Abstract
The effects of human chorionic gonadotropin (hCG) on plasma progesterone (P) levels, size distribution of luteal cells and [ 3H]prostaglandin F 2α ([ 3H]PGF 2α) binding to luteal membranes were determined. In Experiment 1, 8 heifers were randomly assigned at synchronized oestrus to either receive 2500 IU hCG 48 h postoestrus, or remain as controls. Blood samples were taken at 6 h intervals from oestrus until Day 7, when a biopsy of luteal tissue was obtained by laparotomy. In Experiment 2, 16 heifers were assigned randomly at oestrus to one of four groups in a 2 × 2 design with hCG treatment as one factor and day of biopsy (Day 11 or Day 13) as the other. Administration of hCG was the same as in Experiment 1. Blood samples were collected at 12 h intervals from oestrus until luteal biopsy. In Experiment 1, P concentrations were lower ( P < 0.05) in hCG-treated animals than controls at 42, 66, 72, and 84 h post-hCG. In contrast, P concentrations were higher in the hCG-treated heifers than controls at 114 h post-hCG (4.47 ± 0.74 vs 3.41 ± 0.52 ng/ml; mean ± s.e.m.). In Experiment 2, P levels were higher ( P<0.05) in hCG-treated heifers than controls from 96–192 h post-hCG. The proportion of large luteal cells (>25 μm) on Day 7 was lower in the hCG-treated heifers than controls (17.8±4.7% vs 29.0 ± 3.9%; mean ± s.e.m.). The small luteal cells of control heifers on Day 7 had a greater ( P < 0.05) diameter than those of hCG-treated heifers (13.7 ± 0.2 vs 11.9 ± 0.7 μm; mean ± s.e.m.). No differences were observed in the percentage of large cells between treatments on Day 11 or Day 13. The large luteal cells of control heifers on Day 13 had a greater ( P < 0.05) diameter (50.9 ± 1.4 vs 44.5 ± 1.9 μm) than those of hCG-treated animals. Binding of [ 3H]PGF 2α to Day 7 (Experiment 1) luteal membranes was lower ( P < 0.05) in control than hCG-treated heifers (16.3 ± 6.9 vs 54.1 ± 5.3 femtomoles/mg of membrane protein; mean ± s.e.m.). Differences in [ 3H]PGF 2α binding to luteal membranes and size distribution of luteal cells were most apparent during early dioestrus (Day 7) while the stimulatory effects of hCG on P secretion did not occur until 120 h after hCG treatment.
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