Les relations entre les ions de sodium et l'absorption intestinale d'acides aminés

• 1.

  1. The intestinal absorption of L-phenylalanine, determined by measuring the accumulation of the labelled amino acid in the tissue during an incubation in vitro, shows an absolute dependence on the presence of sodium ions in the incubation medium. In the absence of these ions, there exists a barrier against the penetration of the amino acid into the mucosal cells. During gradual ageing of the tissue in vitro, the loss of efficiency of this barrier is coincident with the loss in capacity to transport the amino acid against a concentration gradient.

• 2.

  2. The accumulation of L-phenylalanine by a fragment of intestine is dependent on, but is not directly proportional to, the sodium ion concentration of the incubation medium.

• 3.

  3. After preincubation in a sodium-free medium, a tissue sample is no longer capable of transporting L-phenylalanine against a concentration gradient; the findings resemble those following preincubation under anaerobic conditions. Nevertheless, the tissue continues to respire normally and to maintain the barrier against non-specific entry into the mucosal cells in the absence of these ions. Furthermore, perfusion of a loop of rat intestine in vivo with a sodium-free buffer solution has no deleterious effect on its subsequent ability to absorb L-phenylalanine in vitro. These apparently contradictory results are discussed in the light of the most modern theories of intestinal transport of non-electrolytes.

• 4.

  4. The exit of labelled L-phenylalanine from a tissue previously saturated with this substrate may be stimulated by the presence of the unlabelled amino acid in the surrounding medium. This mechanism is also dependent on the presence of sodium ions in the solution. The full interpretation of these results must wait until it has been shown in which membrane of the cell this exchange mechanism takes place.