Insulin

[Ref: WG21:p337-351]

Also see Carbohydrate metabolism

Basics

Insulin structure

• 2 chains of amino acids linked by disulfide bridges
• Pork insulin differs from human insulin by only one amino acid residue
  --> low antigenicity
• Beef insulin results in low titers of antibodies after 2 months of usage in almost all recipients

Insulin synthesis and secretion

• Synthesized in rough endoplasmic reticulum of B cells
• Packeted in membrane bound granules at Golgi apparatus.
  * granules have "halo" (clear space between insulin packet and mumbrane-lined vesicle)
• Encoded on short arm of chromosome 11
• Connecting peptide (C peptide) facilitates the folding between A chain and B chains, but is detached in the granule before secretion.
  --> C peptide level can be measured as an index of B cell function
• In B cells, insulin forms polymers and complex with zinc

Metabolism

• Half-life of insulin in circulation
  = 5 minutes
• Binds to receptors
  * Some internalised and destroyed by proteases in endosomes

 

Actions

GLUT4

[WG21:p340][See Glucose transporter]

GLUT-4 transporter in muscle and adipose tissue is stimulated by insulin

Normally it is stored in vesicles

When insulin receptors activated
--> Vesicles fuse
--> GLUT4 inserted into membrane
--> Glucose entry into cells

Some pool of GLUT4 are inserted in response to exercise
* Independent of insulin

Effects of insulin (summary)

1. Glucose

• Increase glucose uptake by muscle and adipose tissues
  * Via GLUT-4
• Increase glucose uptake by liver
  * Via induction of glucokinase
  --> Increased phosphorylation
  --> Faster diffusion of free glucose
• Decreased hepatic glucose output

2. Potassium

• K+ entry into insulin-sensitive cells
  --> Reduction of ECF [K+]

3. Anabolic effect

• Increase lipogenesis
• Increase protein synthesis

Effects of insulin by time line

Rapid

• Increased transport into insulin-sensitive cells:
  * Glucose
  * Amino acid
  * K+

Intermediate (minutes)

• Stimulation of protein synthesis
• Inhibition of protein degradation
• Activation of glycolytic enzymes and glycogen synthase
  --> Increased glucose usage and glycogen synthesis
• Inhibition of phosphorylase and gluconeogenic enzymes
  --> Decreased gluconeogenesis and glycogen breakdown

Delayed (hours)

• Increase in mRNA for lipogenic and other enzymes

Effects on insulin by tissues

Adipose tissue

• Increased glucose entry
• Increased fatty acid synthesis
• Increased glycerol phosphate synthesis
• Increased triglyceride deposition
• Activation of lipoprotein lipase
  * Increasing triglyceride uptake from chylomicrons into adipose tissue
  * [WG21:p309,p345]
• Inhibition of hormone-sensitive lipase
  * Decreasing FFA release from adipose tissue
• Increased K+ uptake

Muscle

• Increased glucose entry
• Increased glycogen synthesis
• Increased amino acid uptake
• Increased protein synthesis in ribosomes
• Decreased protein catabolism
• Decreased release of gluconeogenic amino acids
• Increased ketone uptake
• Increased K+ uptake

Liver

• Decreased ketogenesis
• Increased protein synthesis
• Increased lipid synthesis
• Decreased glucose output, due to
  * Decreased gluconeogenesis
  * Decreased glycogen breakdown
  * Increased glycogen synthesis
  * Increased glycolysis

General

• Increased cell growth
  

Mechanism of action

 

Insulin receptor

• Tetramer
• 2 alpha and 2 beta subunits
  * Synthesized on a single mRNA
  * Proteolytically separated
  * Coded on chromosome 19
• Alpha subunits
  * Extracellular
  * Binds to insulin
• Beta subunits
  * Span the membrane
  * Intracellular component have tyrosine kinase activity
• Halflife of insulin receptors
  ~ 7 hours

NB:

• Very similar to IGF-I receptor
  * But different from IGF-II receptor

 

Mechanism of action

Insulin binds to alpha subunits
--> Triggers tyrosine kinase activity of beta subunits
--> Autophosphorylation of the beta subunits
--> Triggers phosphorylation and dephosphorylation of various proteins

 

Regulation

Normal amount
= 1U/hour, with increases after meals
= 40U/day

Factors affecting insulin secretion

[WG21:p348] Selective

Stimulators

• Glucose
• Mannose
• Certain amino acids
  * Arginine, Leucine, and others
• NO
• Intestinal hormones
  * GIP and others
• Glucagon
• Beta-adrenergic agonist
• Sulfonylurea
• Theophylline

Inhibitors

• K+ depletion
• Somatostatin
• Alpha-adrenergic agonist
• Thiazide diuretics
• Phenytoin
• Insulin

 

 

Glucose

Acts directly on pancreatic B cells to increase insulin secretion

Response to glucose is biphasic
* A rapid but brief increase, followed by
* More slowly developing prolonged increase

 

Mechanism

[WG21:p349]

Glucose enters B cells by GLUT2
--> Metabolised
--> ATP generated
--> ATP inhibits ATP-sensitive K+ channels
--> Reducing K+ efflux
--> Depolarizaton of B cells
--> Ca2+ influx (via voltage-gated channel)
--> Exocytosis of granules (with insulin in them)
--> Followed by priming of other secretory granules (via glutamate)

 

Oral hypoglycemic agents

Sulfonylurea

Increase secretion of insulin

Only works when there are B cells remaining

Binds to ATP-inhibited K+ channels in B cells
* Similar mechanism as glucose [see above]

Metformin

Reduce gluconeogenesis
--> Thus decreasing hepatic glucose output

Risk of lactic acidosis

Troglitazone (and other thiazolidinediones)

Increase insulin-mediated peripheral glucose uptake
--> Reducing insulin resistance

 

 

cAMP

Factors that increase cAMP in B cells increase insulin secretion

e.g. glucagon, phosphodiesterase inhibitor (e.g. theophylline)

 

Catecholamine

Inhibits insulin secretion via alpha2-receptor

Stimulate insulin secretion via beta-receptor

Net effect: INHIBITION

In alpha-blockade, effect may be stimulation

 

Intestinal hormones

Stimulation of insulin secretion:

• Glucagon
• Secretin
• Cholecystokinin (CCK)
• Gastrin
• Gastric inhibitory peptide

But,

• GIP is the only one that produces stimulation with normal oral glucose intake

NB:

• Normal oral glucose intake does not produce levels of other hormones high enough to produce stimulation of insulin secretion

K+ depletion

Decreases insulin secretion

Thus,

Primary aldosteronism
--> K+ depletion
--> Diabetic glucose intolerance

Other notes

[WG21:p355]

Thyrotoxicosis aggravates clinical diabetes
* Thyroid increase GIT absorption of glucose and cause some hepatic glycogen depletion

Glucocorticoids can worsen diabetes
* Increase in protein catabolism
* Increased gluconeogenesis in liver
* Increased hepatic glycogenesis and ketogenesis
* Decrease in peripheral glucose utilisation

Growth hormone can also worsen diabetes
* Mobilise FFA --> Ketogenesis
* Decrease glucose uptake in some tissues
* Increased hepatic glucose output
* May decrease tissue binding of insulin
* Growth hormone does not stimulate insulin directly, but the hyperglycaemia it produces may stimulate insulin