LECTURE 7: GI tract Hormones

### Gastrointestinal (GI) Tract Hormones #### Functions of GI Tract Hormones Gastrointestinal hormones, in conjunction with the enteric and autonomic nervous systems, are essential for integrating and coordinating the mechanisms responsible for the movement, digestion, and absorption of ingested meals. Their primary roles include: - **Regulation of Secretion:** GI hormones control both exocrine and endocrine secretions, influencing the release of digestive enzymes, acids, and other substances necessary for digestion. - **Motility Control:** These hormones regulate the motility of the gastrointestinal tract, ensuring efficient movement of food along the digestive system. - **Growth Regulation:** They are involved in promoting growth and maintaining the health of the gastrointestinal tissues. - **Blood Flow Regulation:** GI hormones help regulate blood flow to the digestive organs, which is critical during digestion and absorption. - **Appetite and Feeding Behavior:** They also affect appetite and influence feeding behavior, playing a crucial role in energy balance. #### Routes of GI Tract Hormones GI hormones are released from endocrine cells in the gut and can act via various routes: - **Endocrine Route:** Hormones enter the bloodstream and travel to distant target organs, where they exert their effects. - **Paracrine Route:** Some hormones act locally on nearby cells, influencing their function without entering the circulation. - **Neurocrine Signals:** Peptides released from the nerves of the enteric nervous system can also affect local actions in the gastrointestinal tract. Because many actions of GI hormones are localized and not mediated through the bloodstream, determining the physiologically active concentrations of these hormones can be challenging. Research often employs antibodies that neutralize the action of gut hormones to elucidate their physiological roles. #### Key GI Tract Hormones Several important GI hormones are vital for proper digestive function: - **Gastrin** - Produced by G cells located primarily in the antrum of the stomach (and also in the duodenum and jejunum). - Stimulates gastric glands to secrete pepsinogen (inactive form of pepsin) and hydrochloric acid (HCl), facilitating digestion. - Secretion is stimulated by the presence of food in the stomach. - **Zollinger-Ellison Syndrome** - Characterized by gastrin-secreting tumors leading to hypertrophy of parietal cell mass and excessive acid secretion. - Results in multiple gastric ulcers, which can lead to complications such as bleeding and even death from hemorrhage. ### Cholecystokinin (CCK) and Secretin: Key GI Tract Hormones #### Cholecystokinin (CCK) Cholecystokinin (CCK) is a peptide hormone produced by I cells located in the small intestine, specifically in the duodenum, jejunum, and ileum. This hormone plays a crucial role in the digestion of proteins and fats. Its primary actions include: - **Gallbladder Contraction:** CCK stimulates the contraction of the gallbladder, leading to the release of bile into the small intestine. Bile is essential for emulsifying fats, facilitating their digestion and absorption. - **Relaxation of the Sphincter of Oddi:** CCK causes relaxation of the hepatopancreatic sphincter (sphincter of Oddi), which allows bile and pancreatic juices to flow into the duodenum, aiding in digestion. - **Increased Pancreatic Enzyme Secretion:** CCK enhances the secretion of digestive enzymes from the pancreas, promoting the breakdown of proteins and fats into absorbable units. - **Decreased Gastric Emptying:** CCK slows gastric emptying, ensuring that the contents of the stomach are mixed thoroughly with digestive juices in the small intestine before moving on. #### CCK Receptors CCK exerts its effects through specific receptors, which are categorized into two main types: - **CCK-A Receptors:** Primarily located in the **gastrointestinal tract**, these receptors mediate most of the gastrointestinal actions of CCK. - **CCK-B Receptors:** Found mainly in the **central nervous system (CNS)**, these receptors are involved in signaling satiety and appetite regulation. CCK also acts on the hypothalamus to induce feelings of satiety, contributing to appetite control. Additionally, it enhances the effect of secretin in stimulating the production of alkaline pancreatic juice and exerts a tropic effect on the pancreas, supporting its growth and function. The secretion of CCK is significantly enhanced by the presence of lipids in the intestinal lumen, along with peptones and amino acids. #### Secretin Secretin is a peptide hormone composed of 27 amino acids and is produced by S cells in the duodenum and jejunum. It is initially stored as pro-secretin and is released in response to various stimuli. Its primary functions include: - **Stimulation of Bicarbonate and Fluid Secretion:** Secretin acts through a cyclic AMP (cAMP) signaling pathway to stimulate the secretion of bicarbonate-rich fluid from