Saturable Absorption of Glycerol in the Rat Intestine

The permeability of glycerol, a small hydrophilic solute, across the intestinal membrane would be low, if passive diffusion restricted to the paracellular route is the principal transport mechanism as generally assumed for this class of solutes. However, in the present study using a closed loop of rat small intestine in situ, we found that the absorption of glycerol was faster than that of urea, a probe solute widely assumed to permeate exclusively via the paracellular route. This finding is inconsistent with the paracellular permeation hypothesis, which predicts that the absorption of glycerol, which is larger than urea in terms of molecular size, could not be faster than that of urea. We also found that glycerol absorption was saturable. These findings suggest the involvement of carrier-mediated transport in intestinal glycerol absorption. Glycerol absorption in the colon was also saturable, suggesting the involvement of carrier-mediated transport, although it was much slower than that in the small intestine. Carrier-mediated glycerol transport might play an important role in absorbing glycerol liberated from dietary triglyceride. It would be interesting to further examine the possibility that a carrier-mediated glycerol transport system (or systems) might be involved in drug absorption and also that it might be utilized for oral drug delivery.