Hormonal Control Of Gastrointestinal Motility

Gastrointestinal Physiology General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine Gastrin Motilin Glucose-dependent insulinotropic peptide GIP Cholecystokinin CCK Secretin General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine Secretes by the “G” cells of the antrum of the stomach in response to stimuli associated with ingestion of a meal:  Distention of the stomach  Products of proteins (a.a)  Gastrin-releasing peptide (GIP), which is released by the nerves of the gastric mucosa during vagal stimulation  Ca2+ and epinephrine Gastrin Actions of gastrin Stimulation of gastric acid secretion (Secretion of H ions by Parietal cells) Stimulation of gastric mucosal growth Hypertrophia and hyperplasia of gastric mucosa by gastrin secretin tumors General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine CCK • • Is secreted by “I” cells in the mucosa of the duodenum and jejunum Pancreazymin Stimuli for secretion • Amino acids and small peptides • Fatty acids and monoglycerides General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine CCK Functions • Strongly contracts the gallbladder, expelling bile into the small intestine • Stimulation of enzymatic secretion of pancreas • Inhibits stomach contraction moderately (Fatty meals) • Inhibits appetite • Bicarbonate secretion • Inhibits gastric emptying General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine Secretin • • First gastrointestinal hormone discovered Secreted by the “S” cells in the mucosa of the duodenum in response to acidic gastric juice. Functions • Mild effect on motility of the gastrointestinal tract • Promote pancreatic secretion of bicarbonate • Decrease acid secretion (H+) by parietal cells • Inhibition of gastrin secretion General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine GIP • • • • • Glucose-dependent insulinotropic peptide (gastric inhibitory peptide [GIP]) is secreted by the upper small intestine, in response to • fatty acids • amino acids • to a lesser extent in response to carbohydrate Functions: mild effect in decreasing motor activity of the stomach • slows emptying of gastric contents into the duodenum when the upper small intestine is already overloaded with food products. Stimulation of Insulin secretion Inhibits H+ secretion by parietal cells General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine Motilin •  • • • Is secreted by the stomach and upper duodenum during fasting function Increase gastrointestinal motility Is released cyclically and stimulates waves of gastrointestinal motility called interdigestive myoelectric complexes (MMC) that move through the stomach and small intestine every 90 minutes in a person who has fasted. Motilin secretion is inhibited after ingestion of food by mechanisms that are not fully understood.  Entrogastrone General Principles of Gastrointestinal Function Hormonal Control Of Gastrointestinal Motility Endocrine General Principles of Gastrointestinal Function TYPES OF MOVEMENTS IN THE GASTROINTESTINAL TRACT Mixing movements Propulsive movements—peristalsis Peristalsis is an inherent property of many syncytial smooth muscle tubes (bile ducts, glandular ducts, ureters) Contractile ring appears in the circular muscle Usual stimulus:  Distention of the gut  Chemical or physical irritation Effectual peristalsis requires an active myenteric plexus   • • • •   Peristaltic waves move toward the anus with downstream receptive relaxation—“Law of the Gut.” This complex pattern does not occur in the absence of the myenteric plexus. Therefore, the complex is called myenteric reflex or peristaltic reflex. General Principles of Gastrointestinal Function Gastrointestinal Blood Flow— Splanchnic Circulation • • • It includes the blood flow through the gut plus blood flows through the spleen, pancreas, and liver. The blood that courses through the gut, spleen, and pancreas then flows immediately into the liver by portal vein. In the liver, the blood passes through millions of minute liver sinusoids and finally leaves the liver by way of hepatic veins that empty into the vena cava of the general circulation. This allows the reticuloendothelial cells that line the liver sinusoids to remove bacteria and other particulate matter that might enter the blood from the gastrointestinal tract. General Principles of Gastrointestinal Function Effect Of Gut Activity And Metabolic Factors On Gastrointestinal Blood Flow • • • • Possible causes of the increased blood flow during GI activity Several vasodilator substances (cholecystokinin, VIP, gastrin, and secretin). Some of the gastrointestinal glands also release into the gut wall two kinins, kallidin and bradykinin. Decreased oxygen concentration in the gut wall can increase intestinal blood flow at least 50 to 100 percent. General Principles of Gastrointestinal Function Countercurrent” Blood Flow in the Villi • • oxygen diffuses out of the arterioles directly into the adjacent venules without ever being carried in the blood to the tips of the villi. As much as 80 percent of the oxygen may take this short-circuit route and is therefore not available for local metabolic functions of the villi. General Principles of Gastrointestinal Function Nervous Control Of Gastrointestinal Blood Flow  Parasympathetic stimulation • Increases local blood flow at the same time that it increases glandular secretion. • This increased flow probably results secondarily from the increased glandular activity, not as a direct effect of the nervous stimulation.  Sympathetic stimulation • Direct effect to cause intense vasoconstriction of the arterioles with greatly decreased blood flow. • After a few minutes of this vasoconstriction, the flow often returns to near normal by means of a mechanism called “autoregulatory escape.” General Principles of Gastrointestinal Function Importance of Nervous Depression of Gastrointestinal Blood Flow When Other Parts of the Body Need Extra Blood Flow • • • A major value of sympathetic vasoconstriction in the gut is that it allows shutoff of gastrointestinal and other splanchnic blood flow for short periods during heavy exercise, when the skeletal muscle and heart need increased flow. In circulatory shock, when all the body’s vital tissues are in danger of cellular death for lack of blood flow—especially the brain and the heart—sympathetic stimulation can decrease splanchnic blood flow to very little for many hours. Sympathetic stimulation also causes strong vasoconstriction of the large-volume intestinal and mesenteric veins.