Blockade of Luteal Prostaglandin F Release in vitro During Cloprostenol-Induced Luteolysis in the Pig

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Abstract
Prostaglandin F (PGF) release in vitro by porcine luteal tissue was increased 15-fold 8 h after pigs were injected with 2.5 mg of cloprostenol (ICI 80996 PGF2.alpha. analog) on day 12 of the estrous cycle. Whether increased luteal PGF2.alpha. synthesis associated with Cloprostenol treatment to the pig was required for Cloprostenol to induce luteolysis was determined. Six pigs per group were assigned randomly to 1 of the following treatments: no treatment; i.m. injections of 8 mg of indomethacin /kg BW [body weight] at 1600 and 2000 h on day 12 of the estrous cycle; an i.m. injection of 2.5 mg of Cloprostenol at 2000 h on Day 12; or treatments B and C. Jugular vein blood was collected at 1600 h on day 12 and at ovariectomy at 0800 h on day 13. Luteal tissue was incubated 2 h in Krebs Ringer bicarbonate buffer at 0 or 37.degree. C. Plasma progesterone, 13,14-dihydro-15-keto-PGF (PGFM), luteal progesterone and PGF release into the incubation medium were quantified by radioimmunoassays. Cloprostenol decreased plasma progesterone (A + B vs. C + D; 28.8 vs. 11.2 ng/ml; P < 0.0005) and decreased luteal progesterone release in vitro at 37.degree. C (A + B vs. C + D; 16.0 vs. 4.2 ng/mg tissue; P < 0.0005). Cloprostenol increased luteal PGF release in vitro at 37.degree. C (A vs. C; 34.9 vs. 191.7 ng/100 mg tissue, P < 0.001) and indomethacin blocked the Cloprostenol effect (C vs. D; 191.7 vs. 32.1 ng/100 mg tissue; P < 0.001). Plasma PGFM did not increase significantly after an injection of Cloprostenol; luteal tissue in vivo did not secrete enough additional PGF2.alpha. to be detected as an increase in plasma PGFM. In pigs injected with indomethacin, decreased plasma PGFM (A + C vs. B + D; 302 vs. 29 pg/ml; P < 0.01) on day 13 indicated a significant reduction in PGF2.alpha. secretion. The blockade of increased luteal PGF release in vitro by indomethacin injections indicates that increased luteal PGF2.alpha. synthesis is not required for Cloprostenol-induced luteolysis in the pig.