Pre-eclampsia-like conditions produced by nitric oxide inhibition: effects of L-arginine, D-arginine and steroid hormones.

The aim of this study was to establish that inhibiting nitric oxide (NO) production with NG-nitro-L-arginine methyl ester (L-NAME) results in high blood pressure conditions in chronically treated pregnant rats. To validate the model, the effects of L-arginine (the substrate for NO) and D-arginine (the stereoisomer of L-arginine which is not a substrate for NO synthesis) were studied on blood pressure and fetal weights. The effects of a progesterone agonist, promegestone (R5020) and 17 beta-oestradiol were also explored. The NO synthase inhibitor L-NAME was chronically infused s.c. into pregnant rats from day 17 of gestation, either alone or with the simultaneous infusion of L-arginine and injections of sex steroid hormones (promegestone and oestradiol), compounds that may act in the pathogenic pathways of pre-eclampsia. Systolic blood pressure was measured daily. Weight and mortality of pups were recorded immediately after delivery. Blood pressure was elevated significantly in rats treated with L-NAME for only 1 day following infusion; there was a consistent decline during the next 3 days of pregnancy followed by a dramatic and significant rise just prior to delivery and post-partum. Fetal weights were reduced significantly in the L-NAME-treated rats. Co-treatment of L-NAME-infused rats with L-arginine reversed both the increase in blood pressure and the decrease in fetal weights observed with L-NAME alone. R5020, but not oestradiol, also reduced blood pressure and increased fetal weights in the L-NAME-treated animals. NO appears to play essential roles in the regulation of blood pressure during pregnancy, as well as in fetal perfusion and fetal weights at delivery. This study also indicates that progesterone, and not oestrogen, may regulate the vascular adaptations during normal pregnancy. L-Arginine and progesterone agonists like promegestone may have beneficial effects on the high blood pressure levels and reduced fetal weights associated with pre-eclampsia.