Renal Adaptive Responses of Na+-K+-APTase Subunit Isoforms to Chronic Hypokalemia

Ahn, Kyu Youn; Kim, Sug Chae; Moon, Bum; Kim, Kyung Keun; Kim, Baik Yoon

Abstract

Chronic hypokalemia alters Na+-K+-ATPase gene expression in several tissues. While it is established that Na+-K+-ATPase activity and alpha1 and beta1 subunit protein levels increase during K depletion in the outer medullary collecting duct (OMCD) and do not significantly change in the cortical collecting duct (CCD), little is known about the adaptive responses of the other isoforms in these other nephron segments. Accordingly, this study was performed to characterize the relative levels of expression and cellular distribution of mRNAs encoding the Na+-K+-ATPase subunit isoforms in normal and K-deprived (2 weeks) rats using the Northern analysis and in situ hybridization (ISH). Isoform specific 32P-labeled cDNA (for Northerns) or digoxigenin labeled cRNA (for ISH) probes were used. In normal rats, the order of expression amounts of all isoforms mRNAs from highest was outer medulla > cortex > inner medulla, and that of K-deprived rats was outer medulla > inner medulla > cortex. alpha1 mRNA levels were much greater than those of alpha2 or alpha3 in cortex, outer and inner medulla. mRNA levels for all isoforms were 2~3 folds greater in inner medulla of K-deprived rats compared to controls. In contrasts, the levels of all isoforms mRNAs in cortex and outer medulla were comparable between the two gruops. By ISH, mRNAs for all isoforms were observed in the S3 segment of proximal tubule, the cortical thick ascending limb (CTAL), medullary thick ascending limb (MTAL), distal convoluted tubule (DCT), connecting tuble (CNT), and the entire collecting duct. Both groups exhibited comparable cellular patterns of labeling, but the signal intensity of K-deprived rats was much greater in the proximal portion of the inner stripe of outer medullary collecting duct (OMCDi) and proximal portion of the inner medullary collecting duct (IMCD), and less in the MTAL compared to controls. The signal intensity of alpha1, alpha3, and beta1 isoforms was less in the CTAL, DCT, and CCD of K-deprived rats, but alpha2 isoform was slightly increased. These results suggest that chronic hypokalemia enhances expression of Na+-K+-ATPase subunit isoforms in the proximal portion of OMCDi and proximal IMCD, but not other nephron segments, and that these isoforms may participate in potassium conservation by these segments during potassium deprivation.