GASTROINTESTINAL SECRETIONS V BS 122 2013 Luis A

GASTROINTESTINAL SECRETIONS V BS 122 2013 Luis A Bate

Readings Guyton and Hall Chapter 64

Objectives To familiarize with the different secretory glands of the digestive tract. To understand the role of each secretion in the digestive process. To understand the regulation mechanisms for the different secretory glands.

Secretions Secretory cells from the gastrointestinal tract can release products towards the lumen of the GIT or towards the intracellular space and to circulation F 2 -1

Secretory glands in the digestive tract • Goblet cells in digestive epithelium – Lubrication of surface • Crypts of Lieberkühn – Enzyme secreting cells • Tubular glands – Oxyntic or Parietal cells (acid production) – Peptic or Chief cells (pepsinogen) • Complex glands – Salivary, pancreas, liver F 2 -2

ORAL CAVITY PANCREAS LIVER RECTUM ESOPHAGUS DUODENUM STOMACH SMALL INTESTINE LARGE INTESTINE F 2 -3

Secretions of the oral cavity • Salivary glands – Parotid – Submandibular – Sublingual – Buccal glands • Response to – Parasympathetic (cholinergic) – Sympathetic (catecholamines) F 2 -4

Salivary glands ACINI COLLECTING DUCTS Na+ active absorption PTYALIN Cl- passive absorption MUCUS K+ active secretion EXTRACELLULAR HCOFLUID 3 secretion SALIVA F 2 -5

Role of saliva • Lubricate bolus – Fluid, mucus (especially important in birds) • Contributes digestive enzymes – Salivary amylase, lingual lipase • Antibacterial – Antibodies, lysozyme • Helps thermoregulation – Evapotranspiration in dogs F 2 -6

Regulation of the salivary glands • Autonomic, parasympathetic – Through cholinergic receptors • Classical conditioning – Pavlov • Chewing and test bud stimulation • Sympathetic stimulation – Drooling F 2 -7

Salivary volume and composition • Daily volume ranges widely – 1 litre in an adult pig – 100 -200 L in the adult cow • Ruminants have saliva high in bicarbonate and phosphate – Needed to buffer fermentation acids F 2 -8

Gastric secretions Esophagus Esophageal gland region Cardiac glandular region Pyloric glandular region Fundic glandular region F 2 -9

Gastric secretions • Surface and pits covered with mucus cells – Produce thick protective mucus – Alkaline in nature • Within the pits are tubular glands – Oxyntic or gastric glands – Pyloric glands • Produce slightly different secretions depending on region F 2 -10

Mucus production by stomach wall F 2 -11

Pyloric glands • • Similar to gastric glands Have few chief cells Almost no parietal cells Mainly mucus cells – Like neck cells secrete some pepsinogen – Mucus lubricate food passage – Protect stomach from digestive enzymes • Endocrine cells secreting gastrin F 2 -12

Gastric gland (Oxyntyc gland) SURFACE MUCUS CELLS MUCUS PEPTIC OR CHIEF CELLS PEPSINOGEN MUCUS NECK CELLS MUCUS, PEPSINOGEN OXYNTIC OR PARIETAL CELLS HCl INTRINSIC FACTOR ENDOCRINE HORMONES F 2 -13

Oxyntic cell MUCUS NECK CELL PARIETAL CELL HCl CANALICULI F 2 -14 INTRINSIC FACTOR

Oxyntic cell

Endocrine cell F 2 -15

F 2 -15

Gastrin regulation of stomach p. H LOW p. H HIGH p. H F 2 -16

Pepsinogen secretion • Acetylcholine stimulation of peptic cells – From vagus – From gastric enteric system • Indirect stimulation of peptic cells by local acid concentration F 2 -17

Feedback control by acid • If p. H reaches 3 or below • G cells are blocked – Do not secrete gastrin – Further acid production is reduced • Acid inhibits enteric nervous reflex – Reduces pepsinogen secretion F 2 -18

Phases of gastric secretions • CEPHALIC – VAGUS • GASTRIC – LOCAL REFLEX – VAGAL REFLEX – GASTRIN • INTESTINAL – NERVOUS – HORMONAL F 2 -19

