Lectures 48, 49: Integrated Response to a meal - The Gastric Phase Learning Objectives   At the end of the lectures, the learner will be able to: Define the gastric phase of integrated response to a meal. Describe the most important gastric secretions, the cells where they originate from, their locations in the stomach, and digestive functions. Describe the cellular mechanisms responsible for gastric acid secretion. Define the endocrine, paracrine, and neural controls of gastric acid secretion- explain the negative feedback mechanism. Describe the importance of gastric mucosal barrier. Discuss the significance of digestion in the stomach. Describe the distinct types of motilities in different regions of the stomach and explain their regulation. Explain certain GI patho-physiologies: Peptic ulcers- gastric and duodenal Zollinger-Ellison syndrome (gastrinoma)

Integrated Response to a meal - The Gastric Phase This is when food is in the stomach. The major functions of the stomach are: Storage: temporary storage of meal Secretion of H+: killing microorganisms and conversion of pepsinogen → pepsin Secretion of Intrinsic factor (IF): essential for the absorption of vitamin B12 Secretion of mucus and HCO3- : protection of gastric mucosa Secretion of water: lubrication of bolus, suspension of nutrients in solution Motor Activity: mixing secretions (H+ and pepsin) with bolus. Coordination of motor activity of smooth muscle and emptying of stomach contents into duodenum.

Regulation of Stomach Function Regulation of both motor and secretory responses are via: Neural: Intrinsic and extrinsic Paracrine: histamine is a powerful stimulator of H+ secretion. Endocrine: gastrin (stomach and duodenum)- stimulates gastric acid secretion. somatostatin (stomach, duodenum, and pancreas) - inhibits gastric secretion.

Functional Anatomy of the Stomach

Functional Anatomy of the Stomach 1 2 3 The stomach is divided into 3 regions: The cardia The corpus (or fundus, body) The antrum While discussing physiology, it can also be subdivided into two functional parts: The proximal part (closer to mouth – thin wall) The distal part (further away from mouth- thick wall).

colon appendix esophagus cardiac stomach fundus & body pyloric jejunum ileum duodenum

Cardia - shallow pit, branched and coiled glands Fundus - Shallow pit receiving 3-7 Pylorus -Long pits and short coiled and branched glands

Functional Anatomy of the Stomach The lining of the stomach contains columnar epithelium – folded into gastric pits. Each gastric pit is the opening where gastric glands empty.

Fundus/Body – Gastric glands Shallow Pits Mucosa Long branched tubular glands Muscularis mucosa: But not as thick as in esophagus Note in Pylorus: Long pits and short glands

Fundus/Body Gastric glands at relatively higher magnification Pits Upper glands (mostly parietal cells)

Secretory Cells of the Stomach The gastric mucosa is divided into three distinct regions based on the gland structure: Small cardiac glandular region (below LES) – primarily mucus and HCO3- - secreting gland cells, that provide mechanical and chemical protection of the gastric mucosa The remaining portion of the gastric mucosa is divided into: Oxyntic or parietal gland region (fundus) Pyloric gland region (antrum).

Secretory Cells of the Stomach The fundus and antrum contains 6 types of secretory cells: Parietal or oxyntic cells secrete: HCl: kills bacteria, allows activation of pepsin from pepsinogen, low pH for effective pepsin action. Intrinsic factor: glycoproteins that bind vitamin B12 making it absorbable by the ileum mucosa. It is an ESSENTIAL FACTOR! Mucous Neck cells secrete: Mucus: protection of gastric mucosa.

Secretory Cells of the Stomach The fundus and antrum contains 6 types of secretory cells: 3) Peptic or Chief cells secrete: Pepsinogens: In acidic environment they are cleaved into pepsins (mixture of various proteases). Pepsins are only active in ACIDIC environments. 4) Enterochromaffin-like (ECL) cells secrete: Histamine (paracrine): most powerful stimulator of HCl secretion; 5) D cells secrete: Somatostatin (endocrine) - powerful inhibitor of HCl secretion. 6) G cells (pyloric region) secrete: Gastrin (endocrine): HCl secretagogue.

