23Na nuclear magnetic resonance study of intracellular sodium in vascular endothelial cells

23Na(+)-NMR was used to determine the intracellular mobility and cell membrane permeability of Na+ in porcine vascular endothelial cells. The cells were grown as monolayers onto microcarrier beads and perfused with a medium containing Dy(P3O10)2(7-) to shift the extracellular from the intracellular Na+ resonance. Using triple quantum coherence filtered NMR experiments and spin echoes, it was shown that not all intracellular Na+ ions are in the extreme narrowing limit. The triple quantum coherence filtered experiments resulted in an observed R2f = 2022 +/- 302 s-1 and R2s = 200 +/- 28 s-1. From spin-echo experiments we obtained R2f = 2200 +/- 355 s-1 and a R2s = 145 +/- 15 s-1. These values are similar to those found in other cell systems and indicate water-Na(+)-protein interactions. Using single quantum NMR, the Na+ permeability of the endothelial membrane was determined. To obtain the Na+ transcellular permeability coefficient the cells were treated with 50 microM ouabain in the perfusion medium. Ouabain inhibits the Na-K pump and caused the intracellular Na+ concentration to increase in time. The permeability coefficient was obtained from the time dependence of the intracellular Na+ concentration. Assuming a monolayer of rectangularly shaped cells, we obtained a value of P = 0.02.10-5 cm s-1.