Inhibition of the enterogastric reflex SYSTEMIC CIRCULATION VAGUS PARASYMPATHETIC CCK OTHER HORMONES? ENTERIC p. H NERVOUS OSMOLALITY FAT VAGUS SENSORY F 2 -20

Pancreatic secretions • Similar structure as salivary glands – Enzymes secreted by acini – Large volume sodium bicarbonate secreted by ductules and ducts • Drain into hepatic duct • Contain Islets of Langerhans – Produce hormone insulin – Secreted to circulation F 2 -21

Pancreatic enzymes • Contains enzymes for proteins, carbohydrates and fats • Proteins – Trypsin, chymotrypsin, nucleases, carboxypeptidases, elastases • Carbohydrates – Pancreatic amylase • Lipids – Pancreatic lipase, cholesterol esterase and phospholipase F 2 -22

Pancreatic secretions ISLETS OF LANGERHANS PANCREATIC ACINI PEPTIDASES DUCTULES DUCTS LIPASES AMYLASES SODIUM BICARBONATE TRYPSIN INHIBITOR F 2 -23

Enzyme activation Enterokinase Trypsinogen Chymotrypsinogen Elastase Proelastase Procarboxypeptidase B F 2 -24 Carboxypeptidase A Carboxypeptidase B Procarboxypeptidase A

Regulation of pancreatic secretion • Three main stimuli – Acetylcholine (acinar cells) • Parasympathetic vagus • Cholinergic nerves in enteric nervous system – Cholecystokinin (acinar cells) • Duodenal and jejunal mucosa (food) – Secretin (tubular cells) • Duodenal and jejunal mucosa (acid) F 2 -25

Regulation of pancreatic secretions CCK SEC F 2 -26

Phases of pancreatic secretions • CEPHALIC – VAGUS • SOME ENZYME • GASTRIC – LOCAL REFLEX – VAGAL REFLEX • SOME ENZYME • INTESTINAL – NERVOUS – HORMONAL • SECRETIN, CHOLECYSTOKININ F 2 -27

Liver secretion • Bile is the main product – An 80 Kg animal secretes about 600 -1000 m. L of bile • Produced by hepatocytes • Secreted into duodenum, or • Stored in gallbladder – Concentrated form F 2 -28

Role of bile • • Bile acids help emulsify fat particles Support absorption of digested fat Help neutralize acid Serve to discard waste, toxins – Bile pigments, bilirubin F 2 -29

Enterohepatic circulation F 2 -30

Regulation of bile secretion • Cholecystokinin – Released from duodenal mucosa • Stimulates gallbladder contraction • Relaxation of sphincter of Oddi – Supported by acetylcholine • From vagus and enteric nervous system F 2 -31

Secretions of the small intestine • Brunner´s glands – At beginning of duodenum – Between pyloro and papilla of Vater – Secretes highly alkaline mucus • Tactile stimulation of mucosa • Vagal stimulation • Secretin F 2 -32

Cont. . . • Crypts of Lieberkühn – Throughout the small intestine – Contains some Goblet cells • Secretes mucus to protect surface – Mainly enterocytes • Secretes large volumes of water and electrolytes F 2 -32

Enterocytes • Contains membrane bound enzymes – Peptidases • Peptides to amino acids – Sucrase, maltase, isomaltase, lactase • Disaccharides to monosaccharide – Intestinal lipase • Fat into glycerol and fatty acids F 2 -33

Regulation of SI secretions • • • Presence of chyme in intestine Various enteric nerve reflexes Increase in enteric activity Enhancement of motility Also supported by secretin and cholecystokinin F 2 -34

Secretions of the large intestine • Crypts of Lieberkühn – There is no villi – Mainly mucus producing cells – Responds to tactile stimulation – Parasympathetic stimulation – Emotional disturbances-stress • If irritated mucosa secretes large volumes of water and electrolytes – Diarrhoea F 2 -35

Summary • Got familiar with the salivary, gastric (oxyntic), pancreatic and Bruner’s gland the secretions of the liver • Understand the role of saliva on starches and lipids • Understand the role of the acid and peptidases in the stomach • Reviewed the role of a variety of pancreatic enzymes on CHOs, proteins and fats