Gastric Secretion The cells of the gastric mucosa secrete a fluid called Gastric Juice. The major gastric secretions are: HCl (hydrochloric acid): H+ activates pepsinogen into pepsins. Pepsin initiates protein digestion in the stomach (20%) but that is not essential. Pepsinogen: is the inactive proenzyme of pepsin. Bicarbonate [HCO3]- + MUCUS : important for protection of gastric mucosa against acidic and peptic luminal environment. Intrinsic Factor: glycoprotein, required for normal absorption of vitamin B12 in the ileum. In a healthy human, intrinsic factor is the only essential component of gastric juice, the functions of other components are redundant.

Mechanism of Gastric Acid Secretion A Resting Parietal Cell An Activated Parietal Cell Parietal cells secrete HCl and have distinct ultrastructure. Resting parietal cell cytoplasm contains numerous tubules and vesicles – tubulovesicular system, their membranes contain transport proteins needed for secretion of H+ and Cl-. Intracellular secretory canaliculi are present. Once activated, the tubulovesicular membranes fuse with the plasma membrane of the secretory canaliculi – (increases the number of H+-K+ antiporters on the secretory canaliculi membrane) – and opens to the lumen of the gland.

Cellular Mechanism of Gastric Acid Secretion In the intracellular fluid: Carbonic anhydrase produces H2CO3 from CO2 + H2O. H2CO3 then dissociates into H+ and HCO3-.

Cellular Mechanism of Gastric Acid Secretion In the Lumen (or apical membrane): H+ is secreted via the H+-K+ ATPase – active process that transports both H+ and K+ against electrochemical gradient (uphill). This is inhibited by the drug omeprazole- used to treat ulcers to reduce H+ secretion. Cl- follows H+ by diffusing through Cl- channels. Increased intracellular Ca++ and cAMP increase luminal conduction of Cl- and K+.

Cellular Mechanism of Gastric Acid Secretion In the basolateral membrane: HCO3- is absorbed into the blood via Cl- - HCO3- exchanger. This absorbed HCO3- is the reason for the “alkaline tide” (high pH) in gastric venous blood after a meal. This HCO3- is eventually secreted back in the GI tract by the pancreas. Combination of all these events leads to a net secretion of HCl and net absorption of HCO3-.

Secretion of Bicarbonate (HCO3-) Secretion of HCO3-: The surface epithelial cells of stomach also secrete a watery fluid that contains Na+, Cl-, K+ and HCO3-. Although Na+, Cl- concentrations are similar to plasma, the K+ and HCO3- concentrations are higher. This HCO3- is entrapped by the viscous mucus that coats the stomach lumen – together forms the mucosal barrier.

Secretion of Mucus Mucus- Secretions containing mucins are viscous and sticky – collectively called mucus. Secreted by mucus neck cells. They are ~80% carbohydrate. Intact mucins are tetramers of 4 similar monomers, 500 KD each. The central portion of the mucin tetramer, near the disulfide crosslinks is susceptible to proteolytic digestion by pepsins. Proteolytic fragments do not form gels – dissolves the protective mucus layer.

Gastric Mucosal Protection and Defense Mucus: protects cells from H+ and inactivates pepsins that cannot hydrolyze extracellular portions of membrane proteins Mucus and HCO3- protects the surface of the stomach from H+ and pepsins. The protective mucus layer on the luminal side and the alkaline secretions trapped within this – form the gastric mucosal barrier – protects gastric mucosa. The mucus layer (~0.2mm thick) – separates the HCO3- -rich secretions of the surface epithelial cells from the acidic contents of the gastric lumen.

Regulation of Gastric Secretion Parasympathetic stimulation via vagus nerve is the strongest stimulant for gastric H+ secretion. Extrinsic efferent fibers terminate on intrinsic neurons that innervate: Parietal cells, ECL cells, G cells. Vagal stimulation also secretes pepsinogen, mucus, HCO3- and intrinsic factor.

Stimulation of Gastric Acid Secretion Stimulation: Three substances stimulate H+ secretion by parietal cells: Acetylcholine (neurocrine) Histamine (paracrine) Gastrin (endocrine) Acetylcholine: released from vagus nerve innervating the gastric mucosa binds to muscarinic (M3) receptors on the parietal cells- 2nd messengers are – IP3/Ca++ - stimulates H+ secretion Atropine (inhibits muscarinic receptors) blocks Ach effects on parietal cells. Histamine: released from gastric ECL cells – paracrine effect on parietal cells binds to H2 receptors 2nd messenger cAMP – stimulates H+ secretin by parietal cells Cimetidine blocks H2 receptors – blocks histamine effect.

