hormonal regulation of blood glucose levels (insulin and glucagon)

Question 1

What are the different cells in the pancreatic islet?

Answer 1

• Beta cells (60%)
		○ Insulin
		○ Arranged in a central core
	• Alpha cells (25%)
		○ Secrete glucagon
		○ Sit next to insulin secreting cells
	• D or delta cells
		○ somatostatin
	• PP cells
		○ Pancreatic polypeptide

Question 2

Describe the blood flow to the pancreatic islet

Answer 2

• Fenestrated capillaries, moves from the central core out to the periphery, and thus the cells secreting somatostatin and pancreatic polypeptide are bathed in insulin rich blood

Question 3

Describe the structure of insulin

Answer 3

• Derived from pro-insulin by cleavage of the connecting peptide
  
  • “c peptide”
  • Leaving the A and B chains joined by disulfide bonds

Question 4

Why might you measure C peptide levels in a diabetic?

Answer 4

• Exogenous insulin does not have C peptide, so checking C peptide levels is a way to distinguish between exogenous and native insulin release

Question 5

Describe the synthesis and secretion of insulin

Answer 5

• Pre-pro-insulin is synthesized by the usual pathway for synthesis of secreted proteins
○ Nascent peptide is inserted across the RER membrane where the signal peptide is cleaved
○ Transferred to golgi for packaging into secretory granules
○ C peptide cleaved in secretory vesicles
• Mature secretory vesicles are called secretory GRANULES
○ Insulin is in a crystalline structure made up of insulin hexamers containing Zn atoms
○ Also contain tons of other regulatory proteins
• After secretory vesicle release, the dilution of the insulin crystals will result in crystal degradation release of free insulin

Question 6

What is meant by ‘biphasic’ response of the islet to glucose?

Answer 6

• When islet cells are exposed to high glucose concentrations for 20 minutes or longer, there is an initial surge in insulin release followed by a second phase of prolonged release
  
  • The prolonged release goes until glucose levels drop
  • The initial phase is thought to be from pre-formed and docked vesicles

Question 7

What is the stimulus for insulin secretion?

Answer 7

• Initiators
○ Stimulate insulin release on their own and include glucose, amino acids and drugs like sulfonylureas
§ Glipizide and glyburide
• Potentiators
○ Increase insulin secretion only in the presence of glucose
○ Incretin peptides such as
§ GLP-1 and ach
• Inhibitors
○ Include the drug diazoxide, somatostatin and alpha-adrenergic agents
○ Longstanding hyperglycemia results in a reversible reduction in insulin secretory capacity, or glucose toxicity

Question 8

How does glucose affect insulin release?

Answer 8

• Most important stimulus
  
  • Taken up by beta cell, metabolized via glycolysis and TCA cycle
  • Increases ATP levels which generates the intracellular signal that starts the cascade
  • Increased intracellular calcium will lead to docking of secretory vesicles
  • Anything that results in increased intracellular calcium will result in insulin release
  • Glucose enters through GLUT2, an insulin INDEPENDENT glucose transporter
  • Glucokinase traps it in the cell
  • The cell is depolarized by closing ATP regulated potassium channels
  • Sulfonylureas act directly by blocking the ATP regulated potassium channels

Question 9

How do sulfonylureas mess with insulin release?

Answer 9

• Sulfonylureas act directly by blocking the ATP regulated potassium channels
  
  • Depolarizes the beta cell and leads to increased intracellular calcium and secretory vesicle docking

Question 10

Do amino acids and fats stimulate insulin secretion?

Answer 10

• Just amino acids, not fat

* The amino acids get metabolized and those metabolite intermediates are what stimulates insulin secretion

Question 11

Describe the nervous system regulation of insulin release

Answer 11

• Stimulation of splanchnic nerves inhibits insulin secretion as the catecholamines liberated interact with alpha receptors on the beta cell
  
  • Stimulation of vagal nerve with attendant release of ach increases insulin secretion
  • Modest increase in insulin during the cephalic phase

Question 12

What “other hormones” affect insulin release?

Answer 12

• Somatostatin from the delta or D cells decrease insulin release in a paracrine fashion
  
  • Catecholamines also have effects on insulin secretion
  • Epinephrin inhibits secretion by binding alpha-adrenergic receptors on the beta cells
  • Important during exercise and stress

Question 13

When you see alloxan and streptozotocin what should you think?

Answer 13

• Experimental destruction of islet cells

* Creation of experimental diabetes

Question 14

In general, what does insulin do to the liver, muscle and fat tissue in terms of glucose handling?

