PERIPHERAL AND OVARIAN IGF-I CONCENTRATIONS DURING THE OVINE ESTROUS-CYCLE

IGF-I was measured by RIA in plasma samples collected 8-hourly for 24 days which included two consecutive preovulatory surges of LH. In a se parate study, ovarian venous blood was collected from animals undergoi ng ovariectomy on day 10 of the oestrous cycle, or 36 h later after be ing treated with prostaglandin with or without steroid-free bovine fol licular fluid. Jugular venous blood samples were collected before, dur ing and after surgery. Follicles were dissected from ovaries of these animals and sorted into categories of small, intermediate and large, n on-atretic or atretic, and the follicular fluid was pooled and assayed for IGF-I. From another population of ovaries recovered from the slau ghterhouse, granulosa, theca and corpora lutea were isolated, homogeni zed and assayed for IGF-I. Finally ovarian corpora lutea and granulosa cells were each incubated with tritiated amino acids overnight at 37 degrees C. Thereafter the tissues and media were sonicated, IGF-I extr acted from the supernatant and tritiated IGF-I precipitated using a sp ecific IGF-I antibody. The absence of any significant change in periph eral IGF-I concentrations following ovariectomy and the finding that t he ovarian venous IGF-I concentrations (161 +/- 10 mu g/l) were not si gnificantly different from levels seen in peripheral blood (157 +/- 10 mu g/l) indicated that the ovary is not a net exporter of IGF-I. Howe ver, the ovary does synthesize IGF-I, as evidenced by granulosa and lu teal synthesis, but probably not in quantities in excess of that utili zed by ovarian tissues ber se. Although the plasma IGF-I levels increa sed around the second preovulatory LH surge, the results overall indic ated that the IGF-I concentrations in plasma are not strictly related to any major ovarian event during the oestrous cycle in the sheep. Thi s view is based on the findings that the concentration of IGF-I in fol licular fluid was not related to follicular health but correlated with those in peripheral plasma and that the ovarian venous concentrations did not vary between left and right ovaries irrespective of whether t he ovaries contained a corpus luteum, dominant follicle or neither. Co llectively, these results are consistent with the notion that IGF-I of ovarian origin fulfils an autocrine/ paracrine function and does not have an endocrine role. Moreover, the results show that the concentrat ions of IGF-I in follicular fluid reflect those in peripheral plasma.