Alterations of Aquaporin mRNA and Protein Expression in Hypokalemic Rat Kidney

Kim, Dae Sung; Lee, Chang Bae; Bae, Choon Sang; Kim, Baik Yoon; Park, Sung Sik; Ahn, Kyu Youn

Abstract

There has been a general agreement that potassium depletion causes metabolic alkalosis and substantial morphological changes in kidney structure, and is associated with renal functional abnormalities, including a decrease in urinary concentrating ability. The present study was to examine the alterations of expression and distribution of AQP-1, 2, 3 and 4 mRNAs and proteins in the kidneys of normal and K-depleted rats using RT-PCR, Western blot analysis, and immunohistochemistry. Predicted size of AQP-1, 2, 3, and 4 mRNAs was 119, 822, 539, and 642 bp, respectively. AQP-1 mRNA expression was gradually decreased in K-depleted rats, particularly LK 2W. AQP-2, 3 mRNAs were markedly decreased in K-depleted rats. AQP-4 mRNA expression was markedly increased in K-depleted rats, particularly LK 2W. Western blot analysis demonstrated that AQP-1 protein expression was only decreased in LK 3D and others were comparable with normal rat. AQP-2, 3 proteins expression was markedly decreased in K-depleted rats, compared with normal rat. But, AQP-4 protein expression was markedly increased in K-depleted rats, particularly LK 3W. In immunohistochemistry, AQP-1 was detected in the apical membranes of proximal tubules and thin limb of Henle loop. In potassium-depleted kidney, the pattern of cellular labeling and signal intensity of AQP-1 protein is identical to that of normal rat. AQP-2 was detected in apical region and cytoplasm of the principal cells of entire collecting duct. In potassium-depleted kidney, the pattern of cellular labeling of AQP-2 protein is identical to that of normal rat, but signal intensity is markedly decreased. AQP-3 was detected in the bosolateral plasma membrane of principal cells of entire collecting duct. In potassium-depleted kidney, the pattern of cellular labeling of AQP-3 protein is identical to that of normal rat, but signal intensity is markedly decreased. AQP-4 was detected in the bosolateral plasma membrane of principal cells of entire collecting duct. In potassium-depleted kidney, the pattern of cellular labeling of AQP-4 protein is identical to that of normal rat, but signal intensity is markedly increased in outer and inner medullary collecting ducts. In summary, these results demonstrate that chronic hypokalemia shows the different expression pattern of AQP-1, 2, 3, and 4 mRNAs and proteins. These results suggest that a decrease in urinary concentrating ability is a major factor in the decreased AQP-2, 3 expression, and that is partly compensated by increased expression of AQP-4.