Ischemic-reperfusion injury in the kidney: Overexpression of colonic H⁺-K⁺-ATPase and suppression of NHE-3

Ischemic renal injury is associated with changes in the expression of a number of genes. Although pH regulation is undoubtedly important during the recovery from ischemia, the expression of acid-base transporters during acute ischemic renal failure has not been studied. In the present study, levels of mRNA encoding the colonic H⁺-K⁺-ATPase and four isoforms of the Na⁺/H⁺ exchanger (NHE-1, NHE-2, NHE-3 and NHE-4) were measured by quantitative Northern analysis in rat renal cortex and medulla following ischemia-reperfusion injury. Rats were subjected to 30 minutes of renal artery occlusion and then sacrificed either 12 or 24 hours after the occlusion was released. The most striking changes followed 30 minutes of occlusion and 12 hours of reperfusion and involved the mRNA for NHE-3 (involved in HCO₃^- reabsorption in proximal tubule and thick limb) and colonic H⁺-K⁺-ATPase (involved in HCO₃^- reabsorption in collecting duct). These changes were: (1) a ~75% decrease in NHE-3 mRNA in both cortex and medulla; and (2) an ~8-fold increase in colonic H⁺-K⁺-ATPase mRNA in the cortex. At 12 hours of reperfusion, there was a 66% reduction in the Na⁺/H⁺ exchanger (NHE-3) activity as assayed by acid-stimulated ²²Na⁺ influx into brush border membrane vesicles (P < 0.01). After 24 hours of reperfusion, NHE-3 mRNA remained suppressed while cortical colonic H⁺-K⁺-ATPase mRNA declined to only twice the control level. Medullary colonic H⁺-K⁺-ATPase mRNA did not change significantly. Gastric H⁺-K⁺-ATPase mRNA in cortex or medulla remained the same at 0, 12, and 24 hours after reperfusion. Cortical NHE-1 increased mildly at 12 and 24 hours of reperfusion whereas a moderate decrease in NHE-2 and NHE-4 mRNAs was observed in cortex and medulla after both 12 and 24 hours of reperfusion. We suggest that overexpression of colonic H⁺-K⁺-ATPase in the early phase of renal reperfusion injury may be responsible for compensatory reabsorption of increased HCO₃^- load resulting from suppression of NHE-3. This was supported by a fourfold increase in colonic H⁺-K⁺-ATPase mRNA in rats treated with acetazolamide, which causes renal HCO₃-wasting. Rapid decline in colonic H⁺-K⁺-ATPase expression at 24 hours after reperfusion is likely due to reduced HCO₃^- delivery to distal tubules resulting from decreased GFR. Overexpression of H⁺-K⁺-ATPase may be vital to acid-base homeostasis in the early phase of acute ischemic renal failure.