladenine (1-MA) was blocked by 100 μM TMB-8. Reinitiation of meiosis with germinal vesicle breakdown (GVBD) and the following chromosome condensation did not take place. Maturation-promoting factor (MPF) activity did not increase and GVBD and chromosome condensation did not occur. Ca²⁺ transient observed immediately after 1-MA application in control oocytes was also blocked by TMB-8. When calyculin A, which activate the MPF directly, was applied to the oocytes instead of 1-MA in seawater containing 100 μM TMB-8, GVBD and chromosome condensation were blocked. Cytoplasmic transplantation studies confirmed that MPF was activated, although TMB-8 blocked GVBD. These results show that TMB-8 blocked the increase of MPF activity induced by 1-MA and the process of active MPF inducing GVBD and subsequent chromosome condensation. Together with the above phenomena, it is conceivable that there are two phases of Ca²⁺ requirement during starfish oocyte maturation. These are the activation of MPF, moreover, GVBD, and the subsequent chromosome condensation.