Abstract
Selective pharmacological Na+/H+ exchange (NHE) inhibitors were used to identify functional NHE isoforms in human small intestinal enterocytes (Caco-2) and to distinguish between direct and indirect effects on transport via the intestinal di/tripeptide transporter hPepT1. The relative potencies of these inhibitors to inhibit 22Na+ influx identifies NHE3 and NHE1 as the apical and basolateral NHE isoforms. The Na+-dependent (NHE3-sensitive) component of apical dipeptide ([14C] Gly-Sar) uptake was inhibited by the selective NHE inhibitors with the same order of potency observed for inhibition of apical 22Na+ uptake. However, 5-(N-ethyl-N-isopropyl) amiloride (EIPA) also reduced [14C]Gly-Sar uptake in the absence of Na+ and this inhibition was concentration and pH (maximal at pH 5.5) dependent. NHE3 inhibition by S1611 and S3226 modulates dipeptide uptake indirectly by reducing the transapical driving force (H+ electrochemical gradient). EIPA (at 100 microM) has similar effects, but at higher concentrations (> 200 microM) also has direct inhibitory effects on hPepT1.