MEIOTIC ARREST IN INCOMPETENT RAT OOCYTES IS NOT REGULATED BY CAMP

Fully grown, but not growing, mammalian oocytes spontaneously resume m eiosis in vitro. Resumption of meiosis, also known as oocyte maturatio n, is associated with a drop in intraoocyte concentrations of cAMP fol lowed by activation of the maturation promoting factor (MPF). Microtub ule-associated-protein (MAP) kinase has been suggested as a substrate for the active p34(cdc2) kinase, the catalytic subunit of MPF. Our stu dy was designed to explore the mechanism of regulation of meiotic arre st in growing rat oocytes. Confirming previous observations we showed that in our rat colony oocytes do not acquire the competence to sponta neously resume meiosis earlier than 22 days postpartum We further demo nstrated that follicle-enclosed oocytes from 20-day-old female rats fa il to resume meiosis in response to luteinizing hormone, follicle-stim ulating hormone, a gonadotropin-releasing hormone analog, or forskolin , all of which are known to induce maturation in competent oocytes. Im munoblot analysis using highly specific anti p34(cdc2) antibodies reve aled that incompetent oocytes express the catalytic subunit of MPF at amounts that are not different from that found in competent oocytes. I n addition, highly specific anti MAP kinase antibodies detected the pr esence of similar quantities of two isoforms (42 and 44 kDa) of MAP ki nase in competent and incompetent oocytes. Measurements of cAMP reveal ed that as compared to competent oocytes, incompetent oocytes contain somewhat lower levels of this nucleotide (1.42 +/- 0.3 and 1.17 +/- 0. 07 fmole/oocyte, respectively). However, considering the difference in protein content, the calculated concentrations seem to be similar. Fu rthermore, similar to competent oocytes, intracellular concentrations of cAMP in incompetent oocytes dropped significantly (from 1.17 +/- 0. 07 to 0.77 +/- 0.12 fmole/ oocyte) 2 hr after isolation from the folli cle. We hereby suggest that (a) in mammals, similar to amphibians, the term meiotic incompetence can be extended to include inability to res ume meiosis in response to hormonal stimulation; (b) it is not the lac k of p34(cdc2) or downstream regulatory elements, such as MAP kinase, that prevents growing oocytes from resuming meiosis; and (c) unlike fu lly grown oocytes, resumption of meiosis in growing oocytes is not sub jected to negative regulation by cAMP. (C) 1994 Academic Press, Inc.