Effects of estradiol-17beta on LH-RH/FSH-RH-induced, and spontaneous, LH release in prepubertal female pigs.

The effect of estrogen (E2) on pituitary response to LH-RH/FSH-RH (LH-RH) was examined in immature female pigs (gilts). Eight gilts (9-10 weeks old) received iv injections of 2.5, 5.0, 10.0, 20.0, and 40.0 mug LH-RH, beginning 4 days after placement of indwelling jugular cannulae. Blood samples were drawn at 5-min intervals following LH-RH injection and plasma LH was determined by RIA and expressed as ng R786-3/ml. Pituitary responses were expressed as the increment in LH levels above baseline (delta LH). Pretreatment LH levels were 0.45-1.25 ng/ml. Responses to LH-RH (delta LH plus or minus SE) ranged from 3.7 plus or minus 0.6 (at 5 min) to 8.4 plus or minus 1.4 ng/ml (at 20 min) for 2.5 and 40 mug doses, respectively. Three additional prepubertal gilts were cannulated and implanted with E2. After 3 days plasma estrogens rose from approximately 30 pg/ml to 289 plus or minus 34 pg/ml and the dose-response studies were repeated. Prestimulation LH concentration was consistently below 0.4 ng/ml and delta LH levels ranged from 1.4 plus or minus 0.5 (at 10 min) to 5.7 plus or minus 1.3 ng/ml (at 20 min) for the 2.5 and 40.0 mug doses, respectively. Analysis of the regression equations for delta LH on log LH-RH dose showed that the adjusted means for LH response were significantly lower (P less than 0.02) in E2 implanted animals. However, the regression coefficients were not different (P greater than 0.5). On the basis of these and other analyses, it is concluded that chronic E2 treatment depresses both basal LH secretion and pituitary responsiveness, but that pituitary sensitivity to LH-RH is not modified. Several nonimplanted females showed bursts of LH release subsequent to the LH-RH induced peak, but multiple peaks were never observed in E2 implanted gilts. An explanation was sought and 7 of the nonimplanted gilts were used to study spontaneous LH release by determining plasma LH at 10-min intervals for 20 h. During this period spontaneous episodes of LH release were observed in control animals with a mean frequency of 1.3 LH peaks/h. When 4 animals were given 5 mug E2 iv and then sampled for 12 h, episodic secretion was suppressed and the latency to the first spontaneous LH release was 9.6 plus or minus 0.8 h, significantly greater (P less than 0.02) than the latency to the first peak after saline injections (4.2 plus or minus 1.5 h) in the controls. It is suggested that "multiple" LH peaks occurring after initial LH-RH stimulation may represent the superimposition of spontaneous episodic release upon the induced response.