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Luteinizing Hormone Secretion in Ovariectomized Gilts: Effects of Age, Reproductive State and Estrogen Replacement¹

US Department of Agriculture^,2, Clay Center, NE 68933 and and Iowa State University^,3, Ames 50011 Summary

Abstract

Crossbred gilts were ovariectomized (OVX) at 120, 150, 180 and 210 d of age to determine whether various characteristics of luteinizing hormone (LH) concentrations are influenced by age and reproductive state (prepuberal vs postpuberal). All 120-d-old gilts were prepuberal and all 210-d-old animals were postpuberal, whereas gilts 150 and 180 d old included both prepuberal and postpuberal animals. Blood was collected at 15-min intervals for 2 h, 2 d before OVX (d —2), and 2 (d +2), 8 (d +8) and 14 (d +14) d after OVX. Mean LH concentrations for prepuberal gilts were similar among age groups (P>.05) on d —2 and +2; however, LH increased (P<.05) from d —2 to +2. No change in LH secretion was found in postpuberal gilts during these two periods. After OVX, LH increased from d +2 to +14 in both prepuberal and postpuberal gilts in all age groups. In postpuberal gilts, LH increased linearly (P<.05) between d +2 and +14; rate of increase accelerated with advancing age (P<.01). In prepuberal gilts, LH increased in a nonlinear manner, but it did not increase between d +2 and +8. The increase observed in prepuberal and postpuberal gilts after OVX resulted primarily from an increase in magnitude of peak concentrations of LH. Implants of estradiol-17β (E₂) were used to determine whether the postovariectomy increase in LH is affected differently by E₂ in prepuberal and postpuberal gilts during advancing ages. Estradiol-17β suppressed mean LH concentrations (P<.05)in 120-d-old gilts; whereas in 150-d-old prepuberal gilts mean LH was suppressed (P<.05) on d +2 and +8, but then increased (P<.05) by d +14. In prepuberal gilts, E₂ implants suppressed (P<.05) both frequency and amplitude of LH peaks at 120 d old, but only frequency was suppressed (P<.05) in 150-d-old animals. In postpuberal gilts, 150 and 180 d old, E₂ did not alter (P>.05) either the postovariectomy increase in mean LH, or frequency and amplitude of LH peaks. Observations of the present study suggest that sensitivity of the hypothalamic-pituitary axis to ovarian and estrogen negative feedback regulation of LH secretion decreases as gilts progress from the prepuberal to the postpuberal state.

¹ This study was supported by the USDA, ARS, CSRS, Cooperative Agreement No. 58-519B-9-863. Journal Paper No. J-10840 of the Iowa Agr. and Home Econ. Exp. Sta., Ames, IA; Projects 2443 and 2444. Mention of trade names or companies does not constitute an implied warranty by the USDA or the authors. We thank J. Gray, J. Dague, Julie Smith and Ann Hultine for excellent technical assistance. This investigation was presented, in part, at the 14th Annu. Meeting of the Soc. for the Study of Reprod., Corvallis, OR, 1981, Abstr. 89.

² Roman L. Hruska U.S. Meat Anim. Res. Center, USDA, Agr. Res. Service.

³ Dept. of Anim. Sci.

⁴ Present address: Dept. of Anim. and Range Sci., Montana State Univ., Bozeman 59717.