Regulation of Intestinal Epithelial Brush Border Na+/H+ Exchanger Isoforms, NHE2 and NHE3, in C2bbe Cells

Until recently, studies to characterize the intestinal epithelial Na+/H+ exchangers had to be done in nonepithelial, mutated fibroblasts. In these cells, detection of any Na+/H+ exchange activity requires prior acid loading. Furthermore, most of these experiments used intracellular pH changes to measure NHE activity. Because changes in pHi only approximate Na+/H+ exchange activity, and may be confounded by alterations in buffering capacity and/or non-NHE contributions to pH regulation, we have used 22[Na] unidirectional apical to cell uptake to measure activities specific to NHE2 or NHE3. Furthermore, we performed these measurements under basal, nonacid-stimulated conditions to avoid bias from this nonphysiological experimental precondition. Both brush border NHEs, when expressed in the well-differentiated, intestinal villuslike Caco-2 subclone, C2bbe (C2), localize to the C2 apical domain and are regulated by second messengers in the same way they are regulated in vivo. Increases in intracellular calcium and cAMP inhibit both isoforms, while phorbol ester affects only NHE3. NHE2 inhibition by cAMP and Ca++ involves changes to both KNa and Vmax. In contrast, the same two second messengers inhibit NHE3 by a decrease in Vmax exclusively. Phorbol ester activation of protein kinase C alters both Vmax and KNa of NHE3, suggesting a multilevel regulatory mechanism. We conclude that NHE2 and NHE3, in epithelial cells, are basally active and are differentially regulated by signal transduction pathways.