Effect of osmolarity on the epithelial paracellular permeability in rat jejunum

Studies in vivo have shown an important increase in the substrate passive permeability across small intestine when Na(+)-cotransported substrates, like galactose, are present at the luminal side. The influence of the solution osmolarity on the passive absorption of mannitol or 2-deoxy-D-glucose across rat jejunum and on the galactose-increasing effect is now studied both in vivo and in vitro. In vivo, luminal perfusion with 400 or 500 mosm/L solutions does not affect passive absorption of 10 mmol/L 2-deoxy-D-glucose, although a net fluid secretion towards lumen is observed. Luminal hyperosmolarity, however, prevents the stimulatory action of 25 mmol/L galactose on the passive absorption, a stimulation that is well manifested in the same intestinal segment when the perfusion is made with isoosmotic solutions. However, in vitro results with everted intestinal sacs or with preparations of intestinal wall in Ussing chambers, indicate that hyperosmolarity (500 mosm/L) of the solutions clearly increases net passive mucosal to serosal flux of D-mannitol or 2-deoxyglucose. With low osmolarity solutions (180 or 160 mosm/L by diminution of NaCl) the in vivo passive mannitol absorption is not affected and the stimulatory action of 25 mmol/L galactose is not observed, although net water absorption is enhanced. Moreover, the passive absorption stimulation by the galactose cotransport in isoosmotic solutions is dependent on Na+ levels requiring at least 80 mmol/L. Results suggest that passive paracellular absorption across the small intestine may be modified by changes in the luminal content composition.