NA-K+-ATPASE IN ENDOTHELIAL-CELL ENERGETICS - NA-23 NUCLEAR-MAGNETIC-RESONANCE AND CALORIMETRY STUDY()

Sodium flux rate and energy consumption of the Na+-K+ pump in vascular endothelial cells of porcine aorta grown on microcarrier beads were s tudied using a combination of nuclear magnetic resonance spectroscopy of intracellular Na-23 and microcalorimetry. The Na+ flux into the cel ls was determined in the presence of the shift reagent Dy(P3O10)(2)(7- ), while the Na+-K+ pump was inhibited with ouabain. Basal Na+ influx was 17 +/- 3 nmol min(-1) mg protein(-1), and intracellular Na+ concen tration was 23.5 + 3.8 mM, resulting in a complete exchange of intrace llular Na+ within 5-6 min. Spin-lattice relaxation time (T-1) measurem ents of intracellular Na+ showed a T-1 of 19 +/- 1 ms under basal cond itions and a T-1 of 26.2 +/- 1.6 ms after pump inhibition with 50 mu M ouabain. Such an increase is typical for a system in which the total amount of Na+ increases but where the amount of bound Na+ remains cons tant. The Na+ ionophore nystatin maximally increased the Na+-K+ pump r ate about twofold, whereas the amount of intracellular Na+ only increa sed 14%. With microcalorimetry a cellular heat flux of 183 +/- 18 mu W per mg endothelial protein was determined, which relates to 7.6 mu W/ mg endothelial protein generated by the Na+-K+ -adenosinetriphosphatas e. Our data demonstrate that small intracellular changes of Na+ can st imulate the endothelial Na+-K+ pump activity. The contribution of the Na+-K+ pump to total endothelial energy expenditure is similar to 4-5% .