Abstract
Regulation of cellular Mg^2+^ levels by insulin has been shown in various tissues. However, the mechanisms for hormonal regulation of cellular Mg^2+^ have not been well described. We studied the effect of insulin on Na^+^/Mg^2+^ exchange in normal human cells, measuring Na^+^/Mg^2+^ exchange activity as net total Mg^2+^ efflux driven by an inward Na^+^ gradient in Mg^2+^‐loaded red blood cells (RBCs). Na^+^/Mg^2+^ exchange was increased significantly by the addition of 2.4 nmol/L of insulin to the flux medium (from 0.60 ± 0.06 mmol/L cell × h to 0.75 ± 0.08 mmol/L cell × h [P = 0.0098, n = 44]). A dose‐response curve for the effects of insulin on the exchanger activity gave an estimated EC~50~ for insulin of 0.95 ± 0.31 nmol/L and a V~max~ of 0.86 ± 0.12 mmol/L cell × h (n = 7). Kinetics of the Na^+^/Mg^2+^ exchange were characterized by measuring its activity as a function of Mg^2+^ and Na^+^ concentrations. The K~0.5~ for cellular Mg^2+^ was not affected by incubation with insulin. However, the K~0.5~ for extracellular Na^+^ decreased from 69.9 ± 6.3 to 40.3 ± 8.4 mol/L (n = 5, P = 0.03) in the presence of insulin. We also studied the effect of wortmannin (WT), a PI 3‐kinase inhibitor, on activity of the exchanger. WT significantly blocked the insulin‐stimulated Na^+^/Mg^2+^ activity (n = 6, P = 0.048), with an IC~50~ of 0.5 nmol/L. LY294002, another PI 3‐kinase inhibitor, likewise blocked the insulin‐stimulated activity of the exchanger. Therefore, insulin regulates cellular Mg^2+^ metabolism in part via an increase in the affinity for Na^+^ of the Na^+^/Mg^2+^ exchange and PI 3‐kinase activation, suggesting another role for the PI 3‐kinase pathway in insulin‐mediated cellular events. © 2003 Wiley‐Liss, Inc.