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Cyclic AMP is one of the key regulators of mammalian meiosis. In the present work, realization pathways of the previously revealed modulating influence of prolactin (PRL) on the cAMP-dependent mechanisms of meiosis regulation in bovine oocytes were studied. A comparative investigation of individual and combined effects of PRL (50 ng/ml) and an activator of adenylate cyclase forskolin (FK. 20 microM) on the meiotic reinitiation and nuclear maturation completion in cumulus-enclosed and cumulus-free oocytes was performed. It has been shown that the pattern of PRL influence on the meiotic resumption in oocytes devoid of cumulus cells depends on the presence of FK in the culture medium. Furthermore, realization of this influence is not associated with the activation of cytoplasmic isoforms of protein kinase C. It has been also found that PRL inhibits the retarding action of FK on the completion of oocyte nuclear maturation in both the presence and absence of cumulus cells. The findings suggest that PRL may modulate the functioning of the cAMP-dependent mechanism of meiosis regulation by the direct action on bovine oocytes, with realization of the action being independent of the metabolic coupling of oocytes with cumulus cells.

Prolactin (PRL) is one of the pituitary hormones participating in the control of mammalian folliculo- and oogenesis. In the present study, the joint effect of PRL (50 ng/ml) and dibutyryl cAMP (dbcAMP, 1 mM) on oocyte maturation and the morphologic-functional state of surrounding cumulus cells was investigated in vitro. It has been shown that PRL totally suppresses the braking impact of dbcAMP on meiosis reinitiation and the completion of oocyte nuclear maturation. Furthermore, PRL partly inhibited cumulus expansion induced by dbcAMP, although it exerted the opposite effect in the control medium. In the presence of PRL, the inhibitory impact of dbcAMP on the proliferative activity of cumulus cells and on the PRL-elicited braking of destructive processes in the cells has been found. In cumulus cells, mRNA expression of PRL receptor long isoform was revealed by the RT-PCR method. The data obtained suggest an interaction of signal cascades induced by PRL and cAMP in bovine oocyte-cumulus complexes, with the coupling site of these cascades in oocytes being apparently different from that in cumulus cells.

Comparative investigations of somatotropin and prolactin contents in the fluid of antral follicles and blood serum of cows in different phases of the oestrous cycle were performed. The somatotropin concentration in the fluid was shown to rise with increasing the follicle diameters from 3-5 to 6-10 mm in the follicular phase and to decrease in follicles of diameter 11-20 mm in the luteal phase. The prolactin concentration was higher in the fluid of follicles 11-20 mm in diameter than in those of 3-5 mm in diameter in the follicular phase and did not depend on the follicle size in the luteal phase. Concurrently, the prolactin content in follicles 3-5 mm in diameter was higher in the luteal than follicular phase of the cycle. As compared to the follicular phase, an increase in the prolactin concentration in the bovine blood serum during the luteal phase was also found. The data obtained indicate that changes in the somatotropin and prolactin contents in the follicular fluid are related to processes regulating growth and development of antral follicles depending on the phase of oestrous cycle and to changes in the blood hormone concentrations as well.

Mammalian oocytes mature in follicular fluid (FF), surrounded by follicular cells. In the present study, in vitro maturation of bovine oocytes cultured in FF from dominant follicles 15-17mm in diameter (with various forms of heat pretreatment) and supplementation with follicular wall from follicles 3-5mm in diameter (FW1) were examined. Heat pretreatment of FF was as follows: (1) no treatment (FF1); (2) 56 degrees C for 30min (FF2); and (3) 100 degrees C for 20s (FF3). After IVM in FF1, oocytes underwent IVF and IVC and embryo development was assessed (up to the morula stage). The rate of oocyte maturation was decreased in pure FF1 versus control (44.5% versus 62.8%, P<0.001). In the control medium, FW1 did not significantly affect nuclear maturation. By contrast, the addition of FW1 to FF1 increased the rate of matured oocytes approximately two-fold (85.9% versus 45.6%, P<0.001). Furthermore, the maturation rate in the FF+FW1 system declined (from 85.9 to 71.0%, P<0.001), whereas that in the FF system increased (from 45.6 to 71.6%, P<0.001) with increased temperature of the FF treatment. Supplementation of the control medium with FW1 increased the yield of morulae (42.6% versus 13.7%, P<0.001). However, the stimulatory effect of FW1 on the morula rate was much higher in pure FF1 (72.5% versus 31.7%, P<0.001). These findings indicated, for the first time, the stimulatory impact of FW1 on in vitro maturation and early developmental capacity of bovine oocytes cultured in pure FF from dominant follicles. We also inferred that bovine FF constituents affecting bovine oocyte maturation and the meiosis-promoting ability of the FW were heat-labile.

The effect of arginine-vasotocin (50 ng ml-1) on the in vitro production of progesterone, testosterone, and oestradiol by isolated cow and chicken ovarian follicles and mouse ovaries was studied. This peptide (1) enhanced the release of progesterone into the medium by the ovarian tissues of all three species of animal; (2) inhibited the production of oestradiol by mouse and cow ovaries; (3) had no significant effect on testosterone secretion in any of the species studied. The results obtained indicate that arginine-vasotocin directly influences steroid production in mammalian and avian gonads.

The growth of ovary follicules was accompanied by a considerable decrease in the secretion of progesterone, testosterone and estradiol, as well as by a decrease of the estradiol share in total hormone production,--both in hens and in cows. The hens also revealed a decrease of the testosterone share and an increase of the progesterone share in total hormone production. the data obtained suggest a functional atresia of the ovary follicules in hens as well as in mammals. the hen mature follicules, however, differ from those of mammals in that the progesterone secretion is higher in them as compared with other steroids, the progesterone being the main ovulatory hormone in birds.

The effects of diurnal and nocturnal injections of various hormonal preparations on the dynamics of moult and ovulation rate have been compared in hens kept under the conditions of a shortened (6-hour) illumination period. It was found that ACTH, pituitrin and arginine-vasotocin inhibited the moult if they were injected during night time. Prolactin and diethylstilbestrol enhanced the moult only after nocturnal injections, whereas diurnal injections of these hormones did not affect the shedding. Nocturnal injections of deoxycorticosterone were twice effective with respect to moulting rate as compared with diurnal injections. Thyroxine stimulated the moult irrespectively of the time of injection. Nocturnal injections of ACTH caused retardation of the recovery of egg laying in hens. Nocturnal administrations of arginin-vasotocine enhanced the recovery of ovulation. Diurnal injections of ACTH and vasotocine did not affect the egg laying not capacity of hens. Reproductive function was inhibited by prolactine irrespectively of the period of its administration. The data obtained indicate the existence of diurnal changes in the sensitivity of target tissues of hens to some of the hormones. At night time this sensitivity is usually increased.