Effect of ischemia-reperfusion on the renal brush-border membrane sodium-dependent phosphate cotransporter NaPi-2

To understand the mechanisms underlying ischemia-reperfusion-induced renal proximal tubule damage, we analyzed the expression of the Na+-dependent phosphate (Na+/P-i) cotransporter NaPi-2 in brush border membranes (BBM) isolated from rats which had been subjected to 30 min renal ischemia and 60 min reperfusion. Na+/P-i cotransport activities of the BBM vesicles were also determined. Ischemia caused a significant decrease (about 40%, P < 0.05) in all forms of NaPi-2 in the BBM, despite a significant increase (31 +/- 3%, P < 0.05) in the Na+/P-i cotransport activity. After reperfusion, both NaPi-2 expression and Na+/P-i cotransport activity returned to control levels. In contrast with Na+/P-i cotransport, ischemia significantly decreased Na+-dependent glucose cotransport but did not affect Na+-dependent proline cotransport. Reperfusion caused further decreases in both Na+/glucose (by 60%) and Na+/proline (by 33%) cotransport. Levels of NaPi-2 were more reduced in the BBM than in cortex homogenates, suggesting a relocalization of NaPi-2 as a result of ischemia. After reperfusion, NaPi-2 levels returned to control values in both BBM and homogenates. These data indicate that the NaPi-2 protein and BBM Na+/P-i cotransport activity respond uniquely to reversible renal ischemia and reperfusion, and thus may play an important role in maintaining and restoring the structure and function of the proximal tubule.