Stimulation of Gastric Acid Secretion Gastrin: Secreted into circulation by stomach G cells – endocrine effect on parietal cells Binds to cholecystokinin B (CCKB) receptors on the parietal cells The 2nd messengers are IP3/Ca++. Potentiation: The rate of H+ secretion can be regulated by each of these independently as well as by interactions among the three – this is called potentiation – the reason is unclear - might be due to the fact that each agent works via a different receptor and in case of histamine via a different 2nd messenger. Potentiation consequences: Histamine potentiates the actions of Ach and gastrin- cimetidine will: block direct histamine effects block histamine-potentiated effect of Ach and gastrin. Ach potentiates the actions of histamine and gastrin- atropine will block the direct effects of Ach block Ach-potentiated effects of histamine and gastrin.

Stimulation of Gastric Acid Secretion

Stimulation of Gastric Acid Secretion by Parietal Cells Cephalic and Oral phase 30% of total HCl secretion stimuli are smelling, tasting, chewing, swallowing and conditioned reflexes (in anticipation of food). Several mechanisms that promote HCl secretion during this phase: Direct stimulation of the parietal cells by vagus via Ach (muscarinic receptors) – neural effect. Indirect stimulation of the G cells by vagus via GRP (Gastrin-releasing peptide) to produce gastrin- endocrine effect. Indirect stimulation of the ECL cells by vagus (Ach) and gastrin (CCKB receptors)- to produce histamine- paracrine effect.

Stimulation of Gastric Acid Secretion by Parietal Cells

Stimulation of Gastric Acid Secretion by Parietal Cells Gastric phase: 60% of total HCl secretion Stimuli are distension of stomach, presence of amino acids, small peptides. All the mechanisms that are present in cephalic, oral phase also operate here. Two additional mechanisms exist: distension of the stomach activates vagovagal reflexes – stimulate gastrin release. A direct effect of amino acids and small peptides on G cells - stimulate gastrin release. Alcohol and caffeine also stimulate HCl secretion. Intestinal phase: Only 10% of total HCl secretion and is mediated by products of protein digestion.

Inhibition of Gastric Acid Secretion by Parietal Cells Feed-back Inhibition: Acidic chyme in the distal stomach (antrum) initiates a negative feedback loop – inhibits acid secretion. When the pH of lumen is below 3, somatostatin (SS) is released (by endocrine cells) – inhibits via direct and indirect pathways: In direct pathway, SS binds to receptors in parietal cells – inhibits stimulatory effect of histamine.

Inhibition of Gastric Acid Secretion by Parietal Cells Feed-back Inhibition: In indirect pathway, SS inhibits - histamine release from ECL cells and gastrin release from G cells. Prostaglandins – inhibit stimulatory effect of histamine.  

Digestion in the Stomach Some digestion occurs in the stomach, but it is not required. Pepsin digests proteins (20%) but that is not essential; Some amylase digestion in the stomach occurs. Amylase is inactive at low pH but not when the active site is occupied by carbohydrates (substrate protection). Yet, that digestion is not important; The digestion of lipids start in the stomach (10%)- Gastric Lipases attach to the surface of lipid droplets – generates free fatty acids and monoglycerides, but this is not essential. However, in pancreatic disorders the action of gastric lipases may be important.

Gastric Motility Based on the differences in motility, the stomach can be divided into 2 regions: Orad (proximal)- contains fundus, proximal part of body – thin walled. Caudad (distal) – contains distal part of the body and antrum – thick walled. Contractions in the caudad region (generated by strong muscles) mix the food and propel it into the small intestine.

Gastric Motility Receptive Relaxation: Distension of the lower esophagus by food causes relaxation of LES – also relaxation of the orad stomach - called receptive relaxation. This is via vagovagal reflex- mechanoreceptors detect stomach distension, relay this to CNS via sensory neurons. CNS sends efferent information to smooth muscle of orad stomach to relax. Neurotransmitter released from postganglionic peptidergic vagal fibers is VIP.

Gastric Motility Mixing and Digestion: Caudad region- waves of strong contraction start in the mid portion of stomach – move distally towards pylorus, increasing in strength towards pylorus. They mix the gastric contents and periodically propel a portion of these in the duodenum through pylorus- followed by closure of pylorus. Much of the chyme (not emptied into duodenum) is propelled back to stomach for further mixing and break-down – this is known as retropulsion.

