Insulin

Question 1

What is glucose used for?

Answer 1

By cells to synthesise ATP

Question 2

What are the words used to describe when blood glucose levels are too high and too low?

Answer 2

Too high - hyperglycaemia

Too low - hypoglycaemia

Question 3

What are normal fasting blood glucose concentrations?

Answer 3

3.5-5.5 mmol/L

Question 4

What is glucose used for post feeding?

Answer 4

ATP formation (powers cellular functions)
Stored in specific organs then converted into molecules which can be stored (glycogen, triglycerides)

Question 5

What organs store glucose?

Answer 5

Skeletal muscle
Liver
Adipose tissue

Question 6

How is glucose metabolised during starvation?

Answer 6

Glycogen broken down into glucose

TG broken down into glycerol and free fatty acids -> Glycerol converted into glucose

Question 7

What organ is the most important in glucose homeostasis? What is its role?

Answer 7

Pancreas - regulates insulin secretion to promote glucose storage after meals, vs glucose output from liver during fasting

Question 8

Where is insulin synthesised?

Answer 8

In the pancreas within the islets of langerhans

Question 9

What do the islets of Langerhans contain?

Answer 9

alpha cells - produce glucagon
beta cells - produce insulin
delta cells - produce somatostatin
PP cells - produce pancreatic polypeptide
epsilon cells - produce ghrelin

Question 10

What are the properties of insulin?

Answer 10

• 2 chains (one 21 aa, one 20 aa) linked by 3 disulphide bridges
• monomers form dimers when insulin conc increases and when released
• in presence of Zn2+ and at specific pH dimers form hexamers (storage form of insulin)
  once hexamers secreted insulin dissociates into monomeric form

Question 11

How is insulin synthesised?

Answer 11

Initially in preproinsulin in pancreatic beta cells
peproinsulin processed into proinsulin about 5-10 mins after assembly in endoplasmic reticulum
proinsulin undergoes maturation into active insulin through endopeptidases within Golgi
endopeptidases cleave off C peptide from insulin by breaking bonds between peptides

Question 12

How is endogenous insulin production regulated?

Answer 12

• transcription from insulin gene
• mRNA stability
• mRNA translation
• post-translational modifications

Question 13

How is insulin synthesis and insulin secretion independent?

Answer 13

insulin and C-peptide are stored after synthesis awaiting secretion

Question 14

How is insulin secreted?

Answer 14

• glucose enters beta cells through GLUT 1 transporter
• glucokinase converts glucose into glucose-6-phosphate and acts as glucose sensor for insulin secretion
• Km of glucose carrier and glucokinase ensures initiation of insulin secretion by glucose occurs only when glucose levels are higher
• glucose is converted to glucose-6-phosphate and pyruvate (glycolysis)
• pyruvate through Krebs cycle and ETC generates ATP leading to rise in ATP:ADP ratio within cell
• Katp channels open so resting membrane potential is maintained at hyperpolarised level (-70mV)
• increased ATP/ADP ratio results in Katp closure and membrane depolarisation
• voltage gated Ca2+ channels open, intracellular concentration of Ca2+ increases and so insulin secretion triggered

Question 15

How do pancreatic beta cells release insulin in phases?

Answer 15

1st phase - release is rapidly triggered in response to increased blood glucose levels
2nd - sustained slow release of newly formed vesicles

Question 16

How do amino acids potentiate insulin release?

Answer 16

• increased intracellular catabolism of amino acids increases ATP/ADP ratio
• leucine acts through allosteric activation of glutamate dehydrogenase and transaminated to be converted into acetyl-coA
• amino acids such as arginine can directly depolarise plasma membrane

Question 17

What other signals potentiate insulin release?

Answer 17

• fatty acids
• PS release of acetylcholine
• cholecystokinin
• gastrointestinally derived incretins glucagon like peptide 1
• glucose dependent insulinotropic peptide

Question 18

What is special about insulin responsive cells?

Answer 18

• insulin responsive cells express specific receptor at plasma membrane
• insulin receptor is a transmembrane receptor, activated by insulin, IGF-I and IGF-II, belong to large class of tyrosine kinase receptors

Question 19

How is the insulin receptor activated?

Answer 19

• insulin binds to extracellular portion of alpha subunits
• causes conformational change activating tyrosine kinase domain residing on intracellular portion of beta subunits
• this autophosphorylated tyrosine residues on C-terminus of receptor and within adaptor protein

Question 20

How does insulin stimulate glucose uptake in muscles and adipocytes?

Answer 20

• glucose cannot cross plasma membrane and uptake requires specific transporters
• GLUT4 contained in intracellular vesicles in absence of insulin
• Akt activation due to insulin stimulates GLUT4 translocation to plasma membrane = glucose uptake

Question 21

How does insulin stimulate glycogen synthesis in muscles?

Answer 21

Akt phosphorylates and inactivates glycogen synthase kinase allowing activation of glycogen synthase

Question 22

What is the effect of insulin on fats?

Answer 22

• stimulates lipogenesis in adipocytes (TG break down into glycerol and fatty acids)
• inhibits lipolysis by inhibiting hormone sensitive lipase

Question 23

What is the function of malonyl-CoA?

Answer 23

inhibits transport of free fatty acids into mitochondria via CPT-1 (inhibits Beta oxidation)

Question 24

What effect does insulin have in the liver?

Answer 24

• enhances glucose uptake (increases glucokinase activity)
• increases glycogen synthesis
• increases lipogenesis (lipids exported as lipoproteins)
  (essentially inhibits gluconeogenesis)

Question 25

What other functions does insulin have?

Answer 25

- promotes protein synthesis and storage - stimulates transport of amino acids into the cells - increases translation of mRNAs - inhibits catabolism of proteins - promotes K+ intracellular uptake

Question 26

How does metabolism change during fasting?

Answer 26

- no insulin secretion = reduced blood glucose levels - liver releases glucose into bloodstream - amino acids and glycerol used as precursors for gluconeogenesis - other metabolic adaptations provide substrates for gluconeogenesis (lactate from Cori cycle) - acetyl-CoA accumulation that cannot enter TCA cycle converted into ketone bodies

Question 27

What mechanisms can switch insulin signalling off?

Answer 27

- endocytosis and degradation of receptor bound to insulin - decrease in number of receptors -> reduced insulin signalling - dephosphorylation of tyrosine residues by tyrosine phosphatases - serine/threonine kinases reduce activity of insulin receptor