PROPIONATE ACTIVATES MULTIPLE ION-TRANSPORT MECHANISMS IN THE HT29-18-C1 HUMAN COLON CELL-LINE

Short-chain fatty acids (SCFAs) are the major solutes and the major an ions in the colonic lumen. We studied the response of suspended HT29-1 8-C1 cells (an epithelial cell line derived from a human colon carcino ma) to SCFA exposure. Cellular response was evaluated by measurement o f cell volume (Coulter counter), intracellular pH [pH(i); measured flu orometrically with 2',7'-bis(2-carboxyethyl)-5-(6)-carboxyfluorescein (BCECF)], and intracellular Na+, K+, and Cl- content (flame photometry and chloride titrator). Exposure to 130 mM propionate in isosmotic me dium causes a rapid decrease in pH(i) and activates pH(i) recovery via amiloride-sensitive Na-H exchange. In the presence of propionate, Na- H exchange also causes cell swelling to a peak volume 11% above contro l cells and causes a 2.8-fold increase in intracellular Na+ content. A fter peak swelling, a regulatory volume decrease (RVD) significantly r educed volume and intracellular Na+ returned to baseline. Other SCFAs (acetate, butyrate, and valerate) also elicit swelling and RVD. Activa tion of the Na+-K+-adenosinetriphosphatase (ATPase) is required to ret urn Na+ to normal levels and to indirectly provide ion gradients requi red for propionate-induced RVD, but Na+-K+-ATPase activity does not di rectly mediate RVD. When 1 mM -acetamido-4'-isothiocyanostilbene-2,2'- disulfonic acid (SITS) is added in the presence of propionate, RVD was inhibited and cell Na+ content increased. Cl- depletion inhibited pro pionate-induced RVD and diminished the effect of SITS, Under identical gradient conditions, SITS had no effect on Cl- flux in either the pre sence or absence of propionate, suggesting an indirect Cl- requirement . In summary, exposure of cultured colonocytes to propionate activates multiple ion transporters responsible for net cation uptake, recovery of pH(i), and recovery from swelling.