Two Effects of Estradiol That Normally Contribute to the Control of Tonic LH Secretion in the Ewe¹

Two effects of estradiol on tonic LH secretion in the ewe were observed when the steroidal milieu of the estrous cycle was reproduced in the absence of the ovaries by the administration of estradiol and progesterone in Silastic implants. One effect of estradiol was revealed in a model corresponding to the follicular phase, namely following a precipitous decrease in circulating progesterone. Under these conditions, estradiol diminished the magnitude of the increase in tonic LH secretion initiated by progesterone withdrawal. Importantly, basal estradiol levels were fully effective in this regard; increments above basal concentrations produced no further significant inhibition of LH. The second effect of estradiol was revealed under conditions resembling the luteal phase, namely in the presence of graded increases in serum progesterone. Estradiol greatly enhanced the ability of all doses of progesterone to suppress LH. Again, higher estradiol concentrations did not appear to be more effective in this regard than basal concentrations.