Functional characterization of the carrier-mediated transport system for glycerol in everted sacs of the rat small intestine

Abstract

The mechanism of intestinal glycerol transport was investigated by using the in vitro everted sac method involving the rat small intestine. The uptake of glycerol into everted sacs was saturable with a Michaelis constant (K m) of 0.77 mM and a maximum transport rate (Jmax) of 11.5 nmol/min/100 mg wet tissue weight (wtw), suggesting the involvement of carrier-mediated transport, and was accompanied by unsaturable transport (passive transport) with a membrane permeability clearance (CLm,d) of 4.9 μl/min/100 mg wtw. The carrier-mediated uptake of glycerol was inhibited by the removal of Na+ and also by the addition of 2,4-dinitrophenol (DNP) and sodium azide (NaN3), which are metabolic inhibitors. These results suggest that the carrier-mediated glycerol transport is Na +-dependent and secondary active. Since glycerol uptake was also inhibited by p-chloromer-curibenzene sulfonate (pCMBS), a thiol-modifying reagent, cysteine residues, which have a thiol group, seem to play an important role in the function of the carrier. We further found that glycerol uptake was selectively inhibited by glycerol-3-phosphate, chloramphenicol and voglibose, which are alcohol-related compounds analogous to glycerol. Several other compounds that did not inhibit glycerol uptake included D-glucose and 5-fluorouracil, which are known to be transported by specific carriers, and none of the selective inhibitors of glycerol uptake inhibited the uptake of D-glucose and 5-fluorouracil. Therefore, the carriers for these two compounds do not seem to be involved in glycerol uptake. It is likely that the carrier-mediated transport system involved in glycerol uptake is specific to glycerol and, possibly, some analogous compounds with hydroxyl groups. It would be interesting to examine the possibility that the carrier-mediated glycerol transport system might be involved in drug absorption and also that it might be used for oral drug delivery. © 2004 Pharmaceutical Society of Japan.