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1. The fluxes of Na were measured on isolated coprodeal mucosa at 1--220 mM-Na from hens on low (L) and high (H) Na diets with the purpose of finding the location and characteristics of Na sites activated in the cellular pathway by L. 2. The influx across the brush border, JNamc, and the transmural fluxes, JNasm and JNams, were determined. Effects on these fluxes of ouabain, 10(-3) M in the serosal solution, and amiloride, 10(-4) M in the mucosal solution, were studied for both dietary states. 3. JNamc was 5--22 (L) and 0--0.8 (H) muequiv/cm2.hr at 130 mM-Na corrected for the paracellular flux of Na. The JNamc (H) is tenfold smaller than found by Choshniak, Munck & Skadhauge (1977). This discrepancy is at present inexplicable. Amiloride completely inhibited JNamc (L). Preincubation in 0 or 130 mM-Na had no effect on JNamc. Ouabain reduced JNamc (L) by only about 37% after preincubation at 130 mM-Na. The Kt of JNamc was 5.1 (L) and 50.6 (H) mM-Na. 4. JNasm was 50 (H) and 61 (L) n-equiv/cm2.hr at 6.5 mM-Na. Ouabain increased JNasm by 360% in the low Na state. The increased JNasm was inhibited 74--100% by amiloride. This is interpreted as a ouabain induced Na-Na exchange at the basolateral Na-K-ATPase and an almost complete block of JNacm by amiloride. A similar exchange of Na at the basolateral membrane in the high-Na state was revealed by 'opening' the brush border for Na with monensin added to the mucosal solution. Amiloride in itself prevented a 50% recirculation of Na via the paracellular route and back across the cells in the low Na state. 5. JNams was 5.6 (L) muequiv/cm2.hr and 187 (L) microA/cm2 at 6.5 mM-Na. Amiloride reduced these values to 0.4 muequiv/cm2.hr and 5.8 microA/cm2. On addition of amiloride the transmural resistance in (L) coprodea at 130 mM-Na increased from 140 to 190 and it remained unchanged at 260 omega cm2 in (H) coprodea. The resistance of (L) birds, 163, was not affected by ouabain, 166 (L) omega cm2. 6. 20:1 NaCl dilution potentials at the mucosal side of 17--18 mV (L) and nearly zero (H) had half-times around 1 sec. Amiloride eliminated completely these diffusion potentials. The short half-time indicates a location in the brush border of sodium specific sites induced by the low-Na diet. This conclusion is oppsite to that described by Choshniak et al. (1977). 7. Ion selectivity, voltage--current and conductance--concentration relations in the presence of amiloride indicated a weakly cation selective and highly hydrated pathway, which was also thick and with neutral sites. This fits a paracellular route with the limiting barrier for ions at the tight junction.