The Selective Prostaglandin Endoperoxide Synthase-2 Inhibitor, NS-398, Reduces Prostaglandin Production and Ovulation In Vivo and In Vitro in the Rat¹

^a Departments of Obstetrics and Gynecology and Physiology, Göteborg University, S-413 90 G;auoteborg, Sweden ^b Department of Obstetrics and Gynecology, University of Utah, Salt Lake City, Utah 84132 ^c Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030

Two isoforms of prostaglandin G/H synthase, PGS-1 and PGS-2, catalyze the formation of prostaglandins (PG). Nonselective PGS inhibitors, e.g., indomethacin, reduce the number of ovulations and PG levels in many animal models. This study evaluated the effects of the selective PGS-2 inhibitor NS-398, compared to indomethacin, on ovulation number and on PG and steroid production both in vivo and in vitro in the rat.

NS-398 reduced the synthesis of PGE₂ in isolated, LH-stimulated preovulatory follicles incubated in vitro. The inhibition by NS-398 was similar to that of indomethacin. Maximal inhibition was noted from 0.1 µM. Neither progesterone nor cAMP production was affected by NS-398 or indomethacin. The effect of in vivo administration of NS-398 (1, 3, or 10 mg/kg BW, s.c.) to proestrous rats 1 h after the injection of an ovulatory dose of hCG was monitored in follicles extirpated 10 h after hCG. These follicles were incubated in vitro, and NS-398 dose-dependently reduced PGE₂ production. The synthesis of cAMP and progesterone was not altered. In separate experiments, the same doses of NS-398 were injected to determine their effect on ovulation in vivo. The number of ovulations was decreased by the highest dose of NS-398.

In the in vitro ovarian perfusion model, NS-398 (10 µM) reduced the number of ovulations initiated by LH and isobutylmethylxanthine. Lower doses of NS-398 (0.1 and 1 µM) were less effective. The production of prostanoids (PGE₂, PGF₂, and 6-keto-PGF₁) was reduced in a dose-dependent manner by NS-398. The secretion of steroids was not affected.

This study demonstrates that selective inhibition of PGS-2 by NS-398 reduces LH/hCG-stimulated production of prostanoids and the number of ovulations both in vivo and in vitro. These results provide direct evidence to strengthen the role of the inducible, granulosa cell-expressed PGS-2 as one of the key regulators in the ovulatory process and also document that the elevated and perhaps sustained levels of PG are obligatory for ovulation.

¹ Support was provided by the Swedish Medical Research Council (10375, 12605, and 11363 to L.H.; 11607 to M.B.), the National Institutes of Health (NIH HD-16229 to J.S.R.), the Foundations of Hjalmar Svensson, Assar Gabrielsson, Magnus Bergvall, Axel and Margaret Ax:son Johnson, and a visiting scientist fellowship to L.H. from the Fulbright Foundation. A travel grant (to L.H.) was provided by the Swedish Medical Society.

² Correspondence: Lars Hedin, Department of Physiology, Göteborg University, Medicinaregatan 1F, S-413 90 Göteborg, Sweden. FAX: 46 31 773 3531; lars.hedin{at}fysiologi.gu.se

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