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-β-Endorphin-like immunoreactivity (β-END-LI) was measured by radioimmunoassay in porcine ovarian follicular fluid (FF) from small, medium and large follicles throughout the oestrous cycle. The concentration of β-END-LI in FF from small follicles collected on days 1-5 of the cycle was at least tenfold higher than in the fluid from any other follicles independently from their size and the period of the cycle. The level of β-END-LI in small follicles on days 6-10 was drastically decreased. Subsequently, on days 11-16 its concentration was enhanced and reduced again in preovulatory period of the cycle. Concentrations of β-END-LI in FF from medium follicles were relatively equal throughout the cycle (days 6-21). No significant differences in β-END-LI levels were found between small, medium and large follicles from days 17-21. However, β-END-LI concentrations in medium follicles on days 11-13 and 14-16 were statistically lower than those in small follicles. Moreover, the effects of FSH, prolactin (PRL), progesterone (P 4 ), testosterone (T) and 17 β-oestradiol (E 2 ) on β-END-LI release by granulosa cells (GCs) from large follicles and, on the other hand, the effects of the opioid agonist FK 33-824 alone or in combination with FSH, PRL or naloxone (NAL) on follicular steroidogenesis were studied. FSH drastically increased β-END-LI output in a dose-dependent fashion. This stimulatory effect of the gonadotrophin was inhibited by the highest dose of P 4 (10 -5 M). The effect of PRL and the steroids added to the cultures on β-END-LI release was negligible. FSH-or PRL-induced P 4 secretion by GCs was essentially abolished by both FK 33-824 and NAL. However, androstenedione (A 4 ) and testosterone output by the cells was greatly potentiated by FK 33-824. In the presence of NAL, FSH or PRL, A 4 release stimulated by FK 33-824 was suppressed to the basal level. Secretion of E 2 was completely free from the influence of FK 33-824 or NAL; only oestrone (E 1 ) output was modulated by them in cultures where FSH or PRL was present. In conclusion, FSH appears to be the key regulator of β-END-LI secretion by porcine granulosa cells. Moreover, steroidogenesis in pig granulosa cells is modulated by opioid peptides acting both alone and by way of interaction with FSH or PRL.opioid peptides / β-endorphin / porcine granulosa cells / steroid secretion Reprod. Nutr. Dev. 40 (2000) 63-75 63

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