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Cytogenetic data are presented which indicate species differences in the response of oocytes maturing in vitro in the presence of dibutyryl cyclic AMP (dbcAMP). Inhibition of ewe oocyte maturation was noted at germinal vesicle, MI, and MI1 in the presence of 25-750 pg/ml of dbcAMP. Cow oocytes were primarily affected at MI and MI1 by similar concentrations, and only 750 pglml delayed golden hamster oocytes at these stages. These findings highlight the variability between the responses of the oocytes of these mammals and the rat, mouse, or pig.Many lines of evidence have suggested that cyclic AMP (CAMP) is a mediator in gonadotrophin-induced maturation of oocytes (Tsafriri, '79), yet the apparently paradoxical inhibition of in vitro oocyte meiotic maturation by dbcAMP has been reported for mouse (Stern and Wassarman, '74) and rat (Magnusson and Hillensjo, '77). This inhibition has been utilized as a critical step in various experiments designed to study, for example, protein synthesis during maturation (Ekholm and Magnusson, '79) or the effect of the a-adrenergic agonist phenylephrine directly on the mouse oocyte (Zupko et al, '80). Such generalized use of the dbcAMP blocked mouse oocyte as a model for studying specific aspects of meiotic resumption has led to conclusions which by implication pertain to mammalian oocytes in general. The purpose of this report is to call attention to the diversity of meiotic maturational responses of mammalian oocytes to in vitro dbcAMP, thus indicating a need for caution when interpreting rodent experiments as being generally applicable. MATERIALS AND METHODSOocytes were rapidly removed from mediumsized follicles of the ovaries of young adult cycling golden hamsters and mixed breed diestrous ewes and cows. The Bovidae oocytes were obtained from early fall to late winter at a local slaughterhouse. For each species the oocytes were pooled in McCoy's 5A medium (Grand Island Biological Co.), and distributed randomly within 8 minutes of removal to test petri dishes containing McCoy's 5A with various concentrations of dibutyryl cyclic AMP. The total elapsed time from removal of ovaries to the experimental media for the Bovidae was +28 minutes. Three replicates were carried out of each experiment. Maturation was allowed to proceed for the appropriate time periods (Jagiello et al, '76; Gwatkin and Haidri, '73) at 37"C, pH 7.2, in a humid, constant 5% COJair atmosphere. At theend of the incubation time, harvests of oocytes were prepared for cytologic examination of germinal vesicle (GV) diakinesidmetaphase I or metaphase I1 by a modification of the method of Tarkowski ('66) and stained with 2% toluidine blue in resin. Scoring was carried out with bright-field optics using the lOOX Neofluar oil objective of a Zeiss Photomicroscope 11. RESULTSAs can be seen in Table 1, dibutyryl cyclic AMP at concentrations previously shown to be very inhibitory for germinal vesicle breakdown (GVB) of mouse and rat oocytes had quite different effects on golden hamster, cow, and ewe meiotic progressi...