POLARIZED NA+ H+ EXCHANGE FUNCTION IS PLIABLE IN RESPONSE TO TRANSEPITHELIAL GRADIENTS OF PROPIONATE/

Short-chain fatty acids are produced at high concentration in the colo nic lumen and stimulate electroneutral Na+ absorption by activating ap ical Na+/H+ exchange in colonocytes. We used an epithelial cell line d erived from a human colon carcinoma (HT29-18-C-1) to study activation of apical and basolateral Na+/H+ exchange by a short-chain fatty acid, propionate. Confluent cell monolayers on membrane filters were loaded with 2',7'-bis(2-carboxyethyl)-5 (and 6)-carboxyfluorescein (a fluore scent pH indicator) and intracellular pH was monitored with a digital fluorescence imaging microscope. Cells acidified by transient exposure to NH4Cl demonstrated both apical and basolateral Na+/H+ exchange. In this condition, apical Na+/H+ exchange was 50% of the total Na+/H+ ex change activity. Similar results were obtained when cells were bilater ally perfused with apicat and basolateral propionate in an isosmotic m edium (130 mM propionate at each membrane surface). However, apical Na +/H+ exchange was a significantly larger fraction (76%) of the total N a+/H+ exchange activity when cells were acidified by exposure to apica l propionate alone. Conversely, in cells acidified by basolateral prop ionate alone, apical Na+/H+ exchange was 21% of the total Na+/H+ excha nge activity. The change in relative activity was observed in individu al cells which expressed both apical and basolateral Na+/H+ exchange a nd occurred rapidly (within 7 min). In the presence of transepithelial propionate gradients, all Na+-dependent alkalinization was sensitive to 3 mu M 5-(N-ethyl-N-isopropyl)amiloride, a potent Na+/H+ exchange i nhibitor. These results suggest that transepithelial gradients of shor t-chain fatty acids, which occur in vivo, can cause preferential activ ation of apical Na+/H+ exchange.