Control of Gastric Motility (Gastric Phase) Sensory receptors (stretch and receptors sensitive to aminoacids and small peptides) in the gastric mucosa connect directly to intramural plexi. Intramural plexuses connect with sensitive vagal fibers leaving the stomach. Vagovagal Reflexes are initiated to control acid secretion, distension of gastric wall, and gastric motility. Parasympathetic innervation via vagus nerves is excitatory (Ach and substance P) in the lower stomach causing ↑ motility (strong smooth muscle contractility); the rate of antral contractions is set by gastric pacemaker, but the magnitude of contractions is regulated by neural input. In the gastric phase, pylorus is usually closed, the antral contractions mix the gastric contents and reduce size (grinding function).

Gastric Emptying Gastric Emptying: After a meal, the stomach contains about 1.5L of material (solid, liquid, secretions). Emptying of the gastric contents to the duodenum is slow – takes ~3hours. Liquids empty more rapidly, but solids empty after a log phase. Isotonic contents empty more rapidly than either hypotonic or hypertonic contents. Solids must be reduced to particles of <1 mm3 (via retropulsion) to enter duodenum. More details on this will be discussed in the next lecture.

Fundic glands: Stem, Mucous, Parietal and Chief cells.

Gastric (Peptic) Ulcer This is a slide showing gastric ulcer. See how the histology is changed. We do not see normal surface epithelium or glandular structure. Goblet cells are starting showing up in gastrtic metaplasia. There are necrotic tissue – cells are dying, inflammation etc.

Gastroduodenal Junction Pylorus consists of a two ring-like thickenings of circular smooth muscle cells; Coordination of contraction/relaxation follows the general rule in the GI tract: antrum contracts while initial portion of duodenum relaxes. Functions: Filtering large size particles of food Emptying gastric content at a rate consistent with duodenum’s ability to digest chyme Prevention of reflux of bolus into stomach; Control by hormonal, paracrine and nervous factors. Common pathologies - include gastric ulcerations (gastric mucosa is sensitive to bile – basic pH) and duodenum ulcerations (intestinal mucosa is sensitive to gastric secretion- acid pH).

Control of Pyloric Muscle Tone slows gastric emptying! Sympathetic fibers NE Pyloric constriction slows gastric emptying! Parasympathetic fibers : ACh Vagal Excitatory Fibers Pyloric relaxation (speeds gastric emptying) VIP and NO Vagal Inhibitory Fibers Significant Hormonal Effects: CCK, Gastrin, GIP (gastric inhibitory peptide), Secretin SC

Pathologies in the Stomach and Duodenum Peptic Ulcer Disease: Ulcerative lesion of gastric or duodenal mucosa. This is caused by the erosive and digestive action of H+ and pepsin on mucosa. Causes are Loss of protective mucous barrier or other protective factors Excessive H+ and pepsin secretion or other damaging factors A combination of both. Based on location, peptic ulcers can be gastric or duodenal.

Pathologies in the Stomach and Duodenum Gastric Ulcers: Forms primarily because of defects in the mucosal barrier. This allows H+ and pepsin to digest a portion of the mucosa. Major causative agent is gram-negative bacteria H. pylori. They release cytotoxins (e.g. cagA toxin) that destroys the protective barrier and cells underneath. Surprisingly, in these patients, net H+ secretory rates are lower and thus the secretion rate of gastrin is increased (lacking the negative feedback).

Pathologies in the Stomach and Duodenum Duodenal Ulcers: More common than gastric ulcers and form because H+ secretory rates are higher. Excess H+ delivered to the duodenum overpowers the buffering capacity of pancreatic HCO3-. This excess H+ combined with pepsin damages the duodenal mucosa.

Pathologies in the Stomach and Duodenum Zollinger-Ellison Syndrome (gastrinoma): In these patients usually a tumor (or gastrinoma) located in the pancreas – secretes high quantities of gastrin. High gastrin produces two direct effects: Increased H+ secretion by parietal cells Increased parietal cell mass The excess H+ produces duodenal ulcer. This also leads to steatorrhea (fatty stool), since low duodenal pH inactivates pancreatic lipase. Gastrin secretion by the tumor does not undergo feedback inhibition by H+ (as normal gastrin secretion), so gastrin levels continue to increase. Treatment includes inhibitors of H+ secretion – e.g. Cimetidine, Omeprazole Surgical removal of tumor.