Biochimica et Biophhysica Acta, 448 (1976) 426--450 (~ Elsevier/North-Holland Biomedical Press BBA 77464 BIDIRECTIONAL SODIUM ION MOVEMENTS VIA THE PARACELLULAR AND TRANSCELLULAR ROUTES ACROSS SHORT-CIRCUITED RABBIT ILEUM* N. L. SIMMONS and R. J. NAFTALIN The Department of Physiology, King's College, London, WC2R 2LS (U.K.) (Received March 22nd, 1976) SUMMARY 1. It has been confirmed that the agent 2,4,6-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 transepithelial 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 asym- metry 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 (J2~) across the mucosal boundary are 7.97 and 7.13/~mol • cm -2 • h -1 respectively. Entry flux (,/12) is a saturable function of Ringer [Na + ]. The calculated K m is 295 mM and the Vis 17.6/tmol • 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 K,, for this relationship is 1.2 mM and the V 2.17/tmol • cm -2 • h -~. The stoichiometry between sugar and Na +' entry is approximately 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 (J2a) and entry (Ja2) Na+ fluxes across the serosal border are 16.74 and 15.90/zmol. cm -2. h -1. 0.1 mM ouabain markedly * The authors wish to point out the following errors which occurred in a previous paper (BBA 77302, Vol. 433, pp. 597-614). p. 600, Table I: The Vfor Na ÷ activation of 2 mM 3-O-methyl glucose, replace Jl a = 0.33 ±0.54 by Jt a = 0.33 4-0.054 and Jt z = 0.33 4-0.65 by J12 = 0.33 4-0.065. The V uptake of fl-methyl glucose with Ringer [Na+l = 140mM, replace ./13 = 0.884-0.50 by 3"13 = 0.8810.059.