Apical NA⁺/H⁺ exchangers in the mammalian gastrointestinal tract

The Slc9a family of nine Na⁺/H⁺ exchangers (NHE) plays a critical role in neutral sodium absorption in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins mediate the electroneutral exchange of Na⁺ and H⁺ and are crucial in a variety of physiological processes, including the fine tuning of intracellular pH, cell volume control and systemic electrolyte, acid-base and fluid volume homeostasis. Here, we review the role of the Na⁺/H⁺ exchange mechanism as it relates to the physiology of organs and cells involved in nutrient absorption, and we describe physiological and molecular aspects of individual isoforms, including their structure, tissue-, and subcellular distribution, as well as their regulation by physiological stimuli at the transcriptional and post-transcriptional levels. A particular emphasis is placed on Na⁺/H⁺ exchanger isoforms expressed on the apical (brush border) membrane of the epithelial cells, and the consequences of gene-targeted mutation of individual isoforms are discussed in the context of the physiology of digestive organs. Where available, we also provide a review of pathophysiological states related to aberrant expression and/or activity of Na⁺/H⁺ exchangers within the confines of the digestive system.