Effects of medium hypertonicity on water permeability in the mammalian rectum: ultrastructural and molecular correlates

Minute-by-minute net water fluxes (Jw) were measured across the isolated rectal epithelium in rats and rabbits. Five minutes after a serosal (but not mucosal) hypertonic challenge (plus 200 mosmol/l) a significant increase in the basal Jw was recorded in both species [deltaJw, microl min(-1) cm(-2): 0.40+/-0.06 (rats); 0.45+/-0.10 (rabbits)]. At the same time, most epithelial cells shrank markedly while the intercellular spaces were wide open (electron microscopy studies). In freeze-fracture studies multi-strand tight-junction structures (only slightly modified by serosal hypertonicity in rabbits) were observed in control conditions. No structural changes were observed after mucosal hypertonicity (both in rats and rabbits). Immunohistochemical studies showed the expression of aquaporin 3 (AQP3) at the basolateral membrane of epithelial cells in the rat. A first conclusion is that the epithelium of the mammalian rectum is a highly polarized, aquaporin-3-containing, water permeability structure. The Jw increase induced by serosal hypertonicity was sensitive to mercurial agents in both species and no changes in unidirectional [14C]mannitol fluxes (Ps) or transepithelial resistance (RT) were observed during this Jw increase. These observations suggest a transcellular route for the osmotically induced increase in water fluxes. In the rabbit rectum the initial Jw response, associated with serosal hypertonicity, was a transient one. It was followed by a second, slow and HgCl2-sensitive Jw increase (a transient peak in paracellular mannitol permeability was also observed). A second conclusion is that serosal hypertonicity induces an increase in transcellular water permeability in both rat and rabbit rectum.