Measurement of NO synthesis in humans by L-[15N2]arginine: application to the response to vaccination.

Induction of nitric oxide (NO) synthesis is a key element of the inflammatory response in humans. We describe a sensitive gas isotope ratio mass spectrometric (GIRMS) method for measuring urinary [15N]nitrate production during intravenous infusion of L-[guanidino-15N2]arginine and its application to investigate the effects of a controlled inflammatory stimulus, typhoid vaccination, on NO synthesis in humans. Intravenous infusion of L-[15N2]arginine at 5-12 mumol.kg-1.h-1 for 24 h in three subjects was used to determine arginine and nitrate pool kinetics. Eight subjects received primed constant infusion of 2.5 mumol.kg-1.h-1 of L-[15N2]arginine for 12 h once before and again after typhoid vaccination. NO synthesis was calculated from 15N enrichment of plasma arginine and urinary nitrate, measured by gas chromatography mass spectrometry and GIRMS, respectively, and total urinary nitrate excretion. Baseline NO synthesis was 298 +/- 44 nmol.h-1.kg lean body mass-1, representing 0.41% of arginine flux. After vaccination, NO synthesis (267 +/- 77 nmol.h-1.kg-1) was not increased (P = 0.18), despite demonstration of an acute phase response. Typhoid vaccination is not accompanied by accelerated NO synthesis.