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. Na_i, Cl_i and phosphorus_i 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<0.001) respectively while K_i remained unchanged: 122.9±2.2 and 124.2±1.3 mmol · kg^–1 wet weight. The increases in Na_i and Cl_i were in excess of cell shrinkage produced by the hyperosmolal peritubular environment while the unchanged K_i 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 Na_i and a fall in K_i.