Redistribution and increase in cortical inositol 1,4,5-trisphosphate receptors after meiotic maturation of the mouse oocyte.

Mouse oocytes develop sensitivity to inositol 1,4,5-trisphosphate (IP3) during oocyte maturation. We recently reported that a change in the organization of the endoplasmic reticulum (ER) during oocyte maturation may contribute to this enhanced sensitivity (Mehlmann et al., 1995, Dev. Biol. 170, 607-615). Here, we investigated whether there is an increase in the number of available IP3 receptors after maturation and whether there is a redistribution of IP3 receptors similar to the redistribution of the ER that occurs during maturation. Western blot analysis of the IP3 receptor in oocytes and eggs demonstrated a 1.8-fold increase in immunoreactive mass of the IP3 receptor following oocyte maturation. Microinjection of the function-blocking monoclonal antibody 18A10 inhibited IP3-induced Ca2+ release in a concentration-dependent manner in both eggs and oocytes. More antibody was required to inhibit Ca2+ release to the same extent in eggs compared to oocytes when both were injected with the same concentration of IP3, suggesting that eggs contain a greater number of functional IP3 receptors. Immunolocalization of the IP3 receptor revealed that receptors were present in large clusters, 1-2 microm in diameter, in the cortex of the mature egg except in a ring-shaped band of cortex adjacent to the meiotic spindle. In contrast, receptor clusters were located around the entire cortex of the immature oocyte and were much smaller (<1 microm); larger patches were sometimes seen, but they did not display the same spherical organization as those in eggs. These results suggest that the number of cortical IP3 receptors increases during mouse oocyte maturation and that this increase may contribute to enhanced Ca2+ release at fertilization.