Lysine transport across rat jejunum: distribution between the transcellular and the paracellular routes.

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Abstract
Transport of lysine across the rat jejunum was studied measuring transmural fluxes, Jms and Jsm, under short-circuit conditions, influx across the brush-border membrane, Jmc, under open-circuit and voltage-clamp conditions and steady-state uptake by the isolated mucosa. .**GRAPHIC**. can be described as the sum of a saturable process with a Kt [transport constant] of 3 mM and a Jmax of 2.25 .mu.mol/cm2 .cntdot. h and a diffusional component corresponding to a lysine permeability of 0.014 cm/h. .**GRAPHIC**. is well described as the sum of a saturable process and a diffusional contribution described by the same permeability as for .**GRAPHIC**. The effects of the transmural p.d. [potential difference] on .**GRAPHIC**. indicate that at 60 mM this flux includes a diffusional contribution, which corresponds to a lysine permeability of 0.014 cm/h. The passage of an electrical current across the gut wall changes the electrical conductance as expected for a cation-selective epithelium. The effect of a mucosa to serosa current on the Jms value of mannitol provides confirmation of the expected current effect on transepithelial volume flow. These effects on conductance and solute flux, together with the electrostatic effect on lysine movements, suffice to account for the p.d. effects on Jmc, Jms, and Jsm of lysine. .**GRAPHIC**. is in a saturable manner stimulated by increasing concentrations of D-glucose. At higher (10 mM) concentrations of lysine this effect leads to a net secretion of lysine. Qualitatively and quantitatively these effects are consistent with the model of a glucose-induced fluid circuit between the mucosal solution and the lateral intercellular spaces. A paracellular, transepithelial pathway may exist for lysine, which includes the lateral intercellular spaces. The transport of lysine across the basolateral membrane is analyzed. Together the data on transcellular passage of lysine are very similar to those reported for rabbit ileum, except that more than 1 transport process could not be demonstrated.