17 beta-estradiol corrects hemostasis in uremic rats by limiting vascular expression of nitric oxide synthases

Conjugated estrogens shorten the prolonged bleeding time in uremic patients and are similarly effective in a rat model of uremia. We have previously d emonstrated that the shortening effect of a conjugated estrogen mixture or 17 beta-estradiol on bleeding time was abolished by the nitric oxide (NO) p recursor L-arginine, suggesting that the effect of these drugs on hemostasi s in uremia might be mediated by changes in the NO synthetic pathway. The p resent study investigated the biochemical mechanism(s) by which conjugated estrogens limit the excessive formation of NO. 17 beta-estradiol (0.6 mg/kg ), given to rats made uremic by reduction of renal mass, significantly redu ced bleeding time within 24 h and completely normalized plasma concentratio ns of the NO metabolites, nitrites and nitrates, and of NO synthase (NOS) c atalytic activity, determined by NADPH-diaphorase staining in the thoracic aorta. Endothelial NOS (ecNOS) and inducible NOS (iNOS) immunoperoxidase st aining in the endothelium of uremic aortas of untreated rats was significan tly more intense than in control rats, while in uremic rats receiving 17 be ta-estradiol staining was comparable to controls. Thus 17 beta-estradiol co rrected the prolonged bleeding time of uremic rats and fully normalized the formation of NO by reducing the expression of ecNOS and iNOS in vascular e ndothelium. These results provide a possible biochemical explanation of the well-known effect of estrogens on primary hemostasis in uremia, in experim ental animals and humans.