The chloride conductance of tight junctions of rat ileum can be increased by cAMP but not by carbachol.

It is well known, that in mammalian small intestine, cAMP increases Cl- permeability of the apical membrane of enterocytes as part of its secretory action. Paradoxically, this is usually accompanied by an increase of the transepithelial resistance. In the present study we report that in the presence of bumetanide (to block basolateral Cl- uptake) cAMP always decreased the transepithelial resistance. We examined whether this decrease in resistance was due to a cAMP-dependent increase of the paracellular electrolyte permeability in addition to the increase of the Cl- permeability of the apical cell membrane. We used diffusion potentials induced by serosal replacement of NaCl, and transepithelial current passage to evoke transport number effects. The results revealed that cAMP (but not carbachol) could increase the Cl- permeability of the tight junctions in rat ileum. Moreover, we observed a variation in transepithelial resistance of individual tissue preparations, inversely related to the cation selectivity of the tissue, suggesting that Na+ permeability of the tight junctions can vary between preparations.