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Effects of Estradiol-17β on LH-RH/FSH-RH-Induced, and Spontaneous, LH Release in Prepubertal Female Pigs¹

Animal Genetics Laboratory, University of Illinois Urbana Illinois 61801

Abstract

The effect of estrogen (E₂) on pituitary response to LH-RH/FSH-RH (LH-RH) was examined in immature female pigs (gilts). Eight gilts (9–10weeks old) received iv injections of 2.5, 5.0, 10.0, 20.0, and 40.0 µg 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 (LH). Pretreatment LH levels were 0.45–1.25 ng/ml. Responses to LH-RH (ALH ± SE) ranged from 3.7 ± 0.6 (at 5 min) to 8.4 ± 1.4 ng/ml (at 20 min) for 2.5 and 40 µ;g 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 ± 34 pg/ml and the dose-response studies were repeated. Prestimulation LH concentration was consistently below 0.4 ng/ml and LH levels ranged from 1.4 ± 0.5 (at 10 min) to 5.7 ± 1.3 ng/ml (at 20 min) for the 2.5 and 40.0 µg doses, respectively. Analysis of the regression equations for LH on log LH-RH dose showed that the adjusted means for LH response were significantly lower (P < 0.02) in E₂ implanted animals. However, the regression coefficients were not different (P > 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 µ.g E² iv and then sampled for 12 h, episodic secretion was suppressed and the latency to the first spontaneous LH release was 9.6 ± 0.8 h, significantly greater (P < 0.02) than the latency to the first peak after saline injections (4.2 ± 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. (Endocrinology 96: 551, 1975)

Footnotes

¹ This study was supported by grants NIH HD 3043 and the Ford Foundation.

² Dept. of Physiology and Environmental Studies, University of Nottingham School of Agricultural Sciences, Sutton Bonington, Loughborough, Leicestershire, LE12 5RD, England.

³ Dept. of Physiology, The University of Western Ontario, London, Ontario, Canada N6A 5C1.

⁴ To whom reprint requests should be sent.

Received May 29, 1974.