section 12.4
Absorption of Water and Electrolytes
223
colon because they are actively reabsorbed in the ileum.
When the ileum is diseased or resected, sufficient bile salts
enter the colon to inhibit absorption of Na+ and water and
cause diarrhea. Hydroxylated fatty acids (e.g., ricinoleic
acid, the active ingredient of castor oil) also inhibit salt and
water absorption. In disorders with significant mucosal ab-
normalities (e.g., gluten-sensitive enteropathy), fluid and
electrolyte absorption is impaired.
Loss of fluids and electrolytes in
cholera
results from
stimulation of a secretory process. The toxin secreted by
vibrio cholerae
causes a diarrhea of up to 20 L/day, re-
sulting in dehydration and electrolyte imbalance, which
may lead to death. Bacteria in contaminated food attach
chiefly to the ileal mucosa and secrete enterotoxin con-
sisting of one fraction that binds to specific sites on the
cell membrane and another responsible for the character-
istic biochemical activity. The binding moiety consists of
five identical polypeptide subunits (B) (M.W. 11,300) that
surround the active moiety (A). The A-subunit consists
of two unequally sized polypeptides, Ai (M.W. 23,500)
and A
2
(M.W. 5,500), linked by a disulfide bridge. The A
2
polypeptide appears to connect the Ai polypeptide to the
B-subunit. The B-subunit binds rapidly to monosialogan-
gliosides in the membrane
(Gmi
, Chapters 10 and 19). The
Ai polypeptide then migrates through the membrane and
catalyzes transfer of the ADP-ribose group from NAD+
to the stimulatory guanine nucleotide-binding protein (Gs)
that regulates adenylate cyclase activity. The adenylate cy-
clase, which catalyzes the conversion of ATP to cAMP
(Chapter 30) is stimulated or inhibited by the active forms
of the guanine nucleotide-binding protein, Gs and G,, re-
spectively. The Gs protein is activated by the binding of
GTP and inactivated when GTP is converted to GDP by a
GTPase intrinsic to Gs protein. G proteins contain
a-, ft-,
and
y
-subunits. The ADP ribosylation of the a-subunit
decreases GTP hydrolysis and thus leads to sustained acti-
vation of adenylate cyclase activity, increased intracellular
levels of cAMP, and secretion of isotonic fluid throughout
the entire small intestine. Cholera toxin does not cause
fluid secretion in the stomach, has minimal effects on the
colon, and does not affect Na+-dependent absorption of
glucose and amino acids. Its effects can be readily reversed
by oral or intravenous administration of replacement
fluids.
The mechanism by which cholera toxin causes secretory
diarrhea is through continuous stimulation of the CFTR-
regulated CD channel (Figure 12-15). In cystic fibrosis,
CFTR defects cause the abolition of intestinal chloride
secretion without affecting the absorptive capacity. In ho-
mozygous CF patients, the disease is eventually lethal (dis-
cussed earlier). In cholera infections, however, CFTR is
overactivated with fluid and electrolyte losses that lead to
intravascular volume depletion, severe metabolic acidosis,
and profound hypokalemia. These metabolic changes can
result in cardiac and renal failure with fatal consequences.
It has been postulated that persons who are heterozygous
for a CF mutation may have selective advantage during
cholera epidemics. This speculation has been tested in a
mouse model; indeed, homozygous CF mice treated with
cholera toxin did not show intestinal secretion of fluids
despite an increase in intracellular cAMP levels. In het-
erozygous CF mice the intestinal secretion is intermediate
compared to that of controls.
A human pathogen known as
Escherichia coli
0157:H7
may cause nonbloody diarrhea,
hemorrhagic colitis,
hemolytic uremic syndrome,
and death. The interval be-
tween exposure and illness averages only 3 days. The des-
ignation of 0157:H7 derives from the fact that the bac-
terium expresses the 157th somatic (O) antigen and the
7th flagellar (H) antigen. The pathogen is transmitted by
contaminated food (e.g., ground beef, fruit and vegeta-
bles, and water) from one person to another and occasion-
ally through occupational exposure. The pathogenicity of
E. coli
0157:H7 is due to its ability to produce a molecule
composed of an enzymatic subunit (Ai) and a multimer
of five receptor binding (B) subunits. The genome for the
synthesis of the toxin resides on a bacteriophage inserted
into
E. coli
0157 DNA. The Ai-subunit is linked to a
carboxy terminal A
2
fragment by a single disulfide bond.
Shiga toxin produced by
Shigella dysenteriae
has sim-
ilar structural features. The toxin binds to a glycolipid
(Gb3), undergoes endocytosis, and the enzymatic Ai frag-
ment, which is a specific N-glycosidase, removes adenine
from one particular adenosine residue in the 28S RNA
of the 60S ribosomal subunit. Removal of the adenine
inactivates the 60S ribosome, blocking protein synthe-
sis.
Ricin, abrin,
and a number of related plant proteins
inhibit eukaryotic protein synthesis in a similar manner
(Chapter 25).
Several
E. coli
strains also elaborate heat-labile entero-
toxins that cause diarrheal disease (“traveller’s” diarrhea)
by similar mechanisms. In
V. cholerae,
the same entero-
toxin is produced by all pathogenic strains and is chro-
mosomally determined, whereas in
E. coli,
different en-
terotoxins are produced and the toxin genes are carried on
plasmids.
The actions of diphtheria and pertussis toxins are also
mediated by ADP-ribosylation. Diphtheria toxin inhibits
eukaryotic protein synthesis by ADP ribosylation of elon-
gation factor II (Chapter 23). Pertussis toxin inactivates
G; by ADP ribosylation of its A-subunit and causes an
increase in cAMP production. Unlike cholera toxin, per-
tussis and diphtheria toxins gain access to many tissues to
produce diverse biological effects. Severe watery diarrhea
