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It is not known whether the ratio between the concentrations of NO metabolites (NOx) in plasma (pNOx) and in erythrocytes (eNOx) is constant or correlates with chemical parameters of the blood. We measured pH, PO(2), and PCO(2) and calculated bicarbonate concentration in 19 blood samples from the aorta, coronary sinus, and leg veins of 7 dogs. Erythrocytes were then separated by centrifugation and lysed with distilled water, and the lysate was ultrafiltered with a molecular cutoff of 50 kDa to remove the hemoglobin. NOx were measured in plasma and in the ultrafiltrate. NOx concentration was higher in erythrocytes, with eNOx/pNOx ranging from 4.38 to 14.60. Linear and significant correlations were found between the natural logarithm of eNOx/pNOx and PCO(2) (r = 0.70, P < 0.001) or bicarbonate concentration (r = 0.72, P < 0.001). These results demonstrate, for the first time, that plasma NOx cannot be considered as a constant fraction of the total NOx in blood but varies dramatically in proportion to the CO(2)/bicarbonate concentration. To prevent an underestimation of venous-arterial difference of NOx across organs, NOx should be measured in whole blood.
Recchia et al. (2000) studied this question.