NEOMYCIN, AN INHIBITOR OF PHOSPHOINOSITIDE HYDROLYSIS, INHIBITS THE RESUMPTION OF BOVINE OOCYTE SPONTANEOUS MEIOTIC MATURATION

The possibility that the intracellular signals generated upon phosphoinositide hydrolysis are involved in regulating bovine oocyte spontaneous meiotic resumption was investigated. Oocytes were mass-harvested and cultured in 2A-BMOC medium supplemented with 0.5% bovine serum albumin in the presence or absence of neomycin (an inhibitor of phosphoinositide hydrolysis) or phorbol myristate acetate (an activator of protein kinase C). The role of intracellular calcium was examined by preloading with BAPTA/AM (a calcium chelator) prior to culture. Meiotic maturation was scored cytogenetically. 1) Neomycin induces an irreversible inhibition of germinal vesicle breakdown which does not exceed 60% and is apparent at concentrations of 5 mM or above. Progression of meiosis past metaphase I is inhibited at concentrations of 2.5 mM or above. The full effect of neomycin is only apparent if it is presented to the oocytes within 3 h of follicular release, although germinal vesicle breakdown is not observed until 9 h culture under control conditions. 2) PMA alone has negligible effect on germinal vesicle breakdown, but it acts synergistically with 2 mM IBMX to inhibit this process. PMA has a dual effect on the progression of meiosis past metaphase I: 1 nM PMA has a stimulatory effect while 1-mu-M PMA blocks the ability of oocytes to reach anaphase I or beyond. These observations are not found with a non-tumor-promoting phorbol ester. 3) Spontaneous meiotic resumption is not significantly affected in the absence of added exogenous calcium. However, oocytes preloaded with BAPTA/AM exhibit a dose-dependent inhibition of germinal vesicle breakdown, even in the presence of extracellular calcium. These results provide indirect evidence for a role for phosphoinositide metabolism in bovine oocyte maturation; it is suggested that intracellular calcium mobilization is prerequisite for meiotic resumption and protein kinase C may modulate cAMP-dependent inhibition of this response.