Electron microprobe analysis of proximal tubule cellular Na, Cl and K element concentrations during acute mannitol-saline volume expansion in rats: evidence for inhibition of the Na pump.

It has previously been shown that during mannitol-saline volume expansion (VE) Na transport was inhibited 50% by harvested proximal tubular fluid without a change in paracellular shunt pathway permeability to Na. To determine whether this inhibition was due to changes in cellular entry step or an effect on the pump itself, intracellular element concentrations were measured by electron microprobe X-ray ranalysis in proximal tubular cells of control (non-expanded, NE) and VE rats. Nai, Cli and phosphorusi were increased (mean +/- S.E.) from 19.3 +/- 0.8 to 23.4 +/- 0.6, 15.8 +/- 0.4 to 21.3 +/- 0.4 and 124.3 +/- 2.6 to 138.0 +/- 1.8 mmol . kg-1 wet weight (P less than 0.001) respectively while Ki remained unchanged: 122.9 +/- 2.2 and 124.2 +/- 1.3 mmol . kg-1 wet weight. The increases in Nai and Cli were in excess of cell shrinkage produced by the hyperosmolal peritubular environment while the unchanged Ki in the face of cell shrinkage indicates and actual loss. It is concluded that mannitol-saline VE inhibits the Na pump producing a rise in Nai and a fall in Ki.