One of the roles of [Ca2+]; transients in fertilized oocytes is to cause CG exocytosis, which in turn induces a zona reaction that participates in the block to polyspermy in most mammals. CG exocytosis induced by artificial stimulators is also the result of the rise in [Ca2+];. In the present study, we found that CG exocytosis was significantly affected by the amplitude of the Ca2+ transient. A lower concentration of A23187 induced little increase in [Ca2+]i5 which resulted in a small amount of uneven CG exocytosis and nuclear activation of only a few oocytes. By contrast, a higher concentration of A23187 induced a higher ratio of Ca2+ transient, which caused most of the CGs to be released in oocytes, and the cortical reaction prevented sperm penetration in ZP-intact oocytes. However, the plasma membrane block to sperm penetration of oocytes activated by A23187 was not observed in this study. flovent inhaler
These results were consistent with those observed in Xenopus laevis and mice. It has been reported that the plasma membrane block to polyspermy is sperm-dependent, not cortical reaction-dependent. However, recently Tatone et al. reported that the plasma membrane block to sperm penetration of parthenogenetic mouse oocytes is a temporary change in the egg surface which is maintained for only 1.5-2 h. Changes in membrane potential were reported to be involved in the plasma membrane block in echinoderm and anuran oocytes, but the mechanism of this block in mammalian oocytes remains unknown.