Roles of pattern of secretion of luteinizing hormone and the ovary in attainment of puberty in ewes lambs

Abstract

Four experiments were conducted to examine (1) how patterns of luteinizing hormone (LH) change as lambs approach first estrus, (2) whether mimicry of these changes by exogenous LH will initiate events of the pubertal process, (3) aspects of possible roles of the ovary and estradiol in the initiation of an ovulatory surge of LH in prepubertal lambs and (4) the time requirements for the presence of the ovary in induction of that surge of LH. In 19 lambs (Exp. 1), concentrations of LH in plasma were higher at puberty than 7 weeks earlier. Increased concentrations of LH in samples taken every 20 min for 6 hr during the luteal and follicular phases preceding first estrus were attributed to increased amplitude, but not increased frequency, of episodic pulses of LH. In Exp. 2, endocrine events which are included in the natural onset of ovarian cyclic activity (i.e. surge of LH and subsequent rise in progesterone) were initiated in 5 6 , 4 6 , 1 5 , 1 6 , and 3 6 lambs, receiving either injections of 7.5 μg/hr, 15 μg/hr, 30 μg/2hr, 45 μg/3hr or constant infusions of 15 μg/hr of purified ovine LH, respectively. However, these lambs did not exhibit estrus or cyclic ovarian activity. In Exp. 3, the mechanism by which hourly injections of LH prompted surges of LH was determined to be mediated through ovarian stimulation and not through a direct effect of exogenous LH on the hypothalamo-pituitary axis. All the intact (n=5) but no acutely (two weeks) ovariectomized (n=5) lambs had a preovulatory-like surge in response to exogenous LH. Further, (Exp. 4) it was shown that the ovarian signal must be maintained or evoked within 6 hr preceding the surge of LH, because lambs (n=20) ovariectomized at or 9, 18 or 27 hr after initiation of the exogenous LH with the one exception failed to show a surge of LH. It could not be demonstrated that the ovarian signal involved changes in peripheral concentrations of androstenedione, testosterone or estradiol-17β, although patterns of LH in ovariectomized lambs were responsive to both negative and positive feedback effects of estradiol (n=6, Exp. 3) and in general estradiol levels were increasing prior to the induced surge of LH.