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An inverse relationship exists between urinary tissue kallikrein (TK) excretion and blood pressure in humans and rodents. In the kidney TK is synthesized in large amounts in the connecting tubule and is mainly released into the urinary fluid where its function remains unknown. In the present study mice with no functional gene coding for TK (TK-/-) were used to test whether the enzyme regulates apically expressed sodium transporters. Semiquantitative immunoblotting of the renal cortex revealed an absence of the 70-kDa form of gamma-ENaC in TK-/- mice. Urinary Na+ excretion after amiloride injection was blunted in TK-/- mice, consistent with reduced renal ENaC activity. Amiloride-sensitive transepithelial potential difference in the colon, where TK is also expressed, was decreased in TK-/- mice, whereas amiloride-sensitive alveolar fluid clearance in the lung, where TK is not expressed, was unchanged. In mice lacking the B2 receptor for kinins, the abundance of the 70-kDa form of gamma-ENaC was increased, indicating that its absence in TK-/- mice is not kinin-mediated. Incubation of membrane proteins from renal cortex of TK-/- mice with TK resulted in the appearance of the 70-kDa band of the gamma-ENaC, indicating that TK was able to promote gamma-ENaC cleavage in vitro. Finally, in mouse cortical collecting ducts isolated and microperfused in vitro, the addition of TK in the luminal fluid increased significantly intracellular Na+ concentration, consistent with an activation of the luminal entry of the cation. The results demonstrate that TK, like several other proteases, can activate ENaC in the kidney and the colon.