PBL 1 - T1DM Flashcards
What are the cell types within the islet of langerhans and what do they secrete?
β cells → produce Insulin
α cells → produce Glucagon
delta-cells → produce Somatostatin
Describe the structure of insulin
polypeptide hormone
- two short chains (A and B) linked by disulphide bonds
What is proinsulin?
insulin + c peptide
What is pre-proinsulin?
precursor synthesised by the ribosomes of the beta cells from insulin mRNA
How is pre-proinsulin converted to proinsulin?
cleaved in the golgi apparatus
Where is insulin stored?
in secretory granules
What factors can increase insulin release?
- hyperglycaemia
- growth hormone
- cortisol
- beta agonists
What factors can decrease insulin release?
- hypoglycaemia
- somatostatin
- alpha agonists
Why is growth hormone said to have anti-insulin activity?
it supresses the abilities of insulin to stimulate uptake of glucose in peripheral tissues and enhance glucose synthesis in the liver.
However, administration of growth hormone stimulates insulin secretion, leading to hyperinsulinemia
What triggers insulin secretion?
β-cells sense changes in plasma glucose concentration an responds
How is glucose levels detected in the beta cells?
Glucose enters β cells via the GLUT-2 receptor.
○ Glucose transporter 2 (GLUT2), constitutively expressed in β-cells, is the first encountered glucose sensor in β-
cells
○ mobilization of GLUT2 to the plasma membrane is insulin-independent and the transporter protein shows a low
substrate affinity, ensuring high glucose influx
What is the mechanism of insulin secretion?
● Metabolism of the glucose generates ATP => closes the ATP-sensitive Potassium (K+) channels in the membrane
● Closure of these K+ channels causes a depolarisation in the membrane (since K+ is not moving out of the cell).
● Depolarisation causes voltage-gated Calcium channels to open, causing Ca2+ influx.
● Increased intracellular calcium causes fusion of insulin granules with the cell membrane, releasing insulin by exocytosis.
● Insulin is released into the portal circulation and is carried to the liver, its primary target organ
Where does insulin bind?
Insulin binds to Insulin Receptors on the surface of target tissues
Describe the insulin receptors
- tyrosine kinase receptors
- made up of 2 alpha and 2 beta units
Where are insulin receptors mainly found?
- liver
- striated muscle
- adipocytes
How does insulin bind to the insulin receptor?
insulin binds to
the α-unit
What changes occur on insulin binding to the insulin receptor?
conformational change in the β-units, causing autophosphorylation and an intracellular cascade of events.
Insulin-receptor complex is internalised by the cell
> Insulin is degraded
> Receptor is recycled to the cell surface
What is the effect of insulin on the liver?
○ Increases glycogen synthesis
○ Increases fatty acid synthesis
○ Inhibits gluconeogenesis (PEPCK & G6Pase)
What is the effect of insulin on the muscle?
○ Increases glucose transport (GLUT4 translocation to membrane)
○ Increases glycogen synthesis
What is the effect of insulin on the adipose tissue
○ Increases glucose transport (GLUT4)
○ Suppresses lipolysis
○ Increases fatty acid synthesis
What is the effect of insulin on electrolytes?
increases the permeability of many cells to potassium, magnesium and phosphate ions.
How does insulin effect potassium levels?
Insulin activates sodium-potassium ATPases in many cells, causing a flux of
potassium into cells.
What is glycogen synthesised from?
glucose
What is the function of glycogen?
in liver = storage for blood glucose maintenance
in muscle = storage for local energy production (only used by muscle itself)
How is glucagon synthesied?
- synthesized as proglucagon
- processed in alpha cells of pancreatic islets into glucagon
Where is glucagon secreted?
alpha cells in the islet of the pancreas
What is the major stimulus for glucagon secretion?
hypoglycaemia
What causes inhibition of glucagon secretion?
hyperglycaemia (major inhibition)
also inhibited by somatostatin
What can trigger glucagon secretion?
- hypoglycaemia
- elevated blood levels of amino acids
- exercise
Describe glucagon signalling
Glucagon binds to the Glucagon Receptor, a G-protein coupled receptor (GPCR) found in hepatocytes.
○ Increases cyclic AMP (cAMP)
○ cAMP stimulates cAMP-dependent protein kinase (PKA)
What is the effect of glucagon on target tissues?
Only acts on the liver (only site of receptors), to increase blood glucose
How does glucagon increase blood glucose levels?
○ Glucagon stimulates breakdown of glycogen stored in the liver - acts on hepatocytes to activate the enzymes that
depolymerize glycogen and release glucose.
○ Glucagon activates hepatic gluconeogenesis - pathway by which non-hexose substrates such as amino acids are
converted to glucose
What effect does glucagon have on lipolysis?
appears to have a minor effect of enhancing lipolysis of triglyceride in adipose tissue, which could be viewed
as an addition means of conserving blood glucose by providing fatty acid fuel to most cells.
What is the normal range of glucose?
3.5-8.0 mmol/L
Which tissues are more dependent on glucose than others?
- erythrocytes (no mitochondria)
- brain (FA cannot cross BBB)
- retina
- testes - BTB
What is the response of insulin levels in a fasted state?
Insulin concentration is low
- Main action is to regulate glucose release by the liver
> Acts as a hepatic hormone, modulating glucose production – insulin inhibits gluconeogenesis
> Hepatic glucose production rises as insulin levels fall
What is the response of insulin levels in a postprandial state?
Insulin concentrations are high
○ Promote glucose uptake into peripheral tissues (increased glucose utilisation)
○ Suppresses glucose production in the liver
Why is diabetic insulin injection not equivalent to
1 - insulin is secreted directly into the portal circulation and reaches the liver in high concentration
2 - Subcutaneous soluble insulin takes 60-90 minutes to achieve peak plasma levels - onset and offset of action are too slow
3 - Absorption of subcutaneous insulin into the circulation is variable
4 - Basal insulin levels are constant in healthy people. In contrast, insulin injections causes peaks and declines in plasma insulin
levels, causing swings in metabolic control
Describe the factors that influence the aetiology of T1DM
- polygenic genetic susceptability
- environmental factors
- viruses
- geographical factors
- dietary factors
What gene polymorphisms increase susceptibility to T1DM
DR4-DQ8 and DR3-DQ2 are considered susceptibility genes
What gene polymorphisms provide protection from T1DM
DR15-DQ6 considered protective
What is the link between viruses and T1DM?
In a genetically susceptible individual, viral infection may stimulate the production of antibodies against a viral
protein that trigger an autoimmune response against antigenically similar beta cell molecules
○ congenital rubella syndrome
○ human enteroviruses.
What geographical factors are involved in the aetiology of T1DM?
geographical variation in disease prevalence
increasing worldwide incidence
What dietary factors are involved in the aetiology of T1DM?
Potential increased risk with exposure to cow’s milk in infancy, early introduction of cereals, or maternal vitamin
D ingestion increase type 1 diabetes risk.
infant supplementation with vitamin D may be protective.
What autoimmune diseases is T1DM associated with?
○ Autoimmune thyroid disease
○ Coeliac disease - shares the HLA-DQ2 genotype, more common in DM1 patients
○ Addison’s disease (adrenal insufficiency)
○ Pernicious anaemia
What is the main cause of T1DM?
autoimmune pancreatic beta-cell destruction in genetically susceptible individuals.