Regulation of murine oocyte meiosis: evidence for a gonadotropin-induced, cAMP-dependent reduction in a maturation inhibitor.

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Abstract
An assay was developed that can detect relative changes in the amount of a non-cAMP inhibitor of maturation present in cumulus cells. Using this assay in which accelerated maturation of a group of treated cumulus cell-oocyte complexes relative to untreated complexes indicates a decrease in the amount of inhibitor, a possible relationship between elevation of cAMP levels and subsequent decreased amounts of a non-cAMP inhibitor was investigated. Mouse oocytes obtained from cumulus cell-oocyte complexes treated with luteinizing hormone (LH) resumed meiosis prior to oocytes obtained from untreated complexes; the degree of acceleration of maturation was dependent on LH concentration. A similar result was obtained with FSH. Correlated with LH- or FSH-acceleration of maturation was an LH- or FSH-induced elevation of cumulus cell cAMP levels. Inhibiting LH-induced elevation of cumulus cell cAMP levels inhibited LH-induced acceleration of maturation. An initial incubation of complexes in medium containing dibutyryl cAMP (dbcAMP) also promoted acceleration of maturation. Maturation of denuded oocytes was not altered by treatment with either LH, FSH, or dbcAMP. Complexes initially incubated in dbcAMP-containing medium still demonstrated acceleration of maturation after a subsequent 2 h incubation in dbcAMP-free medium. Relative to untreated complexes, none of these treatments disrupted intercellular communication between cumulus cells and the oocytes. Elevating follicle cAMP levels with cholera toxin induced maturation of follicle-enclosed oocytes when cumulus cell-oocyte coupling was still fully maintained. Gonadotropin-mediated acceleration of maturation is via a cAMP-dependent reduction in the level of a maturation inhibitor present in granulosa/cumulus cells.