Answer 14

• Liver
○ GLUT2 is the transporter and that is insulin independent so nothing as far as uptake
○ It does however inhibit gluconeogenesis while stimulating glycogen syntheisis and fat synthesis
• Muscle
○ Increases uptake and glycogen synthesis
○ GLUT4 transporter
• Fat
○ Stimulates glucose uptake and fat synthesis and inhibits fat breakdown
○ Inhibits lipolysis

Question 15

Describe the insulin receptor

Answer 15

• Part of the EGF (epidermal growth factor) family of membrane associated receptors
  
  • Hetero-tetramer with two extracellular alpha chains and two membrane spanning Beta chains
  • Alpha and beta chains are synthesized as one chain then cleaved and reconnected by disulfide bridge
  • Insulin binding happens on the alpha chains

Question 16

What are the “other” or “global” effects of insulin?

Answer 16

• It reduces food intake at the brain level
• Regulates blood flow in vessels
• Regulates salt and water reuptake in kidneys
• Stimulates overall growth in tissues (mitogenic effects)
○ This can be involved in the development of cancer during hyperinsulinemia
*beta chains have inherent tyrosine kinase activity
*insulin binding leads to autophosphorylation and other protein mediators being phosphorylated

Question 17

What role do SH2 domains play in insulin signaling?

Answer 17

• SH2 domains are the particular domains that dock to and are phosphorylated by RTKs
• Since the insulin receptor phosphorylates SH2 domains on tyrosine residues, it is these domains that are important for insulin signaling
• In particular the IRS proteins are impoartant
○ Insulin receptor substrates
○ 1-4

Question 18

What are the key intermediates in the metabolic pathway?

Answer 18

• The second messengers:
○ PI3K - phosphoinositol 3 kinase
○ AKT
• Also, remember the IRS proteins are adapter proteins and provide a docking site for all the SH2-domain containing proteins in the cell

Question 19

What is a key intermediate in the mitogenic pathway?

Answer 19

• MAP kinase
  
  • Also, remember the IRS proteins are adapter proteins and provide a docking site for all the SH2-domain containing proteins in the cell

Question 20

Describe the pathway that ends up putting GLUT4 in the membrane of insulin sensitive tissues

Answer 20

• Insulin stimulates the phosphorylation of IRS1
  
  • This stimulates PI3K activation
  • PI3K causes rapid translocation to the plasma membrane and probably an activation of these GLUT4 transporters as well
  • When insulin is removed the glut-4 transporters are re-endocytosed

Question 21

The mitogenic actions of insulin are carried out by the action of what kinase pathway?

Answer 21

• MAP kinase pathway

Question 22

Does insulin resistance usually involve the insulin receptor?

Answer 22

• NOPE. Only in rare genetic diseases

Question 23

What, if not the receptor, is implicated in most cases of insulin resistance?

Answer 23

• Most insulin resistance is due to problems in the insulin signaling pathways
  
  • Genetic and lifestyle factors combine to change cellular metabolism in a way that favors phosphorylation of serine and threonine residues on signaling molecules which make signaling less effective
  • Serine and threonine phosphorylation reduces insulin signaling while tyrosine phosphorylation increases it

Question 24

Describe the structure of glucagon

Answer 24

• Synthesized in the alpha cells of the pancreatic islets
  
  • Pro-hormone that is cleaved prior to secretion by exocytosis
  • Like insulin, glucagon is secreted into the protal circulation where it’s first target is liver

Question 25

What, in general, is the action of glucagon?

Answer 25

• Interacts with a G-protein coupled cell surface receptor that, when activated by glucagon binding, increases cellular cAMP
  
  • This results in increased glycogenolysis and gluconeogenesis in the liver
  • Reciprocally regulated with insulin, and has pretty much the opposite effects at the level of liver

Question 26

Besides carbohydrate handling, what else is glucagon controlled in the liver?

Answer 26

• Breakdown of triglyceride in adipose tissue
○ Lipolysis
• Generation of ketons by liver
○ Ketogenesis
• These increase the export of fatty acids and ketone bodies which are alternative fuels to glucose that are used during starvation
• Glucagon also inhibits hepatic glycolysis so that the liver istelf must rely on fatty acids as opposed to glucose for energy needs during fasting states

Question 27

Describe how the secretion of glucagon is regulated

Answer 27

• Glucagon is secreted in response to hypglycemia and inhibited by hyperglycemia although the latter requires insulin and normal insulin action to occur
  
  • Specifically, the secretion of glucagon is inhibited by entry of glucose into the alpha cells via insulin sensitive glucose transporters
  • Since insulin is needed for this, glucagon can be inappropriately high if insulin is low (type I diabetes) or if insulin resistance is there (type II)
  • GLP-1 analogues will reduce glucagon levels
  • Newer diabetes management is taking glucagon and insulin both into account and infusing appropriately

Question 28

What are incretins?

