The passive permeability properties of in vivo perfused rat jejunum.

The polarity of the microvillus membrane of rat and rabbit jejunum, determined in vitro by the incremental free energy changes associated with the addition of -CH2 groups to fatty acids or -OH groups to bile acids, has been found to be more polar than other biological membranes. The reason for the difference in polarity is unexplained but could be due to artifacts caused by the in vitro conditions. In the current studies the apparent permeability coefficients were determined for a homologous series of fatty acids and bile acids in in vivo perfused rat jejunum. The true permeability coefficients were derived by correction for diffusion barrier resistance. The incremental free energy changes associated with the addition of a -CH2 group to a fatty acid and a -OH group to a bile acid were -619 and +2069 cal/mol, respectively. These values correspond to values determined in other biological membranes such as erythrocytes and adipocytes. Thus, the rat microvillus membrane is more nonpolar than previously observed in vitro. The reason for the discrepancy between the in vivo and in vitro results is most likely due to an underestimation of the permeability coefficients in the in vitro studies.