Short Communication

Cytosolic free sodium concentration ([Na+]i) was measured in intact human lymphocytes using the novel sodium-sensitive fluorescent dye sodium-binding benzofuran-isophthalate. In the presence of 1 mmol/l external Mg2+ the resting [Na+]i was significantly lower compared to the value in the absence of external Mg2+ (22.7 +/- 1.1 mmol/l vs. 37.6 +/- 1.4 mmol/l; p < 0.0001). The thapsigargin induced [Na+]i increase was significantly lower in the presence of 1 mmol/l external Mg2+ compared to the value in the absence of external Mg2+ (73.4 +/- 6.0 mmol/l vs. 120.7 +/- 5.8 mmol/l; p < 0.001). Since Mg2+ is known to be a cofactor of the membrane Na+,K(+)-ATPase these measurements in intact lymphocytes indicate that deprivation of external Mg2+ causes an increase of [Na+]i.