Biology of Reproduction, Vol 23, 358-362, Copyright © 1980 by Society for the Study of Reproduction

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

¹ Reproduction Laboratory, Animal Science Institute, Science and Education Administration, Beltsville Agricultural Research Center, U.S. Department of Agriculture, Beltsville, Maryland 20705

In a previous experiment, release of prostaglandin F (PGF) in vitro by porcine luteal tissue was increased 15-fold by 8 h after pigs were injected with 2.5 mg of Cloprostenol (ICI 80996 PGF₂ analog) on Day 12 of the estrous cycle. This present experiment was conducted to determine whether increased luteal PGF₂ synthesis associated with Cloprostenol treatment of the pig was required for Cloprostenol to induce luteolysis. Six pigs per group were assigned randomly to one of the following treatments: A) no treatment; B) i.m. injections of 8 mg of indomethacin /kg BW at 1600 and 2000 h on Day 12 of the estrous cycle; C) an i.m. injection of 2.5 mg of Cloprostenol at 2000 h on Day 12; or D) both 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°C or 37°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°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°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; thus, luteal tissue in vivo did not secrete enough additional PGF₂ 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 secretion of PGF₂. The blockade of increased luteal PGF release in vitro by indomethacin injections indicates that increased luteal PGF₂ synthesis is not required for Cloprostenol-induced luteolysis in the pig.

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ACKNOWLEDGMENTS We are grateful to Ms. Kathy Ogle and Ms. Anne Powell for technical assistance and Ms. Linda Neuenhahn for typing this manuscript.

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