Secretion of progesterone and 20 alpha-dihydroprogesterone during the estrous cycle in goats

The secretion of progesterone and 20 alpha-dihydroprogesterone (20 alpha-DHP) during the estrous cycle and the effects of prostaglandin F2 alpha (PGF2 alpha) and 20 alpha-DHP on the secretion of these two steroids during the midluteal phase in goats were studied. The mean progesterone concentration in the peripheral plasma was extremely low (< 0.01 ng/mL) on the day of estrus. The concentration increased to a maximum (7.80 ng/mL) on about day 10 after estrus, and decreased rapidly during the last 3 days of the cycle. The plasma concentration of 20 alpha-DHP also was low (0.86 ng/mL) on the day of estrus, but increased gradually after estrus and decreased gradually in the last 5 days of the cycle (to 3.22 ng/mL). The ratio of progesterone to 20 alpha-DHP was > 1 after day 2 of estrus, but decreased and fell to < 1 during the last 3 days of the cycle. An injection of 3 mg of PGF2 alpha decreased the secretion of progesterone within 30 min, but affected the secretion of 20 alpha-DHP little until 2 h. The ratio of progesterone to 20 alpha-DHP fell to < 1 after 24 h. Three of the five goats given such an injection came into estrus within 2 days, and the other two came into estrus within 3 days. An injection of 15 mg of 20 alpha-DHP did not affect progesterone secretion or the estrous cycle length. These results indicate that in the goat after estrus, there is much 20 alpha-DHP in the peripheral plasma. Progesterone may be catabolized to the biologically inactive steroid, 20 alpha-DHP, during luteolysis.