The effect of insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF), fibroblast growth factor (FGF) and nerve growth factor (NGF) on production of oxytocin and progesterone by cultured bovine granulosa and luteal cells was studied. Secretion of oxytocin was stimulated, in a dose-dependent manner, by IGF-I at 48 and 120 h of culture to levels much higher than those after stimulation with LH, FSH, EGF, FGF or NGF. A similar effect of IGF-I was observed for progesterone but, in contrast to oxytocin, secretion of progesterone was not increased by EGF, NGF or FGF. During primary culture, for 4 h, of dispersed bovine luteal cells obtained from corpora lutea between days 4 and 10 of the oestrous cycle, all the growth factors tested failed to stimulate secretion of oxytocin or progesterone. The data suggest the relevance of growth factors (especially IGF-I) for ovarian physiology and their possible importance for differentiation of follicles and luteinization.

In the bovine ovary there is a delay of 4-6 days between the observed maximum of oxytocin mRNA and the peak in the luteai levels of oxytocin nonapeptide. This implies a maturation process involving components of the post-translational processing pathway. In situ hybridization shows the oxytocin gene to be transcribed exclusively in the large cells of the corpus luteum at the beginning of the estrous cycle.

Immunoreactive oxytocin was measured in ovaries (corpus luteum and follicular fluid) and adrenals of cows, and in testes, seminal vesicles, prostate gland and adrenals of bulls. Secretion of oxytocin was further measured after culture of whole follicles, granulosa cells and theca tissue. Concentrations of oxytocin increased in corpora lutea of cycling cattle until mid-luteal phase (447 ± 93 ng/g wet weight) and decreased afterwards. Low concentrations were found in corpora lutea of pregnant animals (6 ± 3 ng/g wet weight). Follicular fluid contains some oxytocin (on average 42–108 pg/ml) but concentrations were significantly higher in the fluid of ovarian cysts (190 pg/ml). After culture of follicles the amount of oxytocin released into the medium increased indicating de novo synthesis. The granulosa cells were the main source of follicular oxytocin. Production increased during luteinization indicating that luteinization is an important step for the production of oxytocin in ovaries. Tissues of testes (65 ± 10 pg/g wet weight) and adrenals from cows (122 ± 39 pg/g wet weight) and bulls (111 ±2 pg/g wet weight) contained oxytocin but at much lower concentrations compared to corpus luteum tissue. About 10 times higher concentrations of oxytocin were measured in the adrenal medulla (717 ± 96 pg/g wet weight) compared to the cortex (72 ± 11 pg/g wet weight). Seminal vesicles and prostate gland contained no measurable amounts of oxytocin (< 5 pg/g wet weight).

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