Ischemia-induced changes in cell element composition and osmolyte contents of outer medulla. The effect of 60 minutes of ischemia and subsequent reflow on cell electrolyte and water homeostasis in the rat renal outer medulla was studied by determining sodium, potassium, chloride and phosphorus concentrations and dry weights in individual tubule cells using electron microprobe analysis. HPLC was employed to measure glycerophosphorylcholine, betaine, inositol and sorbitol, as well as several free amino acids in cortical and outer medullary tissue. Ischemia caused cell sodium and chloride concentrations to rise and cell potassium and phosphorus concentrations and cell dry weights to fall. These changes were most pronounced in the proximal straight tubule (PST) cells, less in thick ascending limb (MAL) and outer medullary collecting duct (OMCD) dark cells and barely noticeable in OMCD light cells. Except for some PST cells these changes were almost completely reversed 60 minutes after reintroducing blood flow. After 24 hours of reperfusion the number of PST cells exhibiting deranged electrolyte homeostasis was greatly increased. The contents of glycerophosphorylcholine, betaine or inositol in the cortex and outer medulla were not affected immediately following ischemia. After 24 hours of reperfusion, the cortical contents of osmolytes were still normal, while outer medullary contents were reduced. Except for low glycine contents, the ischemia-induced changes in amino acid contents were reversed after 24 hours of reflow in the cortex, whereas in the outer medulla aspartate, glycine and taurine contents were diminished. These results indicate increasing manifestation of PST cell injury in the reflow period. The defective re-accumulation of organic osmolytes and free amino acids in the outer medulla during reflow may reflect reduced interstitial tonicities, or may be due to inappropriate cellular uptake, synthesis or/and release. In view of the known protective properties of glycine, the postischemic depletion of cortical and outer medullary glycine contents possibly contributes to the establishment of postischemic acute renal failure.