Exogenous prostaglandin F2 alpha stimulates utero-ovarian release of prostaglandin F2 alpha in sheep: a possible component of the luteolytic mechanism of action of exogenous prostaglandin F2 alpha.

Exogenous prostaglandin F2 alpha (PGF2 alpha) is luteolytic in sheep, but its mechanism of action is not completely understood. We hypothesized that exogenous PGF2 alpha stimulates the uterine and(or) ovarian secretion of PGF2 alpha and that, when intramuscular doses of PGF2 alpha are minimal, the utero-ovarian unit is a component of the luteolytic mechanism of action of exogenous PGF2 alpha. Thus, this study was conducted to determine whether exogenous PGF2 alpha stimulates the utero-ovarian release of PGF2 alpha. Catheters were positioned in the vena cava at points cranial and caudal to the entry of utero-ovarian blood, and ewes were either hysterectomized and ovariectomized (H/OX) or left intact (Intact). Treatments were in a 2 x 2 factorial arrangement (i.e., H/OX and PGF2 alpha were the main effects), and there were five ewes per treatment group. In Experiment 1, on Day 9 after the onset of estrus, either saline or PGF2 alpha (15 mg) was injected intramuscularly in the neck, and vena caval blood samples were collected frequently for 120 min, then less frequently for 48 hr. In Experiment 2, on Day 9 after estrus or H/OX, either saline of PGF2 alpha (5 mg, then 5 mg 3 hr later) was injected intramuscularly in the neck, and vena caval blood samples were collected frequently for 150 min after each injection. In both experiments, exogenous PGF2 alpha induced immediate and significant increases in the utero-ovarian release of PGF2 alpha. The increases in PGF2 alpha concentrations were considerably more pronounced in vena caval blood samples collected cranial than in those collected caudal to the entry of utero-ovarian blood, and the increase was significantly greater in Intact than in H/OX ewes treated with PGF2 alpha. Vena caval concentrations of 13,14-dihydro-15-keto-PGF2 alpha (PGFM) increased after exogenous PGF2 alpha, but the changes in PGFM were not suitable representations of the changes in vena caval concentrations of PGF2 alpha. Changes in progesterone concentrations indicated that both PGF2 alpha injection regimens were luteolytic. The results from this study indicate that exogenous PGF2 alpha stimulates the utero-ovarian production of PGF2 alpha, and we believe that the utero-ovarian unit is a component of the luteolytic mechanism of action of exogenous PGF2 alpha.