Acute effect of insulin on nitric oxide synthesis in humans: a precursor-product isotopic study

Nitric oxide (NO) is a key regulatory molecule with wide vascular, cellular, and metabolic effects. Insulin affects NO synthesis in vitro. No data exist on the acute effect of insulin on NO kinetics in vivo. By employing a precursor-product tracer method in humans, we have directly estimated the acute effect of insulin on intravascular NOx (i.e., the NO oxidation products) fractional (FSR) and absolute (ASR) synthesis rates in vivo. Nine healthy male volunteers were infused iv with l-[15N2-guanidino]arginine ([15N2]arginine) for 6 h. Timed measurements of 15NOx and [15N2]arginine enrichments in whole blood were performed in the first 3 h in the fasting state and then following a 3-h euglycemic-hyperinsulinemic clamp (with plasma insulin raised to ≈1,000 pmol/l). In the last 60 min of each experimental period, at ≈steady-state arginine enrichment, a linear increase of 15NOx enrichment (mean r = 0.9) was detected in both experimental periods. In the fasting state, NOx FSR was 27.4 ± 4.3%/day, whereas ASR was 0.97 ± 0.36 mmol/day, accounting for 0.69 ± 0.27% of arginine flux. Following hyperinsulinemia, both FSR and ASR of NOx increased (FSR by ≈50%, to 42.4 ± 6.7%/day, P < 0.005; ASR by ≈25%, to 1.22 ± 0.41 mmol/day, P = 0.002), despite a ≈20–30% decrease of arginine flux and concentration. The fraction of arginine flux used for NOx synthesis was doubled, to 1.13 ± 0.35% ( P < 0.003). In conclusion, whole body NOx synthesis can be directly measured over a short observation time with stable isotope methods in humans. Insulin acutely stimulates NOx synthesis from arginine.