Sodium ion transport in isolated intestinal epithelial cells. The effect of actively transported sugars on sodium ion efflux

A technique to measure Na⁺ efflux from isolated intestinal epithelial cells has permitted us to examine the mechanisms responsible for Na⁺ transport in absorptive cells without contamination by other cell types. We examined the effect of actively transported sugars on Na⁺ efflux from isolated rat jejunal epithelial cells to evaluate the mechanism by which actively transported non-electrolytes stimulate Na⁺ absorption. Glucose, galactose and 3-O-methylglucose, sugars known to be actively transported by the small intestine, stimulate total Na⁺ efflux from isolated epithelial cells. This stimulation results from an increase of active Na⁺ transport, since it is inhibited by ouabain. Glucose stimulation is significantly greater than that produced by galactose or 3-O-methylglucose, 2-Deoxyglucose, a sugar that is not actively transported, has no effect on total Na⁺ efflux from isolated cells. Phloridzin, which has no effect on Na⁺ efflux in a sugar-free medium, completely abolishes the effect of galactose. These findings (a) support the hypothesis that the increase in intestinal absorption of Na⁺ in the presence of actively transported non-electrolytes occurs by a transcellular route; and (b) are consistent with the ion-gradient model. The results are not compatible with the direct energy-coupling model.     

Ethanol selectively affects Na+-gradient dependent intestinal transport systems

Moderate concentrations of ethanol reduce the velocity of uptake of three representative Na+-symport systems (D-glucose, L-alanine, L-ascorbate), whether electrogenic (the first two) or electroneutral (L-ascorbate). This 'inhibition' is observed only if these transport systems are tested in the presence of an initial Na+ gradient (out greater than in); no inhibition is found in tracer-equilibrium exchange measurements. A representative Na+-independent system (D-fructose) is not inhibited by ethanol. 'Passive diffusion' (measured as uptake of L-glucose) is increased somewhat by alcohol. All these observations can be rationalized [as suggested by Tillotson et al. (1981) Arch. Biochem. Biophys. 207, 360-370] by an effect of ethanol on passive diffusion, which leads to a faster collapse of the Na+ gradient, with the resulting reduction of the uptake velocities of Na+-dependent transport systems when tested with the added driving force of an Na+ out----in gradient.     

Aldosterone regulation of intestinal Na absorption involves SGK-mediated changes in NHE3 and Na+ pump activity

Aldosterone-induced intestinal Na(+) absorption is mediated by increased activities of apical membrane Na(+)/H(+) exchange (aNHE3) and basolateral membrane Na(+)-K(+)-ATPase (BLM-Na(+)-K(+)-ATPase) activities. Because the processes coordinating these events were not well understood, we investigated human intestinal Caco-2BBE cells where aldosterone increases within 2-4 h of aNHE3 and alpha-subunit of BLM-Na(+)-K(+)-ATPase, but not total abundance of these proteins. Although aldosterone activated Akt2 and serum glucorticoid kinase-1 (SGK-1), the latter through stimulation of phosphatidylinositol 3-kinase (PI3K), only the SGK-1 pathway mediated its effects on Na(+)-K(+)-ATPase. Ouabain inhibition of the early increase in aldosterone-induced Na(+)-K(+)-ATPase activation blocked most of the apical NHE3 insertion, possibly by inhibiting Na(+)-K(+)-ATPase-induced changes in intracellular sodium concentration ([Na](i)). Over the next 6-48 h, further increases in aNHE3 and BLM-Na(+)-K(+)-ATPase activity and total protein expression were observed to be largely mediated by aldosterone-activated SGK-1 pathway. Aldosterone-induced increases in NHE3 mRNA, for instance, could be inhibited by RNA silencing of SGK-1, but not Akt2. Additionally, aldosterone-induced increases in NHE3 promoter activity were blocked by silencing SGK-1 as well as pharmacological inhibition of PI3K. In conclusion, aldosterone-stimulated intestinal Na(+) absorption involves two phases. The first phase involves stimulation of PI3K, which increases SGK-dependent insertion and function of BLM-Na(+)-K(+)-ATPase and subsequent increased membrane insertion of aNHE3. The latter may be caused by Na(+)-K(+)-ATPase-induced changes in [Na] or transcellular Na flux. The second phase involves SGK-dependent increases in total NHE3 and Na(+)-K(+)-ATPase protein expression and activities. The coordination of apical and BLM transporters after aldosterone stimulation is therefore a complex process that requires multiple time- and interdependent cellular processes.     

Influence of luminal Na+ on the intestinal absorption of sugars in vivo

The effect of substituting Na+ with Tris, Li+, K+ or mannitol on the intestinal absorption of sugars, in successive periods of 1 minute duration, has been studied in rat and hamster in vivo. The absorption of 2 mM D-glucose, D-galactose, 3-0-methyl-D-glucose and D-fructose is clearly inhibited in the absence of Na+, up to 70-80%, and returns to its normal value of restoring Na+. The degree of inhibition varies with the sugar, increases on lowering Na+ concentration, reaches maximum values with mannitol as substituent, and minimum with Tris, D-arabinose absorption is not affected by Na+. These results prove once more how important Na+ is in sugar intestinal transport in vivo, while they reveal additional influences of the different substituents on the transport system.     

Evidence for an intestinal Na+: Sugar transport coupling stoichiometry of 2.0

Membrane potentials maintained by normally-energized intestinal epithelium interfere with an accurate determination of the Na⁺: sugar coupling stoichiometry associated with Na⁺-dependent transport systems. The interference is due to the fact that basal Na⁺ influx is itself a potential-dependent event, and sugar transport induces a membrane depolarization which therefore modifies basal Na⁺ entry. New information obtained under circumstances in which the membrane potential is maintained near 0 indicates that the true coupling stoichiometry is 2:1 rather than the commonly-accepted value of 1:1. A 2:1 stoichiometry means that cellular electrochemical Na⁺ gradients are adequate to account for recently observed 70-fold sugar gradients maintained by these cells under certain conditions.