546
Chapter 15
Primary H
+
/K
+
-ATPases in the luminal membrane of
the parietal cells pump hydrogen ions into the lumen of the
stomach (
Figure 15–18
). This primary active transporter also
pumps potassium into the cell, which then leaks back into the
lumen through potassium channels. Excessive vomiting can
lead to potassium depletion due to this leak, and to metabolic
alkalosis due to loss of hydrogen ions. As hydrogen ions are
secreted into the lumen, bicarbonate ions are being secreted
on the opposite side of the cell into the blood, in exchange for
chloride ions.
Increased acid secretion, stimulated by factors described
in the next paragraph, results from the transfer of H
+
/K
+
-
ATPase proteins from the membranes of intracellular vesicles
to the plasma membrane by fusion of these vesicles with the
membrane, thus increasing the number of pump proteins
in the plasma membrane. This process is analogous to that
described in Chapter 14 for the transfer of water channels
to the plasma membrane of kidney collecting-duct cells in
response to ADH.
Four chemical messengers regulate the insertion of H
+
/
K
+
-ATPases into the plasma membrane and therefore acid
secretion: gastrin (a gastric hormone), acetylcholine (ACh,
a neurotransmitter), histamine, and somatostatin (two para-
crine agents). Parietal cell membranes contain receptors for all
four of these molecules (
Figure 15–19
). Somatostatin inhib-
its acid secretion, whereas the other three stimulate secretion.
Histamine is particularly important in stimulating acid secre-
tion because it markedly potentiates the response to the other
two stimuli, gastrin and ACh. As we will discuss later when
considering ulcers, this potentiating effect of histamine is the
reason that drugs that block histamine receptors in the stom-
ach suppress acid secretion.
Not only do these chemical messengers act directly on
the parietal cells, they also infl uence each other’s secretion.
During a meal, the rate of acid secretion increases markedly as
stimuli arising from the cephalic, gastric, and intestinal phases
alter the release of the four chemical messengers described in
the previous paragraph. During the cephalic phase, increased
Gastric
lumen
Mucous cells
(secrete mucus)
Parietal cell (secretes
hydrochloric acid and
intrinsic factor)
Chief cell (secretes
pepsinogen)
Enteroendocrine cell
(secretes gastrin)
Simple
columnar
epithelium
Gastric
pit
Gastric
gland
Figure 15–17
A gastric gland in the stomach. Not shown are enterochromaffi n-like cells, which release histamine, and D-cells, which release somatostatin.
previous page 574 Vander's Human Physiology The Mechanisms of Body Function read online next page 576 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off