ASSOCIATIONS AMONG PROGESTERONE, ESTRADIOL-17-BETA, OXYTOCIN AND PROSTAGLANDIN IN CATTLE TREATED WITH HCG DURING DIESTRUS TO EXTEND CORPUS-LUTEUM FUNCTION

Treatment of cattle during the middle of the middle of the luteal phase with appropriate doses of human chorionic gonadotropin (hCG) causes a 5 d extension of the estrous cycle. Three experiments were conducted to determine how treatment with hCG affected the pattern of secretion of prostaglandin F2α, as indicated by blood levels of 13,14- dihydro-15-keto-prostaglandin F2α (PGFM]). In experiment 1, Holstein cows were given saline (Sal) or hCG (10000 IU, im) on d 10 of the estrous cycle and blood samples were collected over a 6 h period on d 14 and 18 during which oxytocin (10 and 100 IU, iv) was given at 2 and 4 h. Concentrations of PGFM before and after oxytocin were similar between Sal and hCG-cycles, but PGFM was higher on d 18 than 5 14 (P < 0.05). In experiment 2, episodic PGFM was measured from 16 to 20 in cows given Sal or hCG on d 10. There was tendency for hCG to reduce PGFM baseline and pulse amplitude (P=0.22). In experiments 1 and 2, estradiol increased during d 16 to 20 of Sal-cycles, but did not change during this period of hCG-cycles. Therefore, in experiment 3, Holstein heifers were given Sal or hCG (5000 IU, im) on d 10, followed by corn oil (Oil) or estradiol benzoate (EB; 200 μg, im, 2X/day) on d 15 to 18. No difference in progesterone secretion was observed between Sal-Oil and Sal-EB heifers; however, EB hastened luteolysis in hCG-treated heifers (P < 0.05), without causing an increase in PGFM. Although subtle differences were seen in pulsatile PGFM, we conclude that hCG altered the pattern of estrogen secretion, and this led to delayed luteolysis. © 1990.