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Pituitary Concentrations of Gonadotropins and Receptors for GnRH in Suckled Beef Cows at Various Intervals after Calving^1,2,3,

Purdue University⁴, West Lafayette, IN 47907

Abstract

Mature beef cows were slaughtered at 5 (n = 6), 10 (n = 6), 20 (n = 6) or 30 (n = 5) d after calving to identify endocrine events that may affect the duration of postpartum anestrus. Additional cows (n = 6) were slaughtered 12 to 14 d after their first postpartum estrus (luteal phase cows). Anterior pituitary concentrations of luteinizing hormone (LH) were low at d 5 (383 ± 69 µg/g), averaged 445 ± 103 and 682 ± 207 µg/g at d 10 and 20, respectively, and were elevated (P<.05) by d 30 (1,097 ± 174 µg) to a concentration similar to luteal phase cows (1,208 ± 148 µg/g). Concentrations of follicle-stimulating hormone (FSH) averaged 12.4 ± 1.1, 9.6 ± 2, 8.6 ± 1.8 and 7.4 ± 3.3 mg/g at d 5, 10, 20 and 30, respectively. Affinity (1.6 ± .2 x 10⁹ M^–1) of anterior pituitary receptors for the GnRH (gonadotropin-releasing hormone) analog (DAla⁶; des-Gly¹⁰, [D-Ala⁶]-LH-RH ethylamide) and weights (2.1 ± .1 g) of the anterior pituitaries did not differ among groups (P>.05). Number of receptors for GnRH averaged 37 ± 7, 39 ± 9, 25 ± 5 and 23 ± 5 x 10^–14 M/mg protein at d 5, 10, 20 and 30, respectively. Anterior pituitaries from luteal phase cows contained 22 ± 2 x 10^-14 M/mg protein of receptors for GnRH. Anterior pituitary cells prepared from d 5, 10, 20 and 30 cows did not differ (P>.05) in total quantity of LH, amount of LH released into media in the absence of GnRH, or the amount of LH released in response to GnRH. In contrast, anterior pituitary cells prepared from luteal phase cows contained more LH and released more LH (P<.05) in response to GnRH than d 5 cows, but did not differ from d 10, 20 or 30 cows (P>.05). Content of GnRH in hypothalamus proper, preoptic area and pituitary stalk-median eminence did not differ between groups and averaged 4.8 ± .3, 3.4 ± .2 and 89 ± 5 ng, respectively. These data indicate that anterior pituitary concentrations of FSH, hypothalamic concentration of GnRH and numbers of anterior pituitary receptors for GnRH likely are not deficiencies at 5 d after calving. In addition, the ability of pituitary cells to synthesize and release LH was similar to luteal phase cows after 10 d postpartum. The replenishment of pituitary stores of LH may represent one of the initial limitations to the reestablishment of reproductive competence after calving.

¹ Journal Paper No. 9835, Purdue Univ. Agr. Exp. Sta.

² Presented in part at the 75th Annu. Meet. of the Amer. Soc. of Anim. Sci., Pullman, WA, Abstr. #560. The authors gratefully acknowledge J. Rivier, The Salk Institute, San Diego, CA, for the gift of the GnRH Analog (#89-20-25); J. M. Stewart, Univ. of Colorado, for the inhibitory analog of GnRH; G. D. Niswender, Colorado State Univ., for supplying antiserum to progesterone and LH; D. J. Bolt, USDA, Beltsville, MD, for supplying reagents for the FSH radioimmunoassay; T. M. Nett, Colorado State Univ., for supplying antiserum to GnRH; N. R. Mason, Eli Lilly Co., for providing antiserum to estradiol-17β and the National Pituitary Agency for providing iodination grade LH and GnRH.

³ Supported in part by USDA Grant No. 59-2181-1-2-026-0.

⁴ Dept. of Anim. Sci.