An improved everted gut sac as a simple and accurate technique to measure paracellular transport across the small intestine.

An improved everted gut sac system has been developed in which the sacs were carefully prepared from rat small intestine and incubated in tissue culture medium. Under these conditions, the tissue showed good morphology at the electron microscope level, and was metabolically active for up to 2 h at 37 degrees C. Mannitol, an established probe of paracellular transport, was transported from the mucosal to the serosal side of the sac tissue. Excellent kinetic data showed that transport was linear up to 75 min and over a wide range of concentrations (0.025 - (10 mM). Mannitol was not detected in the tissue and transport was enhanced by EGTA, confirming the paracellular route of passage. Sacs prepared from colon also showed mannitol transport, but at a slower rate. Comparisons with Caco-2 cell monolayers showed that the everted sacs exhibited higher levels of paracellular transport than the cultured cell line. The improved everted gut sac system is an inexpensive and relatively simple technique with considerable potential as an in vitro tool to study the mechanisms, kinetics and enhancement of drug absorption across the small intestine at different sites and in the colon.