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 se cretory action. Paradoxically, this is usually accompanied by an incre ase of the transepithelial resistance. In the present study we report that in the presence of bumetanide (to block basolateral Cl- uptake) c AMP always decreased the transepithelial resistance. We examined wheth er 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 diffusio n potentials induced by serosal replacement of NaCl, and transepitheli al current passage to evoke transport number effects. The results reve aled that cAMP (but not carbachol) could increase the Cl- permeability of the tight junctions in rat ileum. Moreover, we observed a variatio n in transepithelial resistance of individual tissue preparations, inv ersely related to the cation selectivity of the tissue, suggesting tha t Na+ permeability of the tight junctions can vary between preparation s.