ESTROGEN INCREASES NITRIC-OXIDE SYNTHASE ACTIVITY IN THE UTERUS OF NONPREGNANT SHEEP

OBJECTIVE: The aim of this study was to characterize nitric oxide synt hase activity in endometrium and myometrium of nonpregnant sheep and t o determine whether estrogen administration affects uterine nitric oxi de synthase activity. STUDY DESIGN: Nonpregnant sheep were castrated d uring synchronized estrus and 4 days after surgery were treated with 1 00 mu g/day of 17 beta-estradiol for 3 days. Nitric oxide synthase act ivity was measured by the citrulline conversion assay. RESULTS: Citrul line generation found in soluble and particulate fractions had all the characteristics of nitric oxide synthase, namely, it was strictly dep endent on reduced nicotinamide adenine dinucleotide phosphate and enha nced by flavin nucleotides and tetrahydrobiopterin. Estrogen administr ation significantly increased Ca++-dependent nitric oxide synthase act ivity in myometrium but not in endometrium. The effect of estrogen was more pronounced in the membrane-associated enzyme activity (approxima tely fivefold). Estrogen treatment increased myometrial nitric oxide s ynthase activity from 9.0 +/- 2.4 to 20.0 +/- 3.7 pmol/mg of protein p er 30 minutes in the soluble fraction and from 12.0 +/- 5.1 to 62.0 +/ - 13.1 pmol/mg of protein per 30 minutes in the particulate fraction ( mean +/- SEM, p < 0.05 by t test). The increase in nitric oxide syntha se activity was not mediated by an increase in tetrahydrobiopterin ava ilability, as shown to be the case in macrophages. CONCLUSION: These d ata show that in the nonpregnant sheep uterus > 90% of the nitric oxid e synthase activity found in myometrium is Ca++ dependent and is up-re gulated by estrogen in a tissue-specific manner.