Hourly administration of luteinizing hormone induces ovulation in prepubertal female sheep

This investigation examined the effects of repeated injections of LH on ovarian function in the immature sheep approximately 12 weeks before the time of the first expected spontaneous ovulation. The frequency of endogenous LH pulses during the pretreatment period was approximately one pulse each 3 h. The first experiment determined that rapid injection (iv) of 15.5 micrograms LH replicated the amplitude of endogenous pulses. Hourly injection of this dose for 48 h to simulate the rapid LH pulse frequency of the follicular phase of the postpubertal female induced a LH surge and ovulation in two of three lambs. By contrast, administration of 33% of the dose over the 48-h period did not [5.2 micrograms/h (three lambs) or 15.5 micrograms each 3 h (three lambs)]. The second experiment (seven lambs) determined the time course of the preovulatory estradiol rise produced in response to hourly LH pulses (15.5 micrograms/injection), as well as the length of the luteal phase after the induced LH surge. Four lambs produced a sustained estradiol rise, a LH surge, and ovulation. The luteal phase was normal (13 days) in one and short in three lambs (6-11 days). In the remaining three prepubertal females that did not ovulate in response to 48-h injections of LH, the estradiol rise was not sustained. Circulating estradiol in five untreated control lambs exhibited only transient increases during the course of the study. The results indicate that hourly administration of physiological quantities of LH over a relatively brief period (48 h) can produce a follicular phase culminating in first ovulation in the immature lamb. In the context of the mechanism proposed for puberty in the female sheep, the findings are consonant with the hypothesis that the hypothalamus, through its modulation of LH pulse frequency, governs the initiation of ovulation.