Changes of Progesterone and Endothelin Concentrations in the Peripheral Plasma of Female Calves and Cycling Cows: Effects of PGF2.ALPHA. Injection.

Abstract

It is well known that an injection of prostaglandin F2α (PGF2 α) or its analogue during the mid luteal phase of the estrous cycle induces a rapid decrease in plasma progesterone (P4) concentration, followed by luteolysis in the cow. There is evidence that a potent vasoactive peptide, endothelin-1 (ET-1), is produced in the bovine corpus luteum (CL), and that it is directly involved in luteolysis. We previously found that ET-1 concentrations in the peripheral plasma increase during the period of luteolysis and ovulation in cows. However, it is not clear whether the elevation of peripheral plasma ET-1 concentration observed during luteolysis and ovulation originates exclusively from the ovary and/or CL. Such a profile of plasma ET-1 concentration may be affected by the age of female calves as well as the activity of the ovary. Thus, we aimed to 1) determine in detail the changes in plasma ET-1 and P4 concentrations during the estrous cycle in the cow, 2) investigate plasma changes in ET- 1 and P4 concentrations in new-born, 120-day-old and 240-day-old female calves, and 3) examine the effect of luteolytic injection of PGF2α analogue on the plasma ET-1 concentrations in the animals in this study. The peripheral plasma ET-1 concentrations in the cycling cows showed a pulsatile increase. They reached the their highest level (13.66 pg/ml) around the time of luteolysis and estrus, dropped significantly during Days 2-12 (early to mid luteal phase) and Days 13-19 (late luteal phase) (p<0.05), and then increased again on Days 20-22 (p<0.05) when the next estrus appeared. In the peripheral plasma of newborn, 120-day- old and 240-day-old female calves, P4 concentrations remained at low levels (0.1-0.2 ng/ml). ET-1 concentrations in these animals were lower than those in the cycling cows, and remained at low levels throughout the experimental period. Moreover, plasma ET-1 concentrations, unlike those in the cycling cow, did not change after a luteolytic PGF2α injection. In conclusion, the results of the present study gave the first detailed information that plasma ET-1 concentrations increase in a pulsatile manner after the onsets of spontaneous luteolysis and PGF2α-induced luteolysis in cycling cows, but not in female calves. The results suggest that the changes in the plasma ET-1 concentrations during the estrous cycle directly correlate with the cyclic changes in the ovarian function and the uterus.