Endocrinology Session 2 Flashcards
What performs the endocrine function of the pancreas?
Islets of Langerhans: small spherical structures scattered through exocrine pancreatic tissue
What cell types do islets of Langerhans contain?*
- Beta cells (insulin) - 75%
- Alpha cells (glucagon) - 20%
How are the products stored in the islets of Langerhans?
Stored intracellularly in membrane-limited vesicles/storage granules
What ultrastructural features make the islets ideal for export?
- RER
- Well defined Golgi
- Many mitochondria
- Good microfilament and microtubule system
What is the exocrine function of the pancreas?*
99% of all function
- Production of digestive enzymes secreted into the duodenum
- Via pancreatic duct
What is the structure of insulin?*
- 2 polypeptide chains (one alpha, one beta)
- Covalently linked by 2 disulphide bonds
- Third intrachain disulphide bond within the A chain
How is insulin synthesised?***
- Starts off as pre-proinsulin on ribosomes of RER
- Pre-part a signal peptide that ensures it will enter cisternal space of ER and removed once it happens
- Proinsulin folds to ensure correct alignment of cysteine residues and correct bond formation
- Proinsulin transported to trans Golgi apparatus and into storage vesicles
- Proteolysis removes C peptide and 4 basic AA (3 arg 1 lys)
- Chain broken into 2 chains held together by disulphide bridge
What happens in patients who have a lack of C peptide?
- Association with type 1 diabetes
- Lower glomerular hyperfiltration
- Lower urinary albumin excretion
What do the insulin vesicles contain after proteolysis?
Equal amounts of insulin and C peptide which are released during secretion
How is insulin stored in B cells?
- Zinc-insulin complex
- Dissolves in plasma
What is margination?
Movement of the vesicle to the surface
What is exocytosis?
The release of the peptide from the vesicle and the fusion with the plasma membrane
