The role of nitric oxide in the uterotrophic action of oestradiol after 6 h or 72 h was studied in immature (19\p=n-\21 days old) female Wistar rats by use of l-arginine, the amino acid from which nitric oxide is synthesized, and N\g=w\-nitro-l-argininemethyl ester (l-NAME), an inhibitor of nitric oxide synthase. Oestradiol at single s.c. doses of 2.5, 5.0 and 10.0 \g=m\gper rat induced dose-dependent uterine oedema in 6 h. l-NAME (10 and 20 mg kg\m=-\1, i.p.) administered 30 min before oestradiol (10 \g=m\gper rat) injection suppressed the formation of uterine oedema in a dose-related manner. This action of l-NAME on oestradiol-induced uterine oedema was effectively blocked by pretreatment of rats with l-arginine (600 mg kg \m=-\1 , s.c.), a precursor of nitric oxide, but not by l-lysine, an amino acid not involved in the generation of nitric oxide. In addition, l-NAME at similar doses significantly prevented oestradiol-induced (3 \g=m\gper rat, s.c. on three successive days) increases in uterine growth after 72 h; however, this effect was mitigated by l-arginine (600 mg kg \m=-\1). These results suggest the involvement of an l-arginine\p=n-\nitric oxide system in the oestradiol-induced uterotrophic effect in immature rats.
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