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

Abstract

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.

Expression and immunolocalization of aquaporin‐7 in rat gastrointestinal tract

“…The relative contribution of these three mechanisms to transepithelial water movement along the gastrointestinal tract is far from being elucidated. Special attention should be paid to the role of the AQPs, involved, as in other organs, in the rapid and substantial bidirectional movement of fluid given that eight different AQPs have recently been identified in the gastrointestinal tract of mammals (Frigeri et al, 1995; Koyama et al, 1999; Ramirez‐Lorca et al, 1999; Kierbel et al, 2000; Calamita et al, 2001a; Elkjaer et al, 2001; Hatakeyama et al, 2001; Tani et al, 2001; Parvin et al, 2002; Okada et al, 2003; Mobasheri et al, 2004). However, their topographical distribution, cellular localization, expression and functioning in the gastrointestinal tract have not yet been completely characterized.…”

Low carbon dioxide permeability of the apical epithelial membrane of guinea‐pig colon

“…The presence of aquaporin 1 in colon epithelium is controversial; it has been localized to the apical epithelial membrane in human colon by Hasegawa et al (1994) and in Octodon degus colon by Gallardo et al (2002), but has been reported to be entirely absent from the epithelial membranes in rat colon (Nielsen et al 1993). In contrast to the apical membrane, the basolateral membrane of rat colonic and rectum surface epithelium expresses aquaporin 3 abundantly (Matsuzaki et al 1999, 2004; Kierbel et al 2000). It appears possible, therefore, that the apical (and also the basolateral) membranes of colon epithelium lack aquaporin 1, which is the only aquaporin isoform that has been associated with CO 2 transport.…”

Release and permeation of histamine in isolated caecum epithelia of pigs

“…To elucidate the situation in the caecum, we investigated histamine and mannitol fl uxes in parallel. Mannitol is an established probe to determine paracellular permeability [33,34]. The positive correlation between histamine and mannitol fl ux rates in both fl ux directions (Fig.…”