The roles of paracellular and transcellular pathways and submucosal space in isotonic water absorption by rabbit ileum.

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Abstract
Water movements have been studied in sheets of isolated rabbit ileum using a method which measures net volume flows across the mucosal and serosal surfaces of the tissue continuously with high resolution. At 35% C, with the tissues incubated in isotonic Ringer solution containing D-glucose (25 mM) on both sides, there is a steady net inflow of fluid at the rate of 24 .+-. 2 .mu.l cm-2 h-1 across the mucosal surface (Jm) and an outflow of 8 .+-. 1 .mu.l cm-2 h-1 across the serosal surface (Js) n = 16). The stable transepithelial p.d. across these tissues is 2.7 .+-. 0.2 mV, serosa positive, Jm can be reversibly inhibited by anoxia. Ouabain (0.1 mM) added to the serosal solution inhibits inflow across the mucosal and serosal surfaces by 75% (n = 7) within 30 min. If phlorizin (0.1 mM) is added to the mucosal Ringer solution containing glucose (20 mM) within 30 min of the commencement of in vitro absorption, Jm is reduced from 37 .+-. 3 to 28 .+-. 2 .mu.l cm-2 h-1 (n = 3). Dilution of the mucosal Ringer solution by 50 mosmol kg-1 (with the serosal solution kept isosmolar) results in a rapid transient increase in mucosal inflow. An increase of 50 mosmol kg-1 in the mucosal Ringer solution with NaCl, sucrose or mannitol causes a transient reversal of mucosal flow, followed by a return of inflow at a reduced level. Rabbit ileum can transport water against gradients of approximately 75 mosmol kg-1 of sucrose, NaCl, or mannitol. Addition of polyethylene glycol (mol. wt. 20,000; 3 mosmol kg-1) causes a sustained reversal of mucosal inflow; inflow can be restored only by removing polyethylene glycol from the mucosal Ringer solution. The tissue can absorb water against an osmotic gradient of 200 mM-glycerol. The above data have been incorporated into a new model to explain isotonic flow of fluid by this epithelium. The main features are that the hydraulic conductivity (Lp) of the mucosal boundary of the lateral intercellular space is approximately 1 .times. 10-8 cm s-1 cmH2O-1. This Lp is too low to sustain isotonicity of the flow emerging from the lateral intercellular space at the observed rates. Hypertonic fluid emerging from the lateral intercellular space is diluted by transcellular water flow generated by the hypertonicity of the submucosa and back-diffusion of solute via mucosal shunt channels. The solute concentration of the submucosa at steady state is estimated to be only 2-5% above the mucosal solution tonicity. Net absorption across the mucosa occurs simultaneously with a back-leak driven by the interstitial pressure into the mucosal solution via wide extracellular pores.