Structural changes during ischemic ARF. ARF is characterized by tubular dysfunction with impaired sodium and water reabsorption and is associated with the shedding and excretion of proximal tubule brush border membranes and epithelial tubule cells into the urine (32) (Figure 3). Approximately 30–70% of these shed epithelial tubule cells in the urine are viable and can be grown in culture (33). Recent studies using cellular and molecular techniques have provided information relating to the structural abnormalities of injured renal tubules that occur both in vitro and in vivo. In vitro studies using chemical anoxia have revealed abnormalities in the proximal tubule cytoskeleton that are associated with translocation of Na+/K+-ATPase from the basolateral to the apical membrane (34) (Figure 3). A comparison of cadaveric transplanted kidneys with delayed versus prompt graft function has also provided important results regarding the role of Na+/K+-ATPase in ischemic renal injury (35). This study demonstrated that, compared with kidneys with prompt graft function, those with delayed graft function had a significantly greater cytoplasmic concentration of Na+/K+-ATPase and actin-binding proteins — spectrin (also known as fodrin) and ankyrin — that had translocated from the basolateral membrane to the cytoplasm (Figure 4). Such a translocation of Na+/K+-ATPase from the basolateral membrane to the cytoplasm could explain the decrease in tubular sodium reabsorption that occurs with ARF. The mechanisms whereby the critical residence of Na+/K+-ATPase in the basolateral membrane, which facilitates vectorial sodium transport, is uncoupled by hypoxia or ischemia have been an important focus of research. The actin-binding proteins, spectrin and ankyrin, serve as substrates for the calcium-activated cysteine protease calpain (36) (Figure 5). In this regard, in vitro studies in proximal tubules have shown a rapid rise in cytosolic calcium concentration during acute hypoxia, which antedates the evidence of tubular injury as assessed by lactic dehydrogenase (LDH) release (37) (Figure 6).