The effects of recombinant bovine somatotrophin (BST) on the dynamics of ovarian follicular growth and development and peripheral insulin concentrations were investigated. Initially, studies were carried out in a population of Hereford × Friesian heifers to validate the ultrasound technique. In the first experiment, 12 heifers were injected daily with either 25 mg BST or vehicle for two oestrous cycles, and the effects on follicular dynamics and peripheral insulin were determined. In a second experiment, 12 heifers were given a single injection of 10 ml saline or 320 mg BST in a sustained-release formulation to examine the temporal relationships between growth hormone (GH), insulin-like growth factor-1 (IGF-1), insulin and the number of follicles. The validation studies demonstrated that small follicles (<5 mm in diameter) could be clearly detected by real-time ultrasound, and that 75% (9 of 12) of heifers showed three waves of dominant follicle development during the oestrous cycle, whereas the remainder had only two waves. The changes in the numbers of follicles of the three size categories (<5 mm, small; 5−10 mm, medium-sized and > 10 mm, large) also displayed a wave pattern similar to that of the dominant follicle, with a marked reduction in the number of subordinate follicles as the dominant follicle grew and reached its maximum size. In Expt 1, BST treatment increased the number of small follicles and caused a rise in peripheral insulin concentrations (P < 0.01) throughout the treatment period. However, there was no effect of BST on the timing for the pattern of follicular waves during the oestrous cycle, nor on the number of medium-sized or large follicles at each follicular wave. In the second experiment, the temporal change in the number of small follicles following BST treatment was positively correlated with the changes in the peripheral IGF-1 and insulin concentrations. IGF-1 and insulin concentrations increased 48 h after BST injection, and the number of small follicles had increased 24 h later and remained higher during the period when peripheral IGF-1 and insulin concentrations were high. These results demonstrate first, that the number of small follicles was reduced as the dominant follicle grew and reached its maximum size and second that BST treatment could enhance the recruitment of small follicles in heifers. This increase in the number of small follicles was positively correlated with peripheral IGF-1 and insulin concentrations. Third, BST did not affect the turnover of follicular waves, nor the inhibitory action of the dominant follicle on its subordinate follicles. We conclude that BST affects the recruitment of small follicles by increasing peripheral concentrations of IGF-1, or of insulin or both concentrations. Moreover, the effect of BST on the small follicle population was not mediated through the mechanism(s) by which the dominant follicle inhibits subordinate follicles.
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