Puberty in Beef Heifers. I. The Interrelationship between Pituitary, Hypothalamic and Ovarian Hormones

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Abstract
Blood serum concentrations of LH, FSH, GnRH, Prolactin, progesterone and estradiol-17ß were measured by radioimmunoassay in samples collected from six half-sib, Angus heifers before and at attainment of puberty. Blood samples were collected daily at 6-hr intervals and once a week at 20-min intervals for 4 hours. Pituitary and hypothalamic hormones were analyzed in each sample; steroid hormones were measured in daily pools. The period of collection started 30 to 64 days before the First preovulatory peak of LH (day 0) and continued until the first mid-cycle. There were no marked changes in FSH, prolactin or GnRH concentrations as puberty approached or during the first cycle. Levels of estradiol-17ß were high prior to day 40, then decreased gradually for 3 to 4 days to a level that was rather constant through the remainder of the sampling period. There was no elevation of estradiol-17ß associated with the LH peaks. Prolactin was negatively correlated with estradiol-17ß (r = —.26) and GnRH (r = —.31) during the prepuberal period, and these negative correlations disappeared after day 0. Prolactin and FSH were positively correlated prior to (r = .15) and after day 0 (r = .63). The prepuberal period was characterized by levels of LH that fluctuated markedly with an average concentration higher than that observed after day 0. Besides pubertal peak of LH (day 0), each heifer showed another peak of LH (priming) of similar duration and magnitude between days —11 to —9. Progesterone levels were very low (300 pg/ml) in the prepuberal period, but there were two distinct elevations in every heifer before day 0. The return of the first elevation to baseline levels was always followed by the priming peak of LH, while the second preceded the pubertal peak of LH. The profile of levels of LH between the two major LH peaks, coincident with the second progesterone elevation, appeared as a transition between prepuberal and postpuberal LH baseline level. This suggested that progesterone plays a key role in the changes leading to the establishment of the phasic LH release characteristic of the cyclic bovine female. Pituitary and hypothalamic hormones were released in bursts. LH bursts appeared of larger magnitude and lower frequency than those of the other hormones. The frequency of LH bursts did not diminish after puberty, but their magnitude was reduced during the mid-cycle. Copyright © 1975. American Society of Animal Science . Copyright 1975 by American Society of Animal Science.

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