Luteolytic effect of prostaglandin F2α and two metabolites in ewes1

Abstract

Twenty-three ewes were used in an experiment to determine the point in metabolism at which prostaglandin (PG) F2α loses its luteolytic activity. Ewes exhibited at least one normal estrous cycle (15 to 19 d) and then were randomly assigned to one of four treatment groups: 1) control, 2) PGF2α, 3) 15-keto-PGF2α, or 4) 13,14-dihydro-15-keto-PGF2α (PGFM). Each ewe received an i.m. injection of the designated treatment (5 mg in 1 mL of sterile saline) during the midluteal phase of the estrous cycle (d 8, 9, 10, or 11 after estrus). An identical second injection was given to each ewe 3 h after the first injection. Beginning at 9 h after injection, concentrations of progesterone were lower (P < .01) in the PGF2α-treated ewes than in ewes in the other three groups. These differences were maintained throughout the duration of the 48-h sampling period in five of the six ewes that received PGF2α. None of the ewes in the other three treatment groups exhibited any change in concentrations of progesterone. Ewes were classified as completing luteolysis if concentrations of progesterone decreased to less than 1 ng/mL within 48 h after the first injection. Five of the six ewes receiving PGF2α completed luteolysis (P < .05), whereas none of the ewes in the other three groups completed luteolysis. The interestrous interval was reduced by approximately 5 d in the PGF2α-treated group compared with the other three treatment groups (P <.01).(ABSTRACT TRUNCATED AT 250 WORDS)