Answer 28

• When glucose is taken orally, insulin secretion is stimulated much more than it is when glucose is infused IV
• This is called the incretin effect and is estimated to be up to 70% of the overall insulin response to oral glucose ingestion
• Caused mainly by tow intestinal insulin-stimulating hormones
○ GLP-1 and GIP
○ GIP = glucose dependent insulinotropic polypeptite
○ GLP-1 is more clinically important because of the diabetic medication analogues

Question 29

How is GLP-1 secreted?

Answer 29

• Stimulated by the presence of nutrients in the lumen of the gut
  
  • Additional neural or endocrine mechanisms to also help
  • Secretion throughout the day is highly correlated to release of insuline
  • GLP-1 is one of the most potent insulin-releasing substances known

Question 30

How is GLP-1 produced?

Answer 30

• GLP-1 is a prroduct of the glucagon gene
  
  • This is in alpha cells of course but also L cells of intestinal mucosa
  • When made by L cells the primary translation product, pro-glucagon, is cleaved to not produce glucagon but to release the C-terminal peptides GLP-1 and GLP-2
  • 50% sequence homology to glucagon

Question 31

What does GLP-1 do?

Answer 31

• GLP-1 acts through a cell membrane associated receptor present on beta cells of the pancreas, but also present on a number of other cells including cells in the brain
• GLP-1 has a number of biological actions that are beneficial in diabetes treatment
○ Potentiates glucose induced insulin secretion
§ Doesn’t cause hypoglycemia
○ Enhances all steps of insulin biosynthesis
○ Up regulates insulin gene expression
○ Up regulates expression of genes essential for beta cell function
○ Mitotic for beta cells
○ Promotes differentiation of duct progenitor cells to beta cells
○ Inhibits apoptosis of beta cells
○ Inhibits glucagon secretion
§ Results in lowering of blood glucose levels
○ Inhibits gastrointestinal secretion and motility
○ Inhibits appetite and food intake

Question 32

Why is the 1/2 life of GLP-1 so quick?

Answer 32

• Measured in minutes b/c broken down by circulating enzyme named dipeptiyl peptidase-4 (DPP-4)
  
  • Thus, native GLP-1 is not useful as a medicaiton, it has to be restructured to avoid DPP-4 degradation

Question 33

When catecholamines are released on the liver, what is the result on carbohydrate handling?

Answer 33

• Catecholamines significantly increase glycogenolysis, gluconeogenesis and ketogenesis
• Both norepi and epinephrine
• They also decrease glycolysis and glycogen formation
○ In this way they have functions similar to glucagon
• However, increased blood glucose is augmented and prolonged by catecholamines
○ Inhibit insulin secretion by interaction with alpha-receptors on beta cells of the pancreatic islets
○ They produce insulin resistance in skelatal muscle by stimulating glycogenolysis
○ Catecholamines are elevated in hypoglycemia
○ They are also elevated in stress or trauma
○ They also contribute to the initiation and/or maintenance of high glucose levels and in states of insulin dpendency can preciptiate ketoacidosis

Question 34

Why is continuous administration of corticosteroids a potentiator of diabetes?

Answer 34

• High cortisol levels mimics anti-insulin effects
  
  • Cortisol also potentiates the physiological actions of glucagon and catecholamines
  • Also look for cushing’s syndrome

Question 35

What does growth hormone do to blood glucose levels?

Answer 35

• Growth hormone release results in increased blood glucose levels
  
  • It’s level in the blood is increased in response to hypoglycemia as well as other forms of stress
  • Long term - promotion of lipolysis and stimulation of protein synthesis
  • Can mimic or even cause a period of insulin sensitivity (watch for kids going through a particularly crazy growth spurt)
  • Also, tumors of pituitary can result in diabetes with inappropriate secretion of growth hormone

Question 36

What is important to know about somatostatin in terms of blood glucose levels?

Answer 36

• Important role in hypothalamus inhibiting GH release
  
  • Potent inhibitor of insulin and glucagon and is postulated to act in a paracrine manner within the pancreatic islet where it is secreted by delta cells
  • Inhibits gut motility, splanchnic blood flow, secretion of digestive enzymes and intestinal absorption