Estrogen increases nitric oxide synthase activity in the uterus of nonpregnant sheep

The aim of this study was to characterize nitric oxide synthase activity in endometrium and myometrium of nonpregnant sheep and to determine whether estrogen administration affects uterine nitric oxide synthase activity.

Nonpregnant sheep were castrated during synchronized estrus and 4 days after surgery were treated with 100 micrograms/day of 17 beta-estradiol for 3 days. Nitric oxide synthase activity was measured by the citrulline conversion assay.

Citrulline generation found in soluble and particulate fractions had all the characteristics of nitric oxide synthase, namely, it was strictly dependent on reduced nicotinamide adenine dinucleotide phosphate and enhanced by flavin nucleotides and tetrahydrobiopterin. Estrogen administration significantly increased Ca(++)-dependent nitric oxide synthase activity in myometrium but not in endometrium. The effect of estrogen was more pronounced in the membrane-associated enzyme activity (approximately fivefold). Estrogen treatment increased myometrial nitric oxide synthase activity from 9.0 +/- 2.4 to 20.0 +/- 3.7 pmol/mg of protein per 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 synthase activity was not mediated by an increase in tetrahydrobiopterin availability, as shown to be the case in macrophages.

These data show that in the nonpregnant sheep uterus > 90% of the nitric oxide synthase activity found in myometrium is Ca++ dependent and is up-regulated by estrogen in a tissue-specific manner.