PSIO 532 Study Guide - Final Guide: Epithelium, Sodium Chloride, Gastric Acid
Graduate Physiology – GI Tract
Inhibition of Gastrin Release
The presence of the acid inhibits the release of more gastrin; the end product inhibition
negative feedback
Also, somatostatin (from D cells) is an inhibitory hormone that also inhibits the release
of gastrin; it also inhibits HCl histamine secretion
Stomach – Secretion of Acid
oThe hydrogen ion concentration in parietal cell secretions
is roughly 3 million fold higher than in blood, and
chloride is secreted against both a concentration and
electric gradient. Thus, the ability of the partietal cell to
secrete acid is dependent on active transport. The key
player in acid secretion is a H+/K+ ATPase or "proton
pump" located in the cannalicular membrane. This
ATPase is magnesium-dependent, and not inhibitable
by ouabain. The current model for explaining acid
secretion is as follows:
oHydrogen ions are generated within the parietal cell
from dissociation of water. The hydroxyl ions formed in
this process rapidly combine with carbon dioxide to form
bicarbonate ion, a reaction cataylzed by carbonic
anhydrase.
oBicarbonate is transported out of the basolateral
membrane in exchange for chloride. The outflow of
bicarbonate into blood results in a slight elevation of
blood pH known as the "alkaline tide". This process serves
to maintain intracellular pH in the parietal cell.
oChloride and potassium ions are transported into the
lumen of the cannaliculus by conductance channels, and
such is necessary for secretion of acid.
oHydrogen ion is pumped out of the cell, into the lumen,
in exchange for potassium through the action of the
proton pump; potassium is thus effectively recycled.
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