Search for carrier-mediated transport systems in the rat colon

Several nutrients and drugs, which are known to be absorbed by specific carrier-mediated transport systems in the small intestine, had their transport investigated in the rat colon, by measuring uptake into everted sacs, to find if carrier-mediated transport systems may also be present in the colon. Among those transported by Na+-dependent carriers in the small intestine, D-glucose and taurocholate were found to be transported in an Na+-dependent manner in the colon, while 5-fluorouracil and ascorbate were not. It was also found that the colonic transports of D-glucose and taurocholate were saturable. These results suggest the presence in the colon of Na+-dependent carrier-mediated transport systems for D-glucose and taurocholate, but not for 5-fluorouracil and ascorbate. For nicotinate and methotrexate, which are transported by H+-dependent carriers in the small intestine, their transport was elevated at a lower pH (5.0) than the pH 7.4 in the colon, but not saturable. Therefore, the elevated transport of these acidic compounds may be explained by an increase in passive flux due to an increase in the fraction of the unionized and/or neutral forms, without postulating the presence of H+-dependent carrier-mediated transport systems in the colon. The transport activity of the suggested colonic transport systems for D-glucose and taurocholate was much lower than those of their respective counterparts in the small intestine. However, it may be possible to use them for oral drug delivery via the colon. Their physiological roles would also be of interest.