Phospholipid turnover and ultrastructural correlates during spontaneous germinal vesicle breakdown of the bovine oocyte: effects of a cyclic AMP phosphodiesterase inhibitor.

The turnover of 32P]orthophosphate in bovine oocyte phospholipids was studied during the early stages of spontaneous meiotic maturation, and during inhibition of this process by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX). Radioactive lipids were separated by TLC and the meiotic stage was determined cytogenetically. Ultrastructure of the nuclear membrane was examined using transmission EM. During the commitment period to meiotic resumption, which precedes germinal vesicle breakdown (GVBD), small localized convolutions appeared in the intact nuclear membrane. This was accompanied by a decrease in 32P]phosphatidic acid (PA) and an increase in 32P]-phosphatidylcholine (PC). This was followed by extensive convolutions, and subsequent dissociation, of the nuclear membrane, concomitant with a tremendous surge in 32P]PC and 32P]phosphatidylethanolamine (PE). The cAMP-mediated maintenance of meiotic arrest involved retention of entire nuclear envelope integrity and total inhibition of the surge in 32P]PC and 32P]PE which accompanied GVBD. The increase in 32P]phosphatidylinositol (PI) associated with all stages of early meiotic resumption was unaffected by IBMX. Microinjection of heparin inhibited GVBD, and injection of inositol 1,4,5-trisphosphate (IP3) overrode IBMX-maintained meiotic arrest in almost 40% of the oocytes. The results suggest that there may be several functions for phospholipid turnover in the regulation of spontaneous meiotic resumption in the bovine oocyte. The first precedes the commitment period, and involves IP3 generation to serve as the primary signal for meiotic resumption. The second occurs concomitant with the commitment period, is unaffected by the level of intracellular cAMP, and is associated with the general turnover of phospholipid. The third is associated with GVBD, and is cAMP-sensitive, and may represent stimulation of de novo synthesis of phospholipid, thereby permitting disruption of the nuclear membrane.