The negative feedback effect of estradiol-17beta on secretion of luteinizing hormone in beef cows.

Thirty-two ovariectomized cows were used to determine the time course for the negative feedback effect of estradiol-17beta (E) on secretion of the luteinizing hormone (LH). The cows were injected with gonadotropin releasing hormone (GnRH; 40 mug) 2.5 or 5 h after pretreatment with E (1 mug/kg body weight) or with a vehicle for control (C). Pretreatment with E resulted in lower serum concentrations of LH at 2.5 h (0.27 vs 0.90 ng/ml; P < 0.01) and at 5 h (0.27 vs 0.67 ng/ml; P < 0.01); less LH was released in response to GnRH at 2.5 h after treatment compared to cows treated with C (10 +/- 4.9 vs 27 +/- 3.8 ng/ml; P < 0.001). However, when GnRH was administered 5 h after E or C, there was no difference in the total amount of LH released (34 +/- 1.8 vs 26 +/- 4.4 ng/ml; P > 0.2). Time to half area (estimate of decay for the induced surge of LH) was longer for cows treated with E when compared to those treated with C (1.3 vs 0.9 h, P < 0.001; 1.5 vs 0.8 h, P < 0.001). Time to half area was not affected by the time of administration of GnRH after E (P > 0.4). These results suggest that E acts in the pituitary to cause the initial decrease in concentrations of LH. Pituitaries in animals pretreated with E regained the capacity to release as much LH at 5 h after treatment as those treated with C at a time when LH concentrations were still suppressed by E. Thus, the hypothalamus or an extra-hypothalamic area may be involved in maintaining the suppression of LH secretion after the initial effect on the pituitary has declined.