Pancreatic Exocrine Secretions Flashcards
Describe the pancreas
Pancreas: ~15cm long, leafy shape, tucked under the liver, near the gallbladder.
Lies across the body behind the stomach. The pancreatic lobules is composed of closely arranged acini (singular: acinus).
Pancreas = dual organ; has exocrine and endocrine portions
What are the acini?
The acini are luminal structures which drain into a complex branching ductal system.
The pancreatic ductular system empties the secretions into the (proximal) duodenum via the pancreatic duct.
Describe the complex branching ductal system of the acini
Acini start w intercalated ducts which drain into intralobular ducts. Both of these are lined w cuboidal epithelium
Intralobular → extralobular → Interlobular ducts → Pancreatic duct. These ducts are all lined w columnar epithelium.
The pancreatic duct enters the duodenum at the common bile duct where secretions are emptied.
Describe the endocrine pancreas
Primary component is Islets of Langerhans: clusters of endocrine cells located throughout the pancreas, but most abundant in the tail – lots of blood vessels Beta cells (70%) secrete insulin, taking glucose into cells Alpha cells (20%) secrete glucagon which increases gluconeogenesis- increases blood glucose Somatostatin: inhibits insulin and glucagon production, inhibits secretin, CCK, gastrin and motilin
Other cell types secrete VIP, and enterochromaffin cells secrete serotonin, motilin and substance P
Label the anatomy of the pancreas
Label this exocrine pancreas
How are the acinar cells and zymogen granules related?
The acinar cells produce digestive enzymes which are stored inactivley in cytoplasmic zymogen granules. The granules are released by exocytosis.
Intercalated and intralobular ducts secrete Na and HCO3-to keep the stored enzymes inactive (pH >7.5).
Premature enzyme activation within acinar cells leads to acute pancreatitis
What are the centroacinar cells and the basal lamina?
Centroacinar cells are aka intercalated ducts. they have nuclei in the centre, and make pancreatic juice more alkaline by secreting bicarbonate
The basal lamina = layer of extracellular matrix secreted by the epithelial cells; the epithelium sits on it.
What are the pancreatic exocrine functions?
Secretes 1.5L of fluid/day (Na and HCO3- rich juice, albumin, globulin and digestive enzymes)
Acinar cells secrete digestive enzymes in inactive form (as zymogens) to prevent auto-digestion. Enzyme activation occurs in the duodenum
Describe the anions and cations secreted by the pancreas
The anions secreted in the pancreatic juice:
HCO3-, Cl-, SO42-, HPO42-
The cations secreted in the pancreatic juice:
Na+, K+, Ca2+, Mg2+
What are the alkaline secretions of the pancreas?
HCO3- rich juice from the pancreas/gallbladder neutralise gastric acid
Juice composition is modified as it travels thru the duct:
Epithelial cells actively exchange Cl- for HCO3- from the duct into the lumen/pancreatic juice
Na+/H+ exchanges H+ from the duct into the blood.
H+ neutralises HCO3- in the blood to form carbonic acid (H2CO3)
HCO3- also combines w C02 +H20 in the blood to form carbonic acid
Therefore HCO3- conc in pancreatic juice is directly proportional to conc of HCO3- produced in the blood
What is the effect of secretin on the composition and volume of pancreatic juice?
Increased secretin means bicarbonate levels come up and then down as the body metabolises it. Cl- levels go down due to the high levels of bicarbonate secreted
What are the major types of enzymes secreted by the pancreas?
Proteolytic enzymes (trypsin, chymotrypsin, carboxypeptidase)
Amylase (75% reaches small intestine)
Lipase (Pancreatic insufficiency → malabsorption of fats
Steatorrhoea)
Ribonuclease
Deoxyribonuclease
Describe the Enterokinase enzyme
Enterokinase: found in the brush border of duodenum; secreted in response to CCK
Converts trypsinogen to trypsin. Trypsin activates other proenzymes and digests proteins so there is risk of autodigestion
Inhibitors of autodigestion:
Kazal inhibitor, enzyme Y. Intracellular pH of zymogen granule itself = acidic (keeps zymogens inactive)
What suggests the bidirectional permeability to digestive enzymes?
Chymotrypsinogen and amylase cross the basolateral membrane of pancreas. This suggests bidirectional permeability of the basolateral membrane to digestive enzymes
Pancreatic secretions are controlled by…
Neuroendocrine signals
Secretin and CCK: stimulate secretion of pancreatic fluid
Vagal (parasymp) stimulation: enhances secretion of enzymes and aqueous components of pancreatic juice
Symp stimulation: inhibits secretion
Control of pancreatic secretions have 3 phases. What is the cephalic phase?
Cephalic phase: sight, taste, or smell of food stimulates the vagus nerve and muscarinic ACh receptors to release digestive enzymes via acinar cells.
To a lesser extent, it stimulates duct cells to secrete HCO3- and fluid.
Control of pancreatic secretions have 3 phases. What is the gastric phase?
Gastric phase: food in the stomach stimulates pancreatic secretions, primarily from the acinar cells, via 2 routes.
First, stomach distension activates a vagovagal reflex, which also causes ACh release.
Second, peptides and aa stimulate G cells in the antrum to release gastrin→ release of enzyme-rich pancreatic juice.
What happens to pancreatic secretions in the intestinal phase?
Intestinal phase: arrival of gastric acid in the duodenum stimulates S cells–>secretin–>HCO3 and fluid from duct cells.
Protein and lipid breakdown products have 2 effects:
they release CCK, which causes acinar cells to release digestive enzymes. 2nd, they initiate a vagovagal reflex that stimulates the acinar cells through M3 cholinergic receptors.
Complete the table
Describe the effects of secretin
Secretin from duodenum and jejunum induces pancreatic duct cells to secrete HCO3- -rich pancreatic juice
But, secretin ↓ enzyme content bc it’s a weak agonist of the acinar cells
Secretin also stimulates production of bile
Describe the effects of CCK
CCK from duodenum and jejunum in response to fatty acids (aa, and peptides) stimulates pancreatic acinar cells.
These release enzyme-rich pancreatic juice.
CCK also contracts the gallbladder and relaxes the sphincter of Oddi (via VIP and NO) → releasing bile and some HCO3- into duodenum
CCK promotes effects of secretin, which is a weak agonist of acinar cells
