CYTOSKELETON DISRUPTION AND APICAL REDISTRIBUTION OF PROXIMAL TUBULE NA+-K+-ATPASE DURING ISCHEMIA

The polar distribution of Na+-K+-ATPase to the basolateral membrane of proximal tubule cells is essential for the efficient and vectorial reabsorption of Na+ and may be dependent on the formation of a metabolically stable, detergent-insoluble complex of Na-K+-ATPase with the actin membrane cytoskeleton. The present studies utilized immunocytochemical techniques to demonstrate and quantify the apical redistribution of Na+-K+-ATPase during mild ischemia (15 min) that occurred in proximal (1.3 +/- 0.9 vs. 4.5 +/- 1.1 particles/100 mum surface membrane, P < 0.01) but not distal tubule cells. Treatment of control apical membranes with 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl) octanoate (A2C), a fluidizing agent, markedly increased membrane fluidity without any effect on Na+-K+-ATPase activity. In brush-border membrane vesicles isolated after ischemia, however, A2C further increased an already elevated Na-K+-ATPase activity. During ischemia, total cellular Na+-K+-ATPase activity remained unaltered, but the Triton X-100-soluble (noncytoskeleton associated) fraction of Na+-K+-ATPase increased significantly following 15 and 30 min. There was a corresponding decrease in the Triton X-100-insoluble fraction of Na+-K+-ATPase, with the ratio of detergent-soluble to -insoluble Na+-K+-ATPase increasing from 13 +/- 2 to 32 +/- 5% (P < 0.01) during 30 min of ischemia. Western blot analysis of the Triton X-100-soluble fraction, following 30 min of ischemic injury, revealed the presence of Na+-K+-ATPase, actin, fodrin, and uvomorulin. However, in a fraction highly enriched for Na+-K+-ATPase, neither actin, fodrin, nor uvomorulin was detected. Taken together, these data suggest that in proximal tubule cells, as in Madin-Darby canine kidney cells, Na+-K+-ATPase exists primarily in a cytoskeletal-associated form. During ischemic injury, the actin cytoskeleton is disrupted, and Na+-K+-ATPase dissociates from the actin cortical cytoskeleton and is then free to redistribute to the apical membrane in proximal but not distal tubule cells.