SUMMARYTransport of lysine across the rat jejunum has been studied measuring transmural fluxes, Jms and Jam, 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.1. Jly can be described as the sum of a saturable process with a Kt of 3 mm and a Jmax of 2.25 ,umole/cm2 . hr and a diffusional component corresponding to a lysine permeability of 0-014 cm/hr. Also Jlys is well described as the sum of a saturable process and a diffusional contribution described by the same permeability as for 3. 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.4. JlYs is in a saturable manner stimulated by increasing concentrations of Dglucose. 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.5. All observations are consistent with a paracellular, transepithelial pathway for lysine, which includes the lateral intercellular spaces.6. The transport of lysine across the basolateral membrane is analysed. Together the data on transcellular passage of lysine are very similar to those reported for rabbit ileum, except that more than one transport process could not be demonstrated.

5. For all the markers used the e.c.s. estimates remained constant between the 40th and 80th min of incubation.6. In the absence of glucose the transepithelial net fluxes of each of the different markers were zero. In the presence of 28 mM glucose the serosato-mucosa fluxes of all markers were dramatically increased. The ratio between the serosa-to-rflucosa and the mucosa-to-serosa fluxes increased in the order [3H]7. The effect of glucose on the flux ratio of the marker substances suggests that glucose-induced net water transport to the serosa side of the gut wall represents the difference between a transcellular net water transport to the serosal side and a significant paracellular net water transport through the lateral intercellular spaces to the mucosal solution.

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Renal plasma clearances (C) of ¹⁴C-tetraethylammonium (TEA) and ρ-aminohippurate (PAH) as estimates of arterial renal plasma flow (ARPF) were evaluated in anesthetized rats during control conditions and during intravenous glucose infusion. Venous renal blood flow was measured directly by means of a servo-controlled pump, keeping the renal venous pressure constant. Arteriovenous extraction fractions (E = 1 – P_renalvenous/P_{renalarterial}) for PAH averaged 88.3 ± (SE) 0.8% in control rats and 82.0 ± 0.9% in glucose-infused rats (p < 0.001); E_TEA averaged 92.0 ± 0.6 and 90.1 ± 0.6%, respectively (p < 0.05). Under both experimental conditions, (C/E)_PAH did not differ significantly from ARPF, while (C/E)_TEA underestimated ARPF; the rate of extraction of TEA exceeded the rate of excretion by 15–20%, probably due to accumulation of TEA in renal tissue. It is concluded that, when corrected for E, C_PAh is in general a more accurate estimate for ARPF than C_TEA. However, under conditions involving changes in plasma glucose levels C_Tea may provide a better estimate of the effective renal plasma flow than C_PAH.