Inhibition of Gastric Secretion
by Other Post-Stomach Intestinal Factors
Although intestinal chyme slightly stimulates gastric secretion
during the early intestinal phase of stomach secretion, it paradoxically
inhibits gastric secretion at other times. This inhibition results from at
least two influences.
1. The presence of food
in the small intestine initiates a reverse
enterogastric reflex, transmitted through the myenteric nervous system as
well as through extrinsic sympathetic and vagus nerves, that inhibits stomach
secretion. This reflex can be initiated by distending the small bowel, by the
presence of acid in the upper intestine, by the presence of protein breakdown
products, or by irritation of the mucosa. This is part of the complex mechanism
for slowing stomach emptying when the intestines are already filled.
2. The presence of acid,
fat, protein breakdown products, hyperosmotic or hypo-osmotic fluids, or any
irritating factor in the upper small intestine causes release of several
intestinal hormones. One of these is secretin,
which is especially important for control of pancreatic secretion. However,
secretin opposes stomach secretion. Three other hormones—gastric inhibitory peptide, vasoactiveintestinal
polypeptide, and somatostatin—also
haveslight to moderate effects in inhibiting gastricsecretion.
The functional purpose of inhibitory gastric secretion by
intestinal factors is presumably to slow passage of chyme from the stomach when
the small intestine is already filled or already overactive. In fact, the
entero-gastric inhibitory reflexes plus inhibitory hormones usually also reduce
stomach motility at the same time that they reduce gastric secretion.
Gastric Secretion During the Interdigestive
Period. Thestomach secretes a few milliliters of gastric juice each hour
during the “interdigestive period,” when little or no digestion is occurring
anywhere in the gut. The secre-tion that does occur usually is almost entirely
of the nonoxyntic type, composed mainly of mucus
but little pepsin and almost no acid.
Unfortunately, emotional stimuli frequently increase interdigestive
gastric secretion (highly peptic and acidic) to 50 milliliters or more per
hour, in very much the same way that the cephalic phase of gastric secre-tion
excites secretion at the onset of a meal. This increase of secretion in
response to emotional stimuli is believed to be one of the causative factors in
develop-ment of peptic ulcers.
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