Water handling in the rat jejunum: effects of acidification of the medium

The minute-by-minute net water movement (Jw) in the rat jejunum was studied in relation to the diffusive water (Pw) and mannitol (Ps) permeabilities with the following results. (a) Jw was a linear function of the applied hydrostatic and osmotic transepithelial gradients (hydrostatic permeability coefficient, Phydr = 0.052 +/- 0.011 cm s-1; osmotic permeability coefficient, Posm = 0.0069 +/- 0.0014 cm s-1. (b) A fraction of this absorptive Jw (transport-associated Jw, Jwt = 0.086 +/- 0.024 microliter min-1 cm-2) was independent of the presence of any osmotic, hydrostatic or chemical gradient. (c) In the absence of Na+, Jwt was not significantly different from zero and there was an increase in Phydr but no change in Posm. (d) In the presence of a hydrostatic gradient (10 cm H2O, mucosal side), acidification of the medium (95% CO2 bubbling, pH 6.2) simultaneously and reversibly increased Jw and decreased Pw. (e) When an osmotic gradient was present (40 mM polyethyleneglycol on the serosal side) a net increase in Jw was observed. CO2 bubbling in these conditions reversibly reduced Jw while increasing Ps. (f) These effects were not observed when the serosal or mucosal pH was reduced in the presence of a nonpermeant buffer (HEPES/TRIS; MES/TRIS). If we accept that Ps is a good marker of paracellular movements and that Pw mainly reflects transcellular water movements, we may conclude that acidification of the medium, in the presence of bicarbonate, modifies both paracellular and transcellular routes. The experimental evidence indicates that an increase in proton concentration opens the paracellular pathway and probably has a blocking effect on a transcellular route.