PLACENTAL-LACTOGEN AND SOMATOTROPIN - HORMONE-BINDING TO THE CORPUS-LUTEUM AND EFFECTS ON THE GROWTH AND FUNCTIONS OF THE OVARY IN HEIFERS

The effects of recombinant bovine placental lactogen (rbPL) and recomb inant bovine somatotropin (rbST) on development of ovarian follicles a nd CL were tested in heifers. Estrus (day = 0) was synchronized and he ifers were treated (Days 0-21) with either saline (control; n = 7), rb ST (25 mg/day; n = 6), or rbPL (50 mg/day; n = 8). Blood was collected daily for analyses of progesterone, estradiol, ST, FL, and insulin-li ke growth factor (IGF)-I; ultrasound was performed daily for measureme nt 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 increa sed 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 rb ST-treated heifers were intermediate for these measures. Largest folli cles 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) wer e 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). Siz e of second largest follicles (mm) was increased for heifers given rbS T (10.0 +/- 0.5) compared to controls (8.2 +/- 0.5; P < 0.02) or to he ifers given rbPL (8.6 +/- 0.4; P < 0.06); and timing of the second fol licular wave was earlier for animals given rbST and rbPL compared to c ontrols. 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, respectiv ely (P > 0.10). From these studies there appeared to be a specific eff ect of rbPL on development of the CL. Microsomal membranes were prepar ed from CL tissue to determine whether there were specific binding sit es for rbPL. Specific binding of I-125-rbPL to CL membranes averaged 8.44%, and membrane preparations displayed high-affinity binding site s for rbPL (dissociation constant = 1.0 x 10(-11) M; total number of b inding sites = 2.2 fmol/mg protein). Specific binding of radiolabeled rbST or recombinant bovine prolactin (rbPRL) was < 0.1% for either lig and. In conclusion, rbPL was stimulatory to the CL and inhibited folli cular function in heifers. Responses of the ovary to rbPL were not ide ntical to those for rbST. Therefore, changes in ovarian function durin g pregnancy may be explained partially by secretion of bPL from the pl acenta