AJP - Gastrointestinal and Liver Physiology, Vol 243, Issue 2 148-G154, Copyright © 1982 by American Physiological Society
Influence of sodium on major diffusion barriers to passive intestinal uptake
A. B. Thomson
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.
