Lessons from genetically engineered animal models VIII. Absorption and secretion of ions in the gastrointestinal tract

Absorption and secretion of ions in gastrointestinal and other epithelial t issues require the concerted activities of ion pumps, channels, symporters, and exchangers, which operate in coupled systems to mediate transepithelia l transport. Our understanding of the identities, membrane locations, and b iochemical activities of epithelial ion transporters has advanced significa ntly in recent years, but major gaps and uncertainties remain in our unders tanding of their physiological functions. Increasingly, this problem is bei ng addressed by the analysis of mutant mouse models developed by gene targe ting. In this review, we discuss gene knockout studies of the secretory iso form of the Na+-K+-2Cl(-) cotransporter, isoforms 1, 2, and 3 of the Na+/M exchanger, and the colonic H+-K+-ATPase. This approach is leading to a cle arer understanding of the functions of these transporters in the living ani mal.