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The Influence of pH and Various Anions on the Distribution of NH4 + in Human Blood

The pH-dependent distribution of ammonia between blood cells and plasma was investigated with oxygenated blood samples from healthy subjects at 37 degrees C. Blood pH was varied between 6.95 and 7.65 by equilibration with different CO2 mixtures. Plasma ammonia concentrations were measured directly with a specific enzymatic method. Ammonia concentrations within blood cells were calculated from a) the concentration changes of ammonia in plasma after addition of 87.7 mumol/l NH4Cl to whole blood and b) the pH-dependent haematocrit. The pH-dependency of the distribution ratio(ammonia) = P-ammonia/cell ammonia (substance concentrations in water spaces) is described by the equation distribution ratio(ammonia) = 3.095 - 0.342 x pHplasma (r = 0.928, n = 36) in good agreement with available literature data on the distribution of H+. A quantitative figure to describe the actual NH4+ concentration in oxygenated whole blood at defined values of P-NH4+, P-pH and haematocrit is given. Values of distribution ratio(ammonia) at pH 7.4 (0.57 or 0.75, ammonia concentrations corrected/not corrected for water content) are higher than those assumed so far in the literature. Addition of non-permeating anions (citrate, EDTA) to whole blood results in a shift of NH4+ from the intra- to the extracellular compartment. In contrast, chaotropic anions like iodide or thiocyanate lower distribution ratio(ammonia). To avoid medically important bias in the measurement of plasma ammonia concentration, the changes in pH or in the ionic composition of the blood sample following pretreatment with anticoagulants or preservatives should not exceed certain limits. Citrate is not a suitable anticoagulant.