TREATMENT WITH RECOMBINANT BOVINE SOMATOTROPIN ENHANCES OVARIAN FOLLICLE DEVELOPMENT AND INCREASES THE SECRETION OF INSULIN-LIKE GROWTH-FACTOR-I BY OVARIAN FOLLICLES IN EWES

To investigate the effect of recombinant bovine somatotrophin (rGH) on ovarian folliculogenesis in sheep, 18 mature Scottish Blackface ewes were assigned randomly to two treatment groups. Starting from day 5 of the synchronised oestrous cycle, animals were injected daily with eit her vehicle (control group) or 12.5 mg rGH (rGH-treated group) for 7 d ays. Blood samples were collected once daily during the experimental p eriod for the measurement of growth hormone (GH), insulin-like growth factor-I (IGF-I), insulin, follicle-stimulating hormone (FSH), luteini sing hormone (LH) and progesterone. At the end of treatment animals we re killed and ovaries collected. Ail follicles at least 1.0 mm in diam eter were dissected out and diameters measured to assess follicular po pulations for individual animals. Five small follicles (1.0-3.4 mm in diameter) and all the large follicles (at least 3.5 mm) from each anim al were incubated in 1 mi of Medium 199 for 1 h. Medium was then chang ed and incubation continued for a further hour. All medium samples wer e assayed for IGF-I, oestradiol, testosterone and progesterone. Treatm ent of ewes with rGH had no effect on the total number of follicles at least 1.0 mm in diameter (control, 34.4 +/- 2.6; rGH-treated, 31.3 +/ - 1.4; P > 0.2). However, when follicles were further classified into different size categories (1.0-2.0, 2.1-3.0, 3.1-4.0, 4.1-5.0, 5.1-6.0 and over 6.0 mm in diameter), the population of follicles 2.1-3.0 mm in diameter was significantly increased by rGH treatment (control, 9.2 +/- 0.7; rGH-treated, 13.8 +/- 1.1; P = 0.02). The number of follicle s of 3.1-4.0 mm diameter in the rGH-treated group tended to be increas ed (P = 0.09), whilst the population of follicles 1.0-2.0 mm in diamet er was reduced (P = 0.07). Treatment of ewes with rGH significantly in creased peripheral concentrations of GH (P < 0.01), IGF-I (P < 0.01), insulin (P < 0.01) and progesterone (P < 0.05). There was no effect of rGH treatment on circulating concentrations of FSH and LH. Both large and small follicles from rGH-treated ewes secreted significantly (P < 0.001) more IGF-I (37.8 +/- 2.2 ng ml h(-1), n = 50) than follicles f rom the control group (26.7 +/- 1.6 ng ml(-1) h(-1), n = 73). However, there was no significant effect of rGH treatment on the secretion of oestradiol, testosterone and progesterone by either large or small fol licles. It is concluded that treatment of mature ewes with rGH can enh ance the development of ovarian follicles to the gonadotrophin-depende nt stages. Furthermore, rGH appears to act through increased secretion of ovarian IGF-I, as well as increased peripheral concentrations of I GF-I and insulin.