Changes in nitrite concentration during blood flow through the human forearm circulation. (A) Infusion of acetylcholine (ACh) (7.5 μg/min) intra-arterially in the human forearm and ischemia-induced flow-mediated dilation causes a 4- to 10-fold increase in forearm blood flow. (B) The stimulation reverses the normal small uptake of nitrite to instead cause a release of nitrite into the circulation, a result consistent with NO-to-nitrite conversion during forearm circulatory passage. Horizontal bars represent mean values. (C) Comparison of nitrite formation under basal conditions (saline infusion) and NOS stimulation via ACh or shear stress. (D) Time profile of nitrite responses in whole venous blood during continuing ACh infusion (representative results). Error bars show standard error of the mean.