Dynamic changes to the inositol 1,4,5-trisphosphate and ryanodine receptors during maturation of bovine oocytes

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and ryanodine receptor (RyR) have been identified as two ligand-gated calcium channels which play a critical role in mediating calcium release in many different types of cells and tissues. The physiological significance of the two receptors in regulation of intracellular calcium during meiotic maturation and fertilization in the bovine oocyte was evaluated. Metabolic labeling of bovine oocytes by Met-Cys S-35 during early and late maturation was followed by immunoprecipitation of both RyR and IP3R using specific antibodies against these two receptors. Results indicate that IP3R is translated throughout the maturation period; in contrast, RyR is only translated during the late maturation period of bovine oocytes. In addition, the experiments reported here investigate the temporal and spatial relationships between these calcium channels and the endoplasmic reticulum (ER) and cortical granules (CG). Immunocytochemistry, fluorescence staining and confocal microscopy were applied at four oocyte developmental stages: the germinal vesicle-intact (GV-intact), metaphase I (MI) and metaphase II (MII) stages of maturation and the fertilized egg at 6 h post insemination (hpi). Although oocytes demonstrated some differences in staining patterns and localization, both receptor types showed apparent dynamic changes during meiotic maturation and dramatic decreases in signals after insemination. These results indicate the changes in the number and distribution of IP3R and RyR may account for the increased intracellular calcium responsiveness at fertilization. The IP3R appears to associate with the ER at the sub-vitelline membrane cortex in bovine oocytes. In addition, RyR appears to associate with the CG. In conclusion, although these two receptors may have different functional roles in regulation of calcium release during meiotic maturation and fertilization, it appears that both IP3R and RyR contribute to the significant increase of intracellular calcium during fertilization and activation in the bovine oocyte.