Element composition of tubule cells in the inner stripe of the renal outer medulla

To obtain further insight into renal medullary function, element concentrations were determined in individual tubule cells of the outer medulla in the rat kidney using electron microprobe analysis on freeze-dried cryosections. In the cells of the thick ascending limb of Henle's loop the Na, P, Cl, and K concentrations (means +/- SEM) were: 9.5 +/- 0.6, 158.4 +/- 6.2, 25.6 +/- 1.2, and 135.3 +/- 4.8 mmol/kg wet weight, respectively. While similar Na, P, and K concentrations were observed in light and dark cells of the medullary collecting duct, Cl was markedly higher--55.0 +/- 2.8 mmol/kg wet weight--in the dark cells. The electrolyte concentrations of the thick ascending limb cells seen in the present study are in good agreement with ion activities reported for the isolated perfused thick ascending limb. The low cell Na and Cl concentrations provide a favorable driving force for passive cell entry of Na, Cl, and K across the apical membrane via the Na-2Cl-K cotransporter even at low tubule fluid NaCl concentrations. Although in hydropenic rats interstitial tonicity of the inner stripe is above isotonicity, electrolyte concentrations of inner stripe cells did not differ from those obtained in cortical tubule cells. This finding suggests that, similar to papillary cells, osmoadaptation of outer medullary cells is, at least partially, accomplished by organic osmolytes.