The influence of estradiol-17β and progesterone on serum levels of LH and FSH in the EWE before and after GnRH treatment

Abstract

The interaction of estradiol and progesterone in controlling gonadotropin secretion was investigated in cyclic and anestrous ewes. The peak response of ewes to 200 ng of [D-Leu 6, des-Gly NH 2 10 proethylamide 9]-GnRH, in terms of serum LH concentrations, was greater on days 7 and 15 of the cycle (65.6 ± 23.60 ng/ml and 72.1 ± 14.82 ng/ml) than on days 2 and 11 (33.1 ± 11.12 ng/ml and 27.7 ± 6.3 ng/ml) (P<0.05). Peak FSH levels did not vary with the stage of the cycle (4.8 ± 0.25 ng/ml) (P>0.05). Using blank silastic rubber implants (no steroid) and various sizes of implants containing estradiol, serum estradiol concentrations of 20.6 ± 2.8 pg/ml, 33.0 ± 1.9 pg/ml, 37.7 ± 1.6 pg/ml, or 48.4 ± 1.8 pg/ml were produced in 4 groups of mid cycle ewes (days 7–9). Three of 5 ewes exhibiting the highest estradiol levels experienced a transient peak in serum concentrations of LH and FSH 24 ± 6 hr after implant insertion (LH 19.1 ± 3.84 ng/ml, FSH 34.3 ± 11.13 ng/ml). These 3 ewes subsequently underwent luteolysis. Estradiol treatment at mid cycle enhanced the response to the GnRH analog (P<0.05). Peak levels of LH and FSH after GnRH administration were 15.4 ± 4.66 ng/ml and 10.0 ± 3.93 ng/ml in control ewes; 44.0 ± 11.54 ng/ml and 32.4 ± 5.18 ng/ml in ewes treated with 1.5 × 0.24 cm implants; 26.3 ± 4.89 ng/ml and 23.0 ± 4.67 ng/ml in ewes treated with 3.0 × 0.24 cm implants; and 22.8 ± 4.99 ng/ml and 34.2 ± 8.87 ng/ml in ewes treated with 3.0 × 0.48 cm implants. Treatment of anestrous ewes (June) with progesterone, estradiol or progesterone and estradiol, depressed the peak response in serum FSH concentrations (6.2 ± 2.94 ng/ml) to the GnRH analogue as compared to ewes not given any steroid (16.3 ± 2.37 ng/ml) (P<0.05). The steroid treatments only depressed the LH response during the first hour after GnRH administration (P<0.05) (steroid treated ewes 5.7 ± 1.58 ng/ml, control ewes 18.4 ± 3.14 ng/ml). In conclusion, the response of serum levels of LH, but not FSH, to GnRH administration varied with the stage of the cycle and was influenced by both estradiol and progesterone. In anestrous ewes both steroids inhibited the response to GnRH. In general, a divergency in control mechanisms for LH and FSH is indicated.