7.2 Pancreatic Secretions Flashcards
How does the products of the pancreas make it to the small intestine
The pancreatic duct joins together with the bile duct to release into the duodenum
Describe the composition of pancreatic juice
The pancreas makes 1.2-1.5 L of pancreatic juice a day. It is comprised of digestive enzymes, water and bicarbonate ions
Describe the positioning of the acini and ductules in the exocrine area of the pancreas
The acinar cells form a circle which is the acinus, the acinus then opens up into the ductule, formed by duct cells
What are the differing roles of acinar cells and duct cells in producing pancreatic juices
The acinus produces a low volume enzyme rich fluid, the ductule then increases the volume and pH of the fluid by adding water and bicarbonate
Describe how acinar cells are specialised to produce lots of enzyme proteins
The cells have lots of rough endoplasmic reticulum and golgi apparatus. From the golgi apparatus, the proteins enter condensing vacuoles which remove all the unnecessary material from the vesicle before it becomes a zymogen granule.
Describe the purpose of zymogen granules in acinar cells
Zymogens are an inactive precursor form of the enzyme that need to become activated outside of the cell. This is to prevent the enzymes from digesting the cell itself
What are the categories of the products of acinar cells
Precursors of proteases, starch digesting enzymes, lipid digesting enzymes/precursors, nucleases and regulatory factors
Describe how enzymes from the acinar cells are activated
The inactivated enzymes are released through the pancreatic duct into the duodenum. Membrane bound enteropeptidases in the epithelium of the duodenum convert the inactive enzymes to their active forms. A particularly important enzyme to activate is trypsin (from its inactive form trypsinogen), the trypsin can then breakdown other enzyme proteins, activating chymotrypsin from chymotrypsinogen, carboxypeptidase from procarboxypeptidase and activating more trypsin.
Is secretion of enzymes from the pancreas constant
No. It must be stimulated at certain points
Describe the different stages of stimulating acinar cells.
Cephalic stimulation, from the brain when thinking about, looking at and tasting food. It is signalled via the vagus nerve
Gastric stimulation, from the stomach when the stomach destends (stretches) when food is in the stomach. It is signalled neuronally via the vagus nerve.
Intestinal stimulation, which is the majority of acinar stimulation. Comes when food is in the intestine. It is due to the intestine distending causing vagus stimulation and food molecules sending signals to receptors in the intestine causing CCK secretion.
Describe how stimulation causes the secretion of enzymes from acinar cells
Stimulation of the vagus nerve releases the neurotransmitter acetylcholine as well as VIP (vasoactive intestinal polypeptide) and GRP (gastrin releasing peptide) . Hormonal signalling uses the hormone CCK. All of these signalling molecules bind to receptors on the acinar cells. This causes an increase in intracellular calcium concentration (and an increase in cAMP from the VIP). This then causes the vesicles to bind to the membranes and exocytosis of the enzymes. It also causes chloride to exit the cells, drawing some sodium and water into the lumen.
Describe the negative feedback mechanism regulating acinar enzyme secretion
Following pancreatic stimulation from vagus nerve after eating, the pancreas secretes trypsinogen which is activated. The trypsin breaks down the amino acids from the food in the lumen of the small intestine. The pancreas also secretes monitor peptides. After eating the presence of not fully digested amino acids and fatty acids in small intestine can be detected, this causes epithelial cells to release CCK-releasing peptide. The CCK-RP and monitor peptides bind to I cells in the epithelium to release CCK into the bloodstream. CCK then travels in the bloodstream to the pancreas to stimulate acinar cells. However, once the trypsin has broken down all the amino acids and is no longer occupied, trypsin has an inhibitory effect on CCK-RP and the monitor peptide, so I cells are no longer stimulated to produce CCK. This acts as a negative feedback mechanism so once the food has been digested, the acinar cells are no longer stimulated to secrete digestive enzymes
Describe how pancreatic duct cells carry out their role in producing pancreatic juices.
The job of the pancreatic duct cells is to add a lot of water volume and increase the pH via bicarbonate ions to the pancreatic juices. The CFTR chloride channel opens up due to an increase in cAMP, Cl exits cell into the duct lumen but then reenters via bicarbonate/Cl antiporter where bicarbonate ions exit the cell into the duct lumen. In the pancreatic duct cells, bicarbonate is produced via the carbonic anhydrase enzyme (water + CO2 = carbonic acid = bicarbonate + H+). Also some bicarbonate from the post prandic alkaline tide is taken up from blood into duct cells alongside sodium ions. This is allowed for by a Na/K pump (pumping 3 Na out for every 2 K in) creating a Na concentration gradient for the Na to then enter the cell from the bloodstream and bring the bicarbonate with it. Using up some of the bicarbonate and producing some H+ that is released into the blood causes an acid tide in the blood which neutralises the alkaline tide. This whole process functions so that bicarbonate can exit the duct cells into the duct lumen, the bicarbonate’s osmotic pull brings sodium ions and then water across with it. Hence this increases the volume and pH of the pancreatic juices
What stimulates duct cells
Secretin stimulates duct cells to secrete enough bicarbonate to neutralise pH. Secretin also inhibits gastric acid secretion.
Describe the pancreatic symptoms of cystic fibrosis
In cystic fibrosis there are mutations causing dysfunction of the CFTR chloride channel of pancreatic duct cells. This reduces the ability of the duct cells to increase the pH and water volume of the pancreatic juice. This causes thickened pancreatic secretions which block up the duct. A blocked pancreatic duct will cause fibrosis of the pancreas due to digestive enzymes breaking down the pancreas. It will also cause malabsorption and malnutrition as digestive enzymes can’t reach the duodenum to break down the food.
