THE EFFECT OF RECOMBINANT BOVINE SOMATOTROPIN ON OVARIAN-FUNCTION IN HEIFERS - FOLLICULAR POPULATIONS AND PERIPHERAL HORMONES

The objective of this study was to investigate the possible effect of recombinant bovine somatotropin (BST) on ovarian folliculogenesis and ovulation rate. Twelve Hereford x Friesian heifers received daily injections of either 25 mg BST (6 heifers) or vehicle (6 heifers) for a period of two estrous cycles until slaughter. Blood samples were collected three times a week for measurements of peripheral growth hormone (GH), insulin-like growth factor I (IGF-I), FSH, LH, estradiol, and progesterone. Serial blood samples were also taken every 10 min for 8 h on Days 12 and 19 of the second estrous cycle to monitor GH, IGF-I, FSH, and LH profiles. At the end of treatment (Day 7 of the third estrous cycle), the heifers were killed and their ovaries were collected. Ovulation rate was determined by counting the number of fresh corpora lutea (CL). All antral follicles greater-than-or-equal-to 2 mm in diameter were dissected to assess antral follicle populations. Granulosa and thecal cells from the three largest follicles and CL from each heifer were collected for FSH and LH binding measurements. All heifers had a single ovulation. The treated heifers had significantly more antral follicles (60.2 +/- 6.7) than did the animals in the control group (33.2 +/- 3.2) (p < 0.001). When follicles were grouped according to diameter, the mean numbers of follicles > 10 mm, 5-10 mm, and 2-5 mm in diameter were 0.8 +/- 0.2, 6.8 +/- 1.4, and 52.5 +/- 6.5 for the treated group, and 0.8 +/- 0.2, 6.5 +/- 1.0, and 25.8 +/- 2.7 for controls. The treated group had significantly higher peripheral concentrations of GH and IGF-I throughout the treatment period (p < 0.01), but there were no significant differences in peripheral estradiol and progesterone concentrations (p > 0.05). FSH and LH concentrations throughout the treatment period and LH pulse frequency and pulse amplitude during the mid-luteal and follicular phases of the estrous cycle also showed no significant differences between the two groups (p > 0.05). There was no effect of treatment on FSH or LH binding to granulosa and thecal cells or on LH binding to CL. In conclusion, BST treatment increased the population of antral follicles 2-5 mm in diameter in mature heifers. This effect did not appear to be mediated through changes in circulating gonadotropin concentrations or gonadotropin receptor levels. Although it is possible that BST may act via increased peripheral IGF-I concentrations, a direct effect of BST at the ovarian level cannot be excluded.