The effect of human chorionic gonadotropin on the development and function of bovine corpus luteum

Abstract

Proceeding with research into human chorionic gonadotropin (hCG) administration, designed to promote pregnancy rates in embryo transfer, hCG was administered to bovine recipients on the day of ovulation in Holstein cows and the day of selection of recipients, ie., day prior to embryo transfer (5 days after the day of ovulation), and the changes in ovaries and the concentrations of plasma progesterone (P) and estradiol-17β (E2) levels after administration were determined. Changes in the ovaries and functions of blood P and E2 levels after administration were determined in order to investigate the effects of hCG on increase in luteinization and functions of P secretion. Nine Holstein dairy cows were divided into two groups. All 4 cows of one group received an intramuscular injection of saline at a dose of 5 ml on the day of ovulation (Day 0-control group: the day of ovulation was designated as Day 0), which was followed by an injection of hCG at a dose of 1,500 IU on the corresponding day of the next estrous cycle (Day 0-hCG group). In a similar way, 4 cows in the other group received an injection of saline at a dose of 5 ml on day 5 (Day 5-control group), then 3 of the 4 cows and the remaining cow received intramuscular injections of hCG at a dose of 1,500 IU on the corresponding day of the next estrous cycle (Day 5-hCG group). These cattle were examined for changes in ovarian follicles and corpus luteum (CL) every day or every other day by rectal palpation and ultrasonography, and blood samples were collected every day for the P and £2 analysis. The estrous cycle lengths in the hCG groups tended to be slightly longer than those in the control group, though the difference was not significant. In the Day 5-hCG group, ovulation was induced in all cows 1 or 2 days after hCG administration, and was followed by the development of new CL (induced CL). Ultrasonographic observations revealed that the total luteal tissue area of the CL periodicum and the induced CL in the Day 5-hCG group were significantly (P<0.05) wider than those in the Day 5- control group from 3 days after hCG administration onward. Plasma P levels started rising 3 hours after hCG administration in the Day 5-hCG group, and they were significantly (P<0.05) higher than those in the Day 5-control group on days 6, 7,12,13 and 14, respectively. Plasma E2 levels decreased rapidly after hCG administration in the Day 5-hCG group, and were lower than those in the Day 5-control group. These results showed enhancement of the luteal activity and a decrease in plasma E2 level by hCG administration 5 days after ovulation, suggesting that the hCG treatment on the day prior to embryo transfer is effective at increasing the pregnancy rate in embryo transfer.