News from the End of the Gut—How the Highly Segmental Pattern of Colonic HCO3− Transport Relates to Absorptive Function and Mucosal Integrity

A number of transport mechanisms in the colonic epithelium contribute to HCO3− movement across the apical and basolateral membranes, but this ion has been largely regarded as a by-product of the transport functions it is involved in, such as NaCl- or short chain fatty acid (SCFA) absorption. However, emerging data points to several specific roles of HCO3− for colonic epithelial physiology, including pH control in the colonic surface microenvironment, which is important for transport- and immune functions, as well as the secretion and the rheological properties of the mucus gel. Furthermore, recent studies have demonstrated that colonic HCO3− transporters are expressed in a highly segmental as well as species-specific manner. This review summarizes recently gathered information on the functional anatomy of the colon, the roles of HCO3− in the colonic epithelium, colonic mucosal integrity, and the expression and function of HCO3− transporting mechanisms in health and disease.