A possible molecular basis of natriuresis during ischemic-reperfusion injury in the kidney

Ischemic renal injury is associated with increased fractional excretion of sodium, suggesting a Na⁺ reabsorption deficiency in renal tubules. To determine whether alterations in expression of the major Na⁺ transporter genes might contribute to the natriuresis that follows ischemic acute renal failure, the expression of these genes was analyzed in renal cortex and medulla after ischemic-reperfusion injury. Rats were subjected to 30 min of renal pedicle clamping and then sacrificed at 12, 24, or 48 h after reperfusion. Serum creatinine increased significantly at 12 and 24 h, indicative of acute renal failure, but decreased substantially by 48 h. mRNA levels for the NHE-3 Na/H exchanger of the proximal tubule, the apical Na-K- 2Cl cotransporter of the thick ascending limb of Henle, the Na-Cl cotransporter of the distal convoluted tubule, the epithelial Na⁺ channel of the collecting duct, and the basolateral Na⁺-K⁺-ATPase were measured by Northern hybridization. NHE-3 mRNA decreased by approximately 75% at 12 h and remained suppressed at 24 and 48 h after reperfusion. Na-K-2Cl cotransporter mRNA decreased by approximately 88% at 12 h and remained suppressed at 24 and 48 h. Na-Cl cotransporter mRNA remained unchanged at 12 h, decreased by approximately 60% at 24 h, and returned to almost control levels at 48 h. mRNA levels for sodium channels (β subunit) remained unchanged. Na⁺-K⁺- ATPase mRNA in the medulla decreased by approximately 35 to 40% at 12 and 24 h and by 70% at 48 h, whereas in cortex it decreased by only <15% at 12 or 48 h after reperfusion. These results suggest that sharp reductions in expression of the NHE-3 Na/H exchanger and the apical Na-K-2Cl cotransporter are major factors in the natriuresis/diuresis that is one of the hallmarks of ischemic acute renal failure. Lasting suppression of these transporters, despite improvement in renal function, could contribute to the deranged NaCl and water excretion that often leads to volume depletion during recovery from ischemic acute renal failure.