The paracellular pathway in the lepidopteran larval midgut: Modulation by intracellular mediators

The features of the paracellular pathway, an important route for the transfer of ions and molecules in epithelia, are in insects still poorly investigated and it has not yet been elucidated how the septate junction (SJ) acts as a transepithelial barrier. In this study, some properties of the paracellular pathway of Bombyx mori larval midgut, isolated in Ussing chambers, were determined and the modulation of SJ permeability by intracellular events disclosed. Diffusion potentials evoked by transepithelial gradients of different salts indicated that the junction bore weak negative charges and that the paracellular pathway was selective with respect to ion charge and size. In standard conditions, the transepithelial resistance was 28.2 ± 2.1 Ω cm2, a value indicating that the midgut is a low resistance epithelium. The modulation of midgut SJ by typical enhancers of mammalian tight junction permeability known to act on the cytoskeleton was studied by measuring the shunt resistance and the lumen-to-haemolymph flux of sucrose. An increase of the intracellular level of cAMP and Ca2+ caused a significant decrease of the shunt resistance and an increase of SJ permeability. The attenuation of Ca2+ effect in the presence of the calcium channel blocker nifedipine indicated that the influx of external Ca2+ into the cytoplasm was important for the opening of the SJ, as well as the release of Ca2+ from the intracellular stores.