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Anovulation in Postpartum Suckled Beef Cows. II. Associations among Binding of ¹²⁵I-Labeled Gonadotropins to Granulosa and Thecal Cells, and Concentrations of Steroids in Serum and Various Sized Ovarian Follicles^1,2,

Michigan State University^,8, East Lansing 48824 and U.S. Department of Agriculture^,6, Miles City, MT 59301

Abstract

To determine if specific binding of ¹²⁵I-labeled gonadotropins to granulosa and thecal cells, or concentrations of steroids in ovarian follicles change during the postpartum anovulatory period, 21 suckled beef cows were slaughtered on d 7, 14, 28, 42 or 56 after parturition (n=4 to 6 per d). After slaughter, 10 to 15 follicles were dissected from each pair of ovaries and categorized by diameter: small (1.0 to 3.9 mm), medium (4.0 to 7.9 mm) or large (8 mm). Progesterone (221 to 612 ng/ml), androstenedione (48 to 94 ng/ml) and estradiol (2.7 to 23.9 ng/ml) did not change (P>.10) in fluid of small or medium follicles from d 7 to 42 to 56 after parturition. Similarly, specific binding of human chorionic gonadotropin (¹²⁵I-hCG) or follicle stimulating hormone (¹²⁵I-oFSH) to homogenates of small, medium or large follicles did not change (P>.05). In contrast, progesterone in fluid of large follicles increased (P<.05) 3.4-fold between d 7 and 14, but decreased (P<.05) 55% between d 14 and 28. Concentrations of androstenedione in fluid of large follicles did not change (P>.10) from d 7 to 42 to 56. Concentrations of estradiol in fluid of large follicles remained constant between d 7 and 14, but increased (P<.05) 4.2-fold between d 14 and 28. We conclude that during the postpartum anovulatory period, there is no change in steroidogenic capabilities of small or medium follicles, both of which predominantly produce progesterone. In contrast, a shift from progesterone to estradiol production occurred in large follicles without coincident changes in luteinizing hormone or FSH binding. This increase in concentration of estradiol in fluid of large follicles may be an essential step toward resumption of ovulatory cycles in postpartum, suckled beef cows.

¹ Michigan Agr. Exp. Sta. Journal Article No. 11597. This research was supported in part by a grant from The Upjohn Company.

² We thank Steven Lyth and Trudy Hughes for technical assistance; Dr. J. D. Gunther, Dept. Large Anim. Clin. Sci., for monitoring health status of experimental animals; Dr. M. R. Sairam, Montreal, Quebec, Canada for supplying highly purified oFSH; Dr. G. D. Niswender, Colorado State Univ., for supplying antiserum to androstenedione (=866); the Natl. Pituitary Agency, NIAMDD, for supplying hCG (CR-119) and various NIH bovine hormone preparations; and Murco, Inc., Plainwell, MI for donation of bovine ovaries.

³ M. S. Hershey Med. Center, Pennsylvania State Univ., Hershey, PA 17033.

⁴ Merck, Sharp and Dohme Res. Lab., Merck and Co., Inc., Rahway, NJ 07065.

⁵ Reprod. Endocrinol. Program, Univ. of Michigan, Ann Arbor, MI 48109.

⁶ LARRS, Miles City, MT 59301.

⁷ Reprint requests.

⁸ Dept. of Anim. Sci.