3 a gastric secretion and its regulation

Gastric Secretion and

Its RegulationPart I

Learning Objectives

• Stomach: Parts, Function and Gastric Glands•Gastric Secretions•Regulation•Phases Of Secretion

Most dilated partBag like organLies in epigastric, umblical & left hypogastric regions of the abdomen

Functions of stomach

• Temporary storage of food• Grinding & mixing of food• Secretion of HCl • Secretion of intrinsic factor (IF) • Secretion of pepsinogen• Absorption of water and lipid soluble substances • Performs its own controlled emptying

• Surface/Neck mucous cells-secrete protective bicarbonate ions • Parietal Cells- most numerous in

the isthmus of the glands, secrete gastric acid (HCL) + intrinsic factor.

Structure of Glands

• Chief/peptic/zymogenic cells- located towards the bases of the gastric glands. Secrete gastric lipase and pepsinogen.

• Neuroendocrine cells- part of the diffuse neuroendocrine system, secrete serotonin and other hormones

• Stem cells- divide continuously to replace all other types of cell in the glands.

Synthesize and secrete the HCl acid responsible for the acidic pH in the gastric lumen.

Synthesize and secrete the protease precursor known as pepsinogen.

Produce alkaline mucus that covers mucosa layer

Contents of Normal Gastric Juice•Volume: 1 -1.5 liters• Nature: acidic pH 0.7 to 4 Constituents: Water (99.5%); solids (0.5%)

Inorganic constituents: anions ( Cl-, PO43-, SO4

2- & HCO3- and

cations (H+, Na+, K+, Ca++, Mg ++)

Organic constituents: pepsinogen, IF, mucin, renin, gastric lipase, gelatinase, carbonic anhydrase and lysozyme.

Structure of parietal cell: resting and stimulated

•H+ - K+ ATPase in inactive form

•When activated , fuse to cell membrane & microvilli project to increase surface area for secretion

Mechanism of HCl Secretion

• Water dissociated into H+ & OH-

• Pumping of H+ out of cell by H+K+ATPase; permits OH- to accumulate & form HCO3

- from CO2

• Water passes into canaliculus by osmosis making gastric juice isotonic to plasma

Postprandial Alkaline Tide

Pyloric gland Oxyntic gland

Regulation of HCl Secretion

Pyloric gland Oxyntic gland

Potentiation

Agents stimulating the parietal cells

• Ach• Gastrin• Histamine

Agents inhibiting the parietal cells

• Acid• Somatostatin

Meal Related Phases of Gastric Secretion

Cephalic phase : 30%

Gastric phase: 60%

Intestinal phase: 10%

sight, aroma, taste, thought

Cephalic phase

Sham feeding

Gastric phase

Intestinal phase

Inter-digestive phase

Enteric hormones that inhibit gastric H+ secretion

Pepsinogen secretion• Secreted by chief cells in gastric

glands, & mucous cells• Pepsinogen : aspartic proteinases,

endopeptidases• Two types: Group I ( chief cells of

corpus)• group II ( chief cells +

mucous neck cells in all parts)

• Zymogen granule compound exocytosis pepsinogen

• Agonist acting through Ca++

M3 muscarinic receptors for AchGastrin/CCk family peptides’ receptors

Stimulated by:

Regulation of Pepsinogen Secretion

Mucus

Secreted by :• Surface mucous cells•Mucous neck cells• Glandular mucous cells

• forms a mucus gel layer with PLs, electrolytes , water

Mucus Secretion & its Regulation

•Vagal stimulation and irritation stimulate gastric mucous cells to secrete mucin

•Gastric surface cells secrete HCO3- , stimulated by Ach, Acids &

PGs.

•Entry of acid into the duodenum induces Secretin from S cells triggering HCO3

- secretion by pancreas/duodenum & neutralizes Gastric acid

Mucus protects the gastric surface epithelium by trapping an HCO3

- rich fluid near the apical border of these cells

Gastric mucosal barrier

MucusBicarbonatesEpithelium- tight junctions+ surface membranesProstaglandins Trefoil peptides

What we learned today….

• Stomach: Parts, Function and Gastric Glands•Gastric Secretions•Regulation•Phases Of Secretion

Structure of Gastric Glands and secretions

Mechanisms

sight, aroma, taste, thought -----> hypothalamus gustatory centers -----> vagal nuclei of medulla -----> vagus nerve (parasympathetic) -----> increased gastric secretion