In vitro diffusion barriers of the mouse jejunum in Ussing chambers.

Epithelial cells and their intercellular junctions play a primary role in the intestinal mucosal diffusion barrier to the invasion of noxious agents. Our previous study has shown that the mouse jejunum, when incubated in Ussing chambers for 4 h, exhibited morphological deterioration of the villi with denudation of the epithelia while it retained cAMP-induced potential difference (PD), suggesting that some subepithelial tissues had taken part in the barrier functions when the primary epithelial barrier on the villous surface was broken. To further characterize the barrier function of the jejunum in Ussing chambers, we measured the unidirectional lucifer yellow flux (J(LY)), which represents the permeation of a medium-sized anion, the transmural electrical conductance (G(t)), which reflects the permeation of electrolytes, mainly NaCl, and forskolin-induced PD. The values of J(LY), G(t) and forskolin-induced PD were not affected by removing the muscularis propria, suggesting that this tissue did not substantially contribute to the barrier. In addition, the values of J(LY), G(t) and forskolin-induced PD were not correlated with the degree of denudation of epithelial cells on the mucosal surface, supporting the notion that the main barrier function is constituted by the subepithelial tissues rather than the epithelium. Loosening the tissue (thereby approximating the free diffusion condition) by removing Ca(2+) from the bathing solution increased both J(LY) and G(t), with the former being larger than the latter. In conclusion, the mucosa propria and/or the submucosa constitute a diffusion barrier that restricted the permeation of lucifer yellow more tightly than NaCl in the injured jejunum incubated in Ussing chambers.