164
ERNEST SCHOFFENIELS
of the turtle (Testudo hermanni G. F. Gmelin) will be sufficient to
illustrate our point.
The permeability characteristics of the intestinal epithelium of the
turtle may be represented schematically as follows (Fig. 2, A and B)
(42, 113).
In the small intestine the membranes in contact with the mucosal side
are permeable to Na and Κ and seem to be the site of an active transport
mechanism for Na. The same situation is found for the serosal membranes in the small intestine and in the colon. On the contrary, the
mucosal membranes of the colon are permeable only to Na, a situation
thus resembling that found in the frog skin. It is interesting to note that
the intestinal epithelium is of endodermic origin whereas that of the
frog skin comes from the ectoderm. This schematic representation (Fig.
2) emerges from studies of electrical potential difference recorded under
various conditions of ionic composition. As pointed out above (Section
II, Β) it is based on the assumption that the ionic composition is without
appreciable effect on the coefficient of relative permeability of the membrane for the ions involved. As discussed elsewhere, this is certainly
not the case (95). If this invites one to be cautious concerning quantitative treatment of the results, it still remains well established, that,
under the same experimental conditions, the epithelium of the small
intestine and the colon behave differently, thus suggesting that the
permeability characteristics are also different. In the present state of our
knowledge, they are best represented by the scheme of Fig. 2.
IV. Relations between Inorganic Ions, Sugar, Amino Acids,
Fatty Acids, and Bioelectric Potentials
The electrical potential difference recorded at the level of the small
intestine of many species is always very small, around 0.5-4 mv., the
lumen being negative with respect to the serosal side (113).
If glucose, certain amino acids, or fatty acids were added in physiological saline, an increase in potential difference was observed (114).
This is well demonstrated by the following experiment (115): L-Alanine
was added at the concentration of 5 ^moles/ml. in the solution bathing
the mucosal side of an isolated piece of turtle small intestine. The
potential difference goes from 0.5 to 5.25 mv. If the concentration of
L-alanine is further increased to 10 /^moles/ml. it reaches the value of
9 mv. (Fig. 5). The same result is observed with other amino acids
known to be actively transported: glycine and L-serine, for instance.
On the contrary, amino acids moving passively across the epithelium,
e.g., L-glutamic acid, L-arginine, L-lysine, are without effect.
The short-circuit current is also affected by these amino acids actively
ERNEST SCHOFFENIELS
of the turtle (Testudo hermanni G. F. Gmelin) will be sufficient to
illustrate our point.
The permeability characteristics of the intestinal epithelium of the
turtle may be represented schematically as follows (Fig. 2, A and B)
(42, 113).
In the small intestine the membranes in contact with the mucosal side
are permeable to Na and Κ and seem to be the site of an active transport
mechanism for Na. The same situation is found for the serosal membranes in the small intestine and in the colon. On the contrary, the
mucosal membranes of the colon are permeable only to Na, a situation
thus resembling that found in the frog skin. It is interesting to note that
the intestinal epithelium is of endodermic origin whereas that of the
frog skin comes from the ectoderm. This schematic representation (Fig.
2) emerges from studies of electrical potential difference recorded under
various conditions of ionic composition. As pointed out above (Section
II, Β) it is based on the assumption that the ionic composition is without
appreciable effect on the coefficient of relative permeability of the membrane for the ions involved. As discussed elsewhere, this is certainly
not the case (95). If this invites one to be cautious concerning quantitative treatment of the results, it still remains well established, that,
under the same experimental conditions, the epithelium of the small
intestine and the colon behave differently, thus suggesting that the
permeability characteristics are also different. In the present state of our
knowledge, they are best represented by the scheme of Fig. 2.
IV. Relations between Inorganic Ions, Sugar, Amino Acids,
Fatty Acids, and Bioelectric Potentials
The electrical potential difference recorded at the level of the small
intestine of many species is always very small, around 0.5-4 mv., the
lumen being negative with respect to the serosal side (113).
If glucose, certain amino acids, or fatty acids were added in physiological saline, an increase in potential difference was observed (114).
This is well demonstrated by the following experiment (115): L-Alanine
was added at the concentration of 5 ^moles/ml. in the solution bathing
the mucosal side of an isolated piece of turtle small intestine. The
potential difference goes from 0.5 to 5.25 mv. If the concentration of
L-alanine is further increased to 10 /^moles/ml. it reaches the value of
9 mv. (Fig. 5). The same result is observed with other amino acids
known to be actively transported: glycine and L-serine, for instance.
On the contrary, amino acids moving passively across the epithelium,
e.g., L-glutamic acid, L-arginine, L-lysine, are without effect.
The short-circuit current is also affected by these amino acids actively