The plasma level of NOx, i.e., the sum of NO2 ؊ and NO3 ؊ , is frequently used to assess NO bioavailability in vivo. However, little is known about the kinetics of NO conversion to these metabolites under physiological conditions. Moreover, plasma nitrite recently has been proposed to represent a delivery source for intravascular NO. We therefore sought to investigate in humans whether changes in NO x concentration are a reliable marker for endothelial NO production and whether physiological concentrations of nitrite are vasoactive. NO 2 ؊ and NO3 ؊ concentrations were measured in blood sampled from the antecubital vein and brachial artery of 24 healthy volunteers. No significant arterial-venous gradient was observed for either NO 2 ؊ or NO3 ؊ . Endothelial NO synthase (eNOS) stimulation with acetylcholine (1-10 g͞min) dose-dependently augmented venous NO 2 ؊ levels by maximally 71%. This effect was paralleled by an almost 4-fold increase in forearm blood flow (FBF), whereas an equieffective dose of papaverine produced no change in venous NO 2 ؊ . Intraarterial infusion of NO2 ؊ had no effect on FBF. NOS inhibition (N G -monomethyl-L-arginine; 4 -12 mol͞min) dosedependently reduced basal NO 2؊ and FBF and blunted acetylcholine-induced vasodilation and NO release by more than 80% and 90%, respectively. In contrast, venous NO3 ؊ and total NOx remained unchanged as did systemic arterial NO 2 ؊ and NO3 ؊ levels during all these interventions. FBF and NO release showed a positive association (r ‫؍‬ 0.85; P < 0.001). These results contradict the current paradigm that plasma NO 3 ؊ and͞or total NOx are generally useful markers of endogenous NO production and demonstrate that only NO2 ؊ reflects acute changes in regional eNOS activity. Our results further demonstrate that physiological levels of nitrite are vasodilator-inactive.endothelium ͉ blood flow ͉ red blood cells ͉ endothelial dysfunction

To determine half-life and turnover of plasma adenosine, heparinized blood from healthy volunteers was incubated with radiolabeled adenosine in the physiological concentration range of 0.1-1 microM. Plasma levels of adenosine in vitro were 82 +/- 14 nM and were similar to those determined immediately after blood collection with a "stopping solution." Dipyridamole (83 microM) and erythro-9(2-hydroxynon-3yl)-adenine (EHNA) (8 microM) did not measurably alter basal adenosine levels but completely blocked the uptake of added adenosine. Inhibition of ecto-5'-nucleotidase with 100 microM alpha, beta-methyleneadenosine 5'-diphosphate (AOPCP) reduced plasma adenosine to 22 +/- 6 nM. For the determination of adenosine turnover, the decrease in specific radioactivity of added [3H]adenosine was measured using a dipyridamole-containing stopping solution. Without altering basal adenosine levels, the half-life was estimated to be 0.6 s. Similar experiments were carried out with washed erythrocytes or in the presence of AOPCP, yielding half-lives of 0.7 and 0.9 s, respectively. When the initial adenosine concentration was 1 microM, its specific activity decreased by only 11% within 5 s, whereas total plasma adenosine exponentially decreased with a half-life of 1.5 s. Venous plasma concentrations were measured after relief of a 3-min forearm ischemia. Changes in plasma adenosine did not correlate well with changes in blood flow but were augmented in the presence of dipyridamole.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of compression stockings to the lower extremities is a widely used therapeutic intervention to improve venous return, but there is little information about the effects of compression on local arterial perfusion. Therefore, we tested the hypothesis that a positive external pressure increases forearm perfusion. The relation of increasing external pressure induced by standardized compression to the arterial inflow and arterial flow reserve of the forearm was critically evaluated in a group of healthy young men (n = 9). Flow was measured with venous occlusion plethysmography after a 10-min application of six different stockings with compression pressure increasing from 13 to 23 mmHg. During compression, the arterial inflow increased significantly from 3.7 +/- 0.85 to 8.8 +/- 2.01 ml.min(-1).100 ml tissue(-1) (P < 0.001) and the arterial flow reserve increased from 17.7 +/- 4.7 to 28.3 +/- 7.0 ml.min(-1).100 ml tissue(-1). The flow increase was persistent after 3 h of constant application of external pressure and also during simultaneous low-intensity hand grip. Similar results obtained with occlusion plethysmography were seen with MRI. During the interventions, forearm temperature was unchanged, and the volunteers reported no discomfort. In conclusion, 1) arterial perfusion of the human forearm increases more than twofold during application of external compression over a pressure range of 13-23 mmHg, and 2) the result is interpreted as an autoregulatory response following the decrease of the vascular transmural pressure gradient.