Involved in pH regulation to eliminate acids generated by active metabolism or to counter adverse environmental conditions. Major proton extruding system driven by the inward sodium ion chemical gradient. Plays an important role in signal transduction (By similarity). Intestine. Shows an apical localization in intestinal epithelial cells. Expression is higher in 2 and 3 week old rats and lower in adults. Belongs to the monovalent cation:proton antiporter 1 (CPA1) transporter (TC 2.A.36) family.