Influence of sodium on major diffusion barriers to passive intestinal uptake

Intestinal active transport processes have been shown to be sodium dependent, but the effect of varying the concentration of sodium in the luminal bulk phase on the passive uptake of lipids has not been established. An in vitro technique was used to measure the uptake of fatty acids and alcohols into the rabbit jejunum under conditions of stirring of the bulk phase selected to reduce the effective resistance of the unstirred water layer (UWL). Sodium withdrawal had no effect on the uptake of a homologous series of fatty alcohols, and thus the UWL was not influenced by varying bulk-phase sodium; in contrast, the uptake of a homologous series of short-, medium, and long-chain-length saturated fatty acids (FA) was progressively inhibited by reducing bulk-phase sodium from 145 to 0 meq/l. There was no difference in FA uptake between the use of magnesium chloride and mannitol for maintenance of isotonicity with sodium withdrawal. At each sodium concentration, the uptake of FA rose with increasing FA chain length. The relation between sodium concentration and incremental change in free energy (delta delta Fw leads to 1) was described by a sigmoid-shaped curve. The uptake of cholesterol was also inhibited by reducing sodium in the bulk phase. In summary, varying bulk-phase sodium has no effect on UWL, and the influence of sodium on the uptake of FA is mediated by altering delta delta Fw leads to 1 and therefore the physicochemical properties of the brush-border membrane.