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Secretion of Steroids after Induced Luteal Regression in Beef Heifers: Effects of PGF₂ and Removal of Corpora Lutea¹

West Virginia University, Morgantown 26506

Abstract

Regulation of the secretion of estradiol-17ß after exogenous prostaglandin F₂ (PGF₂) and the luteolytic potential of intrafollicular PGF₂ were examined in beef heifers. Hereford heifers were observed for estrus twice daily, and after two normal estrous cycles were assigned to randomized treatments on day 11 postestrus.

In Experiment I, the largest follicle on the ovary with the corpus luteum (CL) received a 50 µl injection of PGF₂ (600, 900, 1,200 µg), Tham buffer (TB) or TB plus removal of the CL (six heifers per group). Lengths of estrous cycles (days) of heifers treated with 600 µg PGF₂ (19.0 ± 1.5) or TB (20.0 ± .5) did not differ. Lengths of cycles for heifers receiving 900 (14.5 #x00B1; .5) and 1,200 µg PGFF₂ (15.0 ± .2) or TB plus removal of the CL (14.0 ± .5) were not different, but were shorter (P<.01) than estrous cycles in the other groups. Declines in the concentrations of systemic progesterone among heifers receiving 900 or 1,200 µg PGFF₂ did not differ from those in heifers receiving TB plus removal of the CL, but were not parallel to the profiles of systemic concentrations of progesterone in heifers receiving TB only (P<.01). Profiles of progesterone in heifers receiving 600 µg PGF₂ differed from those in heifers receiving either TB only or TB plus removal of the CL. Increasing concentrations of estradiol-17ß were observed only when concentrations of progesterone declined to low levels. Profiles of estradiol-17ß were parallel among heifers receiving luteolytic doses of PGFF₂ (900 or 1,200 µg) or TB plus removal of the CL, but differed from those in heifers receiving TB only. Changes in concentrations of luteinizing hormone (LH) were observed after concentrations of progesterone declined and before increases in concentrations of estradiol-17ß. Profiles of the concentrations of estrone did not vary among treatments.

In Experiment 2, the corpus luteum was removed from 12 heifers on day 11 postestrus. Six of the heifers were anesthetized with sodium pentobarbital from 4 to 10 hr after removal of the CL. Pentobarbital blocked the changes in concentrations of LH, compared to unanesthetized control heifers. However, the profiles of estradiol-17ß in heifers receiving pentobarbital were parallel to those observed in unanesthetized heifers. It is suggested that increases in the concentrations of estradiol-17ß occur in response to withdrawal of progesterone without any detectable direct influence of exogenous PGF₂. Until 24 hr after removal of the CL these increases in estradiol occurred independently of changes in the concentrations of LH.

¹ Published with the approval of the Director of the Agricultural Experiment Station as Scientific Paper No. 1508. Supported by Hatch Project 224 (NE-72). Division of Animal and Veterinary Sciences and Department of Obstetrics and Gynecology. PGFaa for injection was a gift from Drs. J. E. Pike and J. W. Lauderdale, The Upjohn Company, Kalamazoo, MI.

² Present address: Animal Reproduction Laboratory, Department of Dairy Science, Michigan State University, East Lansing 48824.

³ Present address: Division of Agriculture, Arizona State University: Tempe 85281.