The physiological role of beta-endorphin in porcine ovarian follicles

beta-Endorphin-like immunoreactivity (beta-END-LI) was measured by radioimm unoassay in porcine ovarian follicular fluid (FF) from small, medium and la rge follicles throughout the oestrous cycle. The concentration of beta-END- LI in FF from small follicles collected on days 1-5 of the cycle was at lea st tenfold higher than in the fluid from any other follicles independently from their size and the period of the cycle. The level of beta-END-LI in sm all follicles on days 6-10 was drastically decreased. Subsequently, on days 11-16 its concentration was enhanced and reduced again in pre ovulatory pe riod of the cycle. Concentrations of beta-END-LI in FF from medium follicle s were relatively equal throughout the cycle (days 6-21). No significant di fferences in beta-END-LI levels were found between small, medium and large follicles from days 17-21. However, beta-END-LI concentrations in medium fo llicles on days 11-13 and 14-16 were statistically lower than those in smal l follicles. Moreover, the effects of FSH, prolactin (PRL), progesterone (P -4), testosterone (T) and 17 beta-oestradiol (E-2) on beta-END-LI release b y granulosa cells (GCs) from large follicles and, on the other hand, the ef fects of the opioid agonist FK 33-824 alone or in combination with FSH, PRL or naloxone (NAL) on follicular steroidogenesis were studied. FSH drastica lly increased beta-END-LI output in a dose-dependent fashion. This stimulat ory effect of the gonadotrophin was inhibited by the highest dose of P-4 (1 0(-5) M). The effect of PRL and the steroids added to the cultures on beta- 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 3 3-824. In the presence of NAL, FSH or PRL, A(4) release stimulated by FK 33 -824 was suppressed to the basal level. Secretion of E2 was completely free from the influence of FK 33-824 or NAL; only oestrone (E-1) output was mod ulated by them in cultures where FSH or PRL was present. In conclusion, FSH appears to be the key regulator of beta-END-LI secretion by porcine granul osa cells. Moreover, steroidogenesis in pig granulosa cells is modulated by opioid peptides acting both alone and by way of interaction with FSH or PR L.