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.