Volume, sodium content and sodium efflux of human mononucleated cells: characteristics and methodological problems

New methods for determining the volume and sodium content of human mononucleated cells are presented and the kinetics of sodium efflux are characterized. Sodium efflux obeyed first-order kinetics with a half-life of about 9 min. The ouabain-sensitive sodium pump is responsible for about 60% of the total turnover of this cation. Reliable measurement of cell volume and sodium content requires pre-incubation at physiological conditions. Determination of sodium efflux should be based on the initial (less than 15 min) cellular delay of 22Na-activity in preloaded cells. In normal subjects (n = 34), mean cell volume was 635 +/- 138 nl/10(6) cells and sodium content averaged 14.6 +/- 2.8 mmol/l cell. Total and ouabain-sensitive efflux rate constants were 4.56 +/- 0.66 h-1 and 2.70 +/- 0.47 h-1. Corresponding absolute sodium efflux averaged 65.1 +/- 14.8 and 38.9 +/- 9.6 mmol/l/h, respectively.