Discussion Forum

The vascular endothelium releases two mediators that have a major role in regulation of artery calibre, blood flow and blood pressure. These are nitric oxide (NO), which can be detected and quantified using an amperometric microsensor, and EDHF (endothelium-derived hyperpolarizing factor), which is undefined chemically but can be detected by measuring vascular smooth muscle hyperpolarisation and with pharmacological antagonists. Since NO can also hyperpolarize vascular smooth muscle cells, and NO synthase inhibitors in commonly used concentrations do not cause 100% block of NO synthesis, it can be difficult to evaluate the independent roles of NO and EDHF in cardiovascular physiology. L-Arginine, the substrate for NO synthase, is present within endothelial cells in excess. Nevertheless, the formation of NO from eNOS in rat superior mesenteric artery rings was found to be dependent on extracellular L-arginine, and was optimal at a concentration of Larginine close to the plasma level (carbachol stimulated NO: control 15.7 ± 0.9, L-arginine 100 μM 22.8 ± 1.3 nmol l). Enhancement of NO output by L-arginine was stereospecific, required the cationic amino acid transporter and was dependent of caveolin. Induction of iNOS impaired the NO synthesis from eNOS (100 nmol l carbachol stimulated NO: control 5.7 ± 0.6, iNOS 0.3 ± 0.3 nmol l). 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 μmol l administered simultaneously with carbachol, but not by L-arginine administered during incubation with lipopolysaccharide. 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.