Regulation of Gastric Secretion By Dr Nikhil BhavsarDepartment of PhysiologySRM MCH & RC.

Introduction : 1) Stimuli altering Gastric Secretion - Stimulating factors and their action - Inhibiting factors and their action 2) Regulation. 3) Phases of Gastric Juice Secretion.

Stimuli altering Gastric Acid Secretion

A. Factors increasing HCL secretion

Luminal Factors 1) Distention of stomach. 2) Products of protein digestion

Hormonal Factors 1) Acetylcholine 2) Gastrin 3) Histamine

Neural Factors 1) Vagal stimulation (cholinergic & noncholinergic)

Blood Borne 1) Epinephrine 2) Gastrin

B. Factors that decrease HCL secretion

Luminal Factors 1) Increased acid content (highly acidic chyme)

Hormonal Factors 1) Somatostatin

Blood borne Factors 1) Secretin, Gastric Inhibitory Peptide(GIP), Vasoactive Intestinal polypeptide(VIP), Glucagon

Mechanism of actionOf hormones on parietalCells.

Action of Stimulating factors is mainly through

-Acetylcholine -Gastrin -Histamine

AcetylcholineReleased at nerve endings of vagal cholinergic fibres Acts on M3 cholinergic receptorsDirect action on parietal cell to increase HCL secretionIndirect action on Enterchromafin-like (ECL) and G cells causes secretion of histamine and gastrin respectively

Gastrin-Most potent stimulator-Non cholinergic fibres of vagus release neurotransmitter Gastrin Releasing peptide (GRP)-It stimulates G cells present in antral mucosa-Causing release of Gastrin-Which acts on Gastrin Receptor on Parietal cells-Increasing intracellular Ca++ levels-leading to stimulation of H+K+ ATPase pump-Gastrin also causes release of Histamine from ECL cells.

HistamineActs on Histamine type 2 receptor on parietal cellsReleased by ECL cells Both acetylcholine and gastrin stimulate its release

Mechanical and Chemical factorsStretching of G cells causes gastrin releasePeptides and Amino Acids increase gastrin release

Action of Inhibiting factors

-pH of gastric luminal content- somatostatin- chyme in duodenum

pH of Gastric luminal contentBy negative feed back via G cells and D cellsWhich causes inhibition of gastrin secretion and stimulation of somatostatin secretion SomatostatinSecreted from D cells of gastric mucosa, when pH less than 2It inhibits Gastrin release Chyme in DuodenumAcidic chyme causes release of Secretin from upper intestinal mucosa.It inhibits Gastrin release.Products of carbohydrate and lipid digestion cause release of Gastric inhibitory peptide (GIP) , which also inhibits secretion.

Regulation of Gastric Juice Secretion occurs by 2 mechanisms

Neural mechanisms Hormonal mechanisms Neural Mechanisms are Parasympathetic and Sympathetic influences Short and long Gastro Intestinal Reflexes Central influences mediated via Vagus Hormonal Mechanisms As discussed above.

Phases of Gastric Secretion 1) CEPHALIC PHASE

2) GASTRIC PHASE

3) INTESTINAL PHASE Cephalic Phase

Stimuli : Smell, sight, thought, taste and chewing of food.Via Unconditioned and Conditioned reflexesSecretion occurs before food reaches stomach and stimulates appetite so its called Appetite JuiceAccounts for 20% of total gastric secretion

Experimental design to study Cephalic Phase

-SHAM FEEDING AND PAVLOVS POUCH

GASTRIC PHASE

Stimulus food distending the stomach, also products of digestion like peptides and amino acids. Initiates both central and local reflexesCentral reflex Via vagus (vagovagal reflex) , is both cholinergic and non non cholinergic

Local Myenteric reflex -Stimulation of myenteric plexus in stomach wall. -Release of Acetylcholine, stimulates parietal cell Stretching , Peptides and Amino Acids cause release of gastrin from G cells.Accounts for 70% of gastric secretion Experimental design to Study Gastric PhaseStudied by making 5 types of pouches 1) Pavlovs PouchSmall part of stomach incompletely separated from stomach and made into a pouch.Intact nerve supply and blood supply

2) Heidenhain Pouch Modified pavlov's pouch, pouch is completely separated from stomach. Parasympathetic nerve supply absent Sympathetic supply and blood supply is intact To demonstrate effect of local hormones and sympathetic nerves. 3) Bickel Pouch Totally denervated, and without blood supply To demonstrate role of local hormones. 4) Farrel and Ivy Pouch Complete transplantation for Bickel Pouch to subcutaneous tissue of abdominal wall or thoracic wall in the same animal. To demonstrate the role of local hormones.

INTESTINAL PHASE

Initial stage is stimulatory - chyme stimulates duodenal mucosa to release gastrin, and entero-oxynthin from upper jejunum - these are transported via blood and stimulate gastric secretion Later stage is inhibitory - acidic chyme inhibits gastric secretion via Entero-gastric reflex - stimulates release of secretin which inhibits G cells - secretion of Bulbogastrone from duodenal mucosa which is inhibits parietal cells - products of fat digestion stimulate secretionof GIP and CCK (cholecystokinin) from upper part of small intestine, inturn inhibit parietal cells. - Net effect is inhibition and this phase accounts for 10% of gastric secretions

In Summary Factors Stimulating and Inhibiting the Gastric Secretion Mechanism of action of Hormones Regulation of Gastric Juice Secretion . 3 Phases of Gastric Secretion & Experimental Designs to Study them.

REFERENCES

Textbook of Medical Physiology - G.K.Pal Bern and Levy Physiology Essentials of medical physiology - K Sembulingam Textbook of Medical Physiology - Guyton & Hall Ganongs review of Medical Physiology

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