Changes in nitric oxide synthase activity in the ovary of gonadotropin treated rats: The role of nitric oxide during ovulation

Immature rats receiving equine chorionic gonadotropin (eCG) and human CG (h CG) were used to study the time course changes in nitric oxide synthase (NO S) activity in the ovary during ovulation. To study the role of NO in ovula tion, the effects of intrabursal injection of L-N-G-monomethylarginine (L-N MMA, 125 mu g/20 mu l/bursa), a NOS inhibitor, on the number of ova shed we re also examined. Rats were sacrificed at -48, 0, 3, 6, 9, 12, and 24 h aft er hCG injection, and the ovaries were collected for the NOS activity assay , Western blotting, NADPH-diaphorase histochemistry and immunohistochemistr y. Total NOS and constitutive NOS activities in the ovary increased signifi cantly at 9 h after hCG injection and the values remained high thereafter. Inducible NOS (iNOS) activity was detectable as a small peak at 3 and 6 h a fter hCG injection. Endothelial NOS (eNOS) protein production increased aft er hCG injection with a peak at 12 h, whereas iNOS protein production decre ased at 12 and 24 h after hCG injection. NADPH-diaphorase positive cells in creased at the thecae of growing follicles after hCG injection, appeared at mural granulosa cells before ovulation, and were detected in newly formed corpora lutea, which coincided with the results in eNOS positive cells by i mmunohistochemistry. L-NMMA given to rats at 5 or 7 h after hCG was most ef fective in reducing the number of ova shed. These results indicate that the NOS activity and NOS positive cells increased after hCG injection, and tha t eNOS was likely the main NOS increasing in the ovary during ovulation. It is concluded that NO produced between 5 and 9 h after hCG might play a sup portive role in ovulation.