Placental lactogen and somatotropin: Hormone binding to the corpus luteum and effects on the growth and functions of the ovary in heifers

M. C. Lucy, J. C. Byatt, T. L. Curran, D. F. Curran, R. J. Collier

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Abstract

The effects of recombinant bovine placental lactogen (rbPL) and recombinant bovine somatotropin (rbST) on development of ovarian follicles and CL were tested in heifers. Estrus (day = 0) was synchronized and heifers were treated (Days 0-21) with either saline (control; n = 7), rbST (25 mg/day; n = 6), or rbPL (50 mg/day; n = 8). Blood was collected daily for analyses of progesterone, estradiol, ST, PL, and insulin-like growth factor (IGF)-I; ultrasound was performed daily for measurement of follicles and CL PL in plasma (mean ± SE; ng/ml) averaged 4.1 ± 0.2 for rbPL-treated heifers, and ST in plasma (ng/ml) averaged 2.7 ± 0.3 for rbST-treated heifers. IGF-I in plasma (ng/ml) was increased for rbST-treated (198 ± 10; p < 0.001) and rbPL treated (143 ± 9; p < 0.06) heifers compared to controls (117 ± 9). After Day 9 of the estrous cycle, heifers treated with rbPL had larger CL (p < 0.001) and more progesterone in plasma (p < 0.001) than controls, whereas rbST-treated heifers were intermediate for these measures. Largest follicles were decreased in size (mm) throughout the estrous cycle for rbPL-treated heifers (12.9 ± 0.4) compared to controls (14.2 ± 0.5; p < 0.06) or heifers given rbST (14.0 ± 0.5; p < 0.11). After Day 17 (preovulatory period), concentrations of estradiol in serum (pg/ml) were decreased for rbST-treated (2.7 ± 0.3; p < 0.01) and rbPL-treated (2.9 ± 0.2; p < 0.02) heifers compared to controls (3.8 ± 0.3). Size of second largest follicles (mm) was increased for heifers given rbST (10.0 ± 0.5) compared to controls (8.2 ± 0.5; p < 0.02) or to heifers given rbPL (8.6 ± 0.4; p < 0.06), and timing of the second follicular wave was earlier for animals given rbST and rbPL compared to controls. Estrous cycle length (days) averaged 20.4 ± 0.8, 21.3 ± 0.8, and 21.5 ± 0.7 for the control, rbST, and rbPL groups, respectively (p > 0.10). From these studies there appeared to be a specific effect of rbPL on development of the CL. Microsomal membranes were prepared from CL tissue to determine whether there were specific binding sites for rbPL. Specific binding of [125I]-rbPL to CL membranes averaged 8.44%, and membrane preparations displayed high-affinity binding sites for rbPL (dissociation constant = 1.0 x 10-11 M; total number of binding sites = 2.2 fmol/mg protein). Specific binding of radiolabeled rbST or recombinant bovine prolactin (rbPRL) was < 0.1% for either ligand. In conclusion, rbPL was stimulatory to the CL and inhibited follicular function in heifers. Responses of the ovary to rbPL were not identical to those for rbST. Therefore, changes in ovarian function during pregnancy may be explained partially by secretion of bPL from the placenta.

Original languageEnglish (US)
Pages (from-to)1136-1144
Number of pages9
JournalBiology of reproduction
Volume50
Issue number5
DOIs
StatePublished - Jan 1 1994

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ASJC Scopus subject areas

  • Reproductive Medicine
  • Cell Biology

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