Arteriovenous Differences in NO2- Kinetics in Anesthetized Rabbits

Abstract

The critical role of nitric oxide (NO) in regulating vascular tone is now widely recognized, and the search for reliable and practical indices of endothelial NO formation have been received much attention. 1) Early studies that attempted to use plasma nitrate (NO 3 Ϫ ) or NO x (nitrite [NO 2 Ϫ ] plus NO 3 Ϫ ) as stable metabolites of NO 2,3) encountered too high a degree of interference, resulting from numerous confounding factors (including contamination), for their results to be applied practically. [4][5][6] In addition, as-yet unexplained and paradoxical decreases in NO x following increases in NO formation have been reported. 7,8) In contrast, plasma NO 2 Ϫ has been the source of much interest lately as a promising indicator of NO production following reports that approximately 70-90% of circulating plasma NO 2 Ϫ is derived from endothelial nitric oxide synthase (eNOS) activity in humans and animals. 9,10) Indeed, several studies have shown a close relationship between changes in endothelium-dependent blood flow and plasma NO 2 Ϫ levels. 9,11,12) Furthermore, plasma NO 2 Ϫ is now believed to be a storage site for NO activity. Plasma NO 2 Ϫ is catalyzed by the nitrite reductase activity of deoxygenated hemoglobin (deoxyHb) 13) ; evidence in support of this includes the enhancement of vasodilatory activity of NO 2 Ϫ or production of NO in the presence of deoxyHb. [14][15][16][17][18] To further examine and evaluate these roles for plasma NO 2 Ϫ , accurate and highly sensitive methods for quantifying NO 2 Ϫ and precise information regarding kinetics in vivo are essential. Although there have been some preliminary and incomplete reports regarding NO 2 Ϫ kinetics in vivo, 19,20) to date, there are no systematic data based on standardized methods.Therefore, the goal of the present study was to clarify the kinetic features of plasma NO 2 Ϫ in vivo using a canonical method (NO 2 Ϫ loading study) with an established highly sensitive quantifying technique, 21) taking possible arteriovenous (A-V) differences into consideration. That steady-state NO 2 Ϫ levels might differ between veins and arteries has been subject to debate for many years. 7,12,[21][22][23] MATERIALS AND METHODS Measurement of NO 2؊ and NO 3 ؊We determined NO x levels using a high-performance liquid chromatography (HPLC)-Griess system (ENO10 and ENO20; Eicom, Kyoto, Japan) consisting of a separation column, a flow reactor (with Griess reagent), a reduction column, and a detector at 540 nm, as described elsewhere. 24) Operating under default conditions, the detection limit and sensitivity was 0.1 mM for both NO 2 Ϫ and NO 3 Ϫ with a loading volume of 10 m1. To determine nanomolar concentrations of NO 2 Ϫ , we removed the reduction column and increased the loading volume to 100 m1, which improved the sensitivity and detection limits for NO 2 Ϫ to 1 nM and 2 nM, respectively. 21) In addition, a modified aqueous mobile phase was applied to improve the recovery time of the system. Special attention was paid to excluding possible sources of NO 2 Ϫ co...

Different Disappearance Rates of Plasma Nitrite (NO2-) Contribute to Apparent Steady-State Arterio-Venous Differences in Anesthetized Animals

“…It is also unclear whether plasma NO 2 Ϫ levels reflect in vivo changes in NO 2 Ϫ levels. 10,12,18) Exogenously administered NO 2 Ϫ or NO resulted in a large A-V difference in plasma NO 2 Ϫ , with arterial blood preferentially reflecting the changes, 10,14,17) presumably due to greater contributions by RBCs or tissue as a buffering compartment under a state of lower oxygenation. A possible difference in the uptake rate of RBCs could not be large enough to explain such a large A-V difference, and contributions of tissue factors would be a great concern.…”

“…12) Although we have previously reported a small but significant A-V difference under steadystate conditions, 10,14,17) these observations are complicated by a report that, in isolated blood, uptake of NO 2 Ϫ by RBCs is faster in venous blood than in arterial blood, 15) a scenario that may cause artificial apparent A-V differences when arterial and venous blood samples are treated after some delay. If this situation is indeed the case, our previous work reporting kinetic parameters 10) should also be re-evaluated. It is also unclear whether plasma NO 2 Ϫ levels reflect in vivo changes in NO 2 Ϫ levels.…”

“…However, only limited kinetic data [8][9][10] are presently available, and many problems of NO 2 Ϫ biology await resolution.…”

“…[10][11][12][13][14][15][16] The A-V difference may reflect consumption of NO 2 Ϫ to form NO during peripheral circulation for preservation of blood flow at lower oxygen tension. 12) Although we have previously reported a small but significant A-V difference under steadystate conditions, 10,14,17) these observations are complicated by a report that, in isolated blood, uptake of NO 2 Ϫ by RBCs is faster in venous blood than in arterial blood, 15) a scenario that may cause artificial apparent A-V differences when arterial and venous blood samples are treated after some delay. If this situation is indeed the case, our previous work reporting kinetic parameters 10) should also be re-evaluated.…”

“…When we removed the reduction column and increased the loading volume to 100 ml, the sensitivity and detection limit for NO 2 Ϫ improved to 1 nM and 2 nM, respectively. 10,14) Special attention was paid to exclude possible sources of NO 2 Ϫ and NO 3 Ϫ contamination during the entire procedure. Unless otherwise specified, all laboratory wares were washed five times with pure water (resistance above 18.3 MW and almost completely free of NO x contamination; MILLI-Q SP, Millipore, Bedford, MA, U.S.A.) to minimize NO 2 Ϫ and NO 3 Ϫ contamination.…”