NHE3-dependent cytoplasmic alkalinization is triggered by Na+-glucose cotransport in intestinal epithelia

Cytoplasmic pH (pHi) was evaluated during Na+-glucose cotransport in Caco-2 intestinal epithelial cell monolayers. The pHi increased by 0.069 ± 0.002 within 150 s after initiation of Na+-glucose cotransport. This increase occurred in parallel with glucose uptake and required expression of the intestinal Na+-glucose cotransporter SGLT1. S-3226, a preferential inhibitor of Na+/H+ exchanger (NHE) isoform 3 (NHE3), prevented cytoplasmic alkalinization after initiation of Na+-glucose cotransport with an ED50 of 0.35 μM, consistent with inhibition of NHE3, but not NHE1 or NHE2. In contrast, HOE-694, a poor NHE3 inhibitor, failed to significantly inhibit pHi increases at <500 μM. Na+-glucose cotransport was also associated with activation of p38 mitogen-activated protein (MAP) kinase, and the p38 MAP kinase inhibitors PD-169316 and SB-202190 prevented pHi increases by 100 ± 0.1 and 86 ± 0.1%, respectively. Conversely, activation of p38 MAP kinase with anisomycin induced NHE3-dependent cytoplasmic alkalinization in the absence of Na+-glucose cotransport. These data show that NHE3-dependent cytoplasmic alkalinization occurs after initiation of SGLT1-mediated Na+-glucose cotransport and that the mechanism of this NHE3 activation requires p38 MAP kinase activity. This coordinated regulation of glucose (SGLT1) and Na+ (NHE3) absorptive processes may represent a functional activation of absorptive enterocytes by luminal nutrients.