Bidirectional sodium ion movements via the paracellular and transcellular routes across short-circuited rabbit ileum

1. It has been confirmed that the agent 2,3,6-triaminopyrimidine decreases Na+ conductance in the paracellular pathway of rabbit ileum. 2. Triaminopyrimidine has been used as a means of measuring transcellular bidirectional Na+ flux, and also, of assessing the contribution of the paracellular pathway to transepithlial Na+ flux. 3. Reduction of Ringer [Na+] to 25 mM or incubation with 0.1 mM ouabain reduces paracellular Na+ permeability. This effect may be due to lateral space collapse. Ringer galactose increases serosa to mucosa Na+ flux by a stimulating reflux through the tight junctions. A proportion of net Na+ flux in control tissues is due to asymmetry generated in the paracellular pathway. It is likely that this passive asymmetry results from an osmotic pressure gradient across the tight-junction. 4. Measurement of the tissue isotope specific activity ratio together with bidirectional transcellular Na+ fluxes allows calculation of the four unidirectional fluxes across the mucosal and serosal boundaries. Values obtained for Na+ entry (J12) and exit (J21) across the mucosal boundary are 7.97 alnd 7.13 mumol-cm(-2)-h(-1) respectively. Entry flux (J12) is a saturable function of Ringer [Na+]. The calculated Km is 295 mM and the V is 17.6 mumul-cm(-2)-h(-1). Na+ entry flux is insensitive to ouabain (0.1 mM). Ouabain results in elevation of exit (J21) flux of Na+ across the brush border. D-Galactose causes a saturable increase in Na+ flux (J12) across the mucosal boundary; the Km for this relationship is 1.2 mM and the V 2.17 mumol-cm(-2)-h(-1). The stoichiometry between sugar and Na+ entry is applixmately 1:1. In contrast to the effect of galactose on entry flux, no change in Na+ efflux across the mucosal boundary is observed when Ringer [galactose] is raised. This finding is dissonant with the prediction of the Na+ -gradient hypothesis. The calculated values of exit (J23) and entry (J32) Na+ fluxes across the serosal border are 16.74 and 15.90 mumol-cm(-2)-h(-1). 0.1 mM ouabain markedly reduces both these unidirectional fluxes. This result is consistent with a serosal location of the Na+-pump. Serosal Na+ exit flux J23 increases as a hyperbolic function of Ringer [galactose]. A small galactose-dependent decrease in entry (J32) is also observed. 0.1 mM ouabain abolishes these galactose-dependent changes. 5. The present findings together with those in the previous paper are discussed in relation to the convective-diffusion model for sugar transport.