EXOGENOUS PROSTAGLANDIN-F2-ALPHA STIMULATES UTEROOVARIAN RELEASE OF PROSTAGLANDIN-F2-ALPHA IN SHEEP - A POSSIBLE COMPONENT OF THE LUTEOLYTIC MECHANISM OF ACTION OF EXOGENOUS PROSTAGLANDIN-F2-ALPHA

Exogenous prostaglandin F-2 alpha (PGF(2 alpha)) is luteolytic in shee p, but its mechanism of action is not completely understood. We hypoth esized that exogenous PGF(2 alpha) stimulates the uterine and(or) ovar ian secretion of PGF(2 alpha) and that, when intramuscular doses of PG F(2 alpha) are minimal, the uteroovarian unit is a component of the lu teolytic mechanism of action of exogenous PGF(2 alpha). Thus, this stu dy was conducted to determine whether exogenous PGF(2 alpha) stimulate s the utero-ovarian release of PGF(2 alpha). Catheters were positioned in the vena cava at points cranial and caudal to the entry of utero-o varian blood, and ev;es were either hysterectomized and ovariectomized (H-OX) or left intact (Intact). Treatments were in a 2 x 2 factorial arrangement (i.e., WOX and PGF(2 alpha) were the main effects), and th ere were five ewes per treatment group. In Experiment 1, on Day 9 afte r the onset of estrus, either saline or PGF(2 alpha) (15 mg) was injec ted intramuscularly in the neck, and vena caval blood samples were col lected frequently for 120 min, then less frequently for 48 hr. In Expe riment 2, on Day 9 after estrus or WOX, either saline or PGF(2 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 af ter each injection. In both experiments, exogenous PGF(2 alpha) induce d immediate and significant increases in the utero-ovarian release of PGF(2 alpha). The increases in PGF(2 alpha) concentrations were consid erably more pronounced in vena caval blood samples collected cranial t han in those collected caudal to the entry of utero-ovarian blood, and the increase was significantly greater in Intact than in H/OX ewes tr eated with PGF(2 alpha). Vena caval concentrations of 13,14-dihydro-15 -keto-PGF(2 alpha) (PGFM) increased after exogenous PGF2 alpha, but th e changes in PGFM were not suitable representations of the changes in vena caval concentrations of PGF(2 alpha). Changes in progesterone con centrations indicated that both PGF(2 alpha) injection regimens were l uteolytic. The results from this study indicate that exogenous PGF(2 a lpha) stimulates the utero-ovarian production of PGF(2 alpha), and we believe that the utero-ovarian unit is a component of the luteolytic m echanism of action of exogenous PGF(2 alpha).