The formation of NO from endothelial nitric oxide synthase (eNOS) in rat superior mesenteric artery rings was dependent on extracellular L-arginine, and was optimal at a concentration of L-arginine close to the plasma level (carbachol-stimulated NO: control 15.7+/-0.9, L-arginine 100 micro M 22.8+/-1.3 nM). 2. Enhancement of NO output by L-arginine was stereospecific, required the cationic amino-acid transporter and was dependent on caveolin. 3. Induction of inducible nitric oxide synthase (iNOS) impaired the stimulated NO synthesis from eNOS (100 nM carbachol-stimulated NO: control 5.7+/-0.6, iNOS 0.3+/-0.3 nM). 4. The interaction between iNOS and eNOS was reversed by the superoxide scavenger MnTMPyP. Impairment of eNOS by iNOS was also prevented by L-arginine 100 micro M administered simultaneously with carbachol, but not by L-arginine administered during incubation with lipopolysaccharide. 5. These data provide functional evidence that supplementing L-arginine from the extracellular medium optimises the formation of NO from eNOS and suggests that the impairment of eNOS by iNOS is caused by excess formation of superoxide by NO synthase, which can be prevented by L-arginine. These results provide an explanation for the observations that extracellular L-arginine can enhance endothelium function only when the endothelium is impaired or when iNOS has been induced.
