Induction of Ovulation and Normal Luteal Function by Pulsed Injections of Luteinizing Hormone in Anestrous Ewes

Abstract

To determine whether failure of the final stages of follicular maturation during anestrus in sheep was due to inadequate LH stimulation, two experiments were carried out in midanestrus, 1 yr apart, in six Finn-Merino ewes with an ovary or an ovary and uterus autotransplanted to the neck. In the first experiment, iv injections of ovine LH were given at a rate (one injection per 3 h for 24 h, one per 2 h for 24 h, and one per hour for 24 h) equivalent to the natural increase in frequency of LH pulses which occurs in the preovulatory period of the estrous cycle during the breeding season. In the second experiment, the frequency of LH pulses was kept constant at one per 3 h for 72 h. Each injection resulted in a pulse of LH which was qualitatively and quantitatively similar to an endogenous LH pulse. Jugular and ovarian venous samples were collected and assayed for LH, FSH, PRL, 17β-estradiol, and progesterone. An increase in the secretion of 17β-estradiol, a preovulatory LH peak, and normal corpus luteum function were induced in all four ewes in the first experiment and in two of the four ewes in the second experiment. In these animals, the pattern of secretion of LH, FSH, 17β-estradiol, and progesterone was similar to that occurring spontaneously in the estrous cycle. In both experiments, the concentration of PRL remained elevated at a level consistent with midanestrus. It is concluded that 1) FSH levels are adequate for, and raised PRL levels do not directly interfere with, follicular development in anestrus; 2) an increase in the number of pulses of small physiological amounts of LH alone is sufficient to induce follicular maturation in anestrus, resulting in ovulation and formation of a corpus luteum with normal function; and 3) the failure of normal preovulatory follicular development in the anestrous ewe is due mainly to an inadequate frequency of the pulsatile release of LH.