Topic 3: Glucose and Insulin; Hypoglycemia and Diabetes Mellitus

What is the main roles of insulin on different organs?

anabolic hormone → builds things

Adipose

• increase glucose uptake , lipogenesis

• decrease lipolysis

Muscle

• increased glucose uptake, glycogen synth, protein synth

Liver

• decreased gluconeogenesis

• increased glycogen synth, lipogenesis

How is glucose taken up by cells? By which mechanism>

since glucose is too large to diffuse through membranes, it moves by facilitated transport (ATP-independent)

• flip flop mechanism

• import into most tissues

• export possble for liver and kidney

1. Glucose binds GLUT in “outside-open” conformation

2. GLUT-glucose changes to “inside-open” conformation

3. glucose is release into cell and GLUT returns to “outside-open” conformation

How is glucose taken up by the gut? Which receptor

uptake of glucose in SI occurs via sodium-glucose cotransport 1 (SGLT1)

• SGLT1 localised to luminal membrane of enterocytes

export of glucose from basolateral side of enterocytes into circulation is mediated by GLUT2

where are each GLUT proteins located and what are their special properties?

GLUT1

• most cells: kidney, colon, RBC, brain microvessels

• high affinity + high capacity

GLUT2

• liver, pancreatic beta cells, basolateral membrane of SI, kidney

• low affinity + high capacity

• glucose sensor in beta cells

• carrier for fructose

GLUT3

• brain

• high affinity and high capacity

GLUT4

• fat, skeletal and cardiac muscle

• activated by insulin

• high affinity

• mediates insulin-stimulated glucose uptake in adipose + muscle

GLUT5

• intestine, testes, kidney

• primarily fructose carrier in intestine

Where is insulin synthesised and secreted?

islet of langerhans/pancreatic islets containing clusters of endocrine cells

beta cells secrete insulin

how does glucose mediate secretion of insulin? Which organ and which receptor is responsible for this? Include a diagram

1. pancreas detects rise in blood glucose >5mM

   1. GLUT2: has high capacity - low affinity for glucose

   2. glucokinase has high Km and phosphorylates glucose → glucose-6-phosphate

   3. this breakdown of glucose increases ATP prod → increases ATP/ADP ratio

2. this blocks ATP sensitive K+ channel → prevents K+ efflux→ membrane depolarises

3. Ca2+ channels open → Ca2+ influx

4. Ca2+ stimulates release of stored insulin in vesicles which are exocytosed

Describe the structure of the insulin receptor protein tyrosine kinase

• monomer of alpha and beta chains

  • receptor exists as a dimer is alpha 2 beta 2

  • chains are joined by cysteine disulfide bridges

• extracellular ligand binding domain

• transmembrane domain

• intracellular tyosine kinase and phosphorylation sites

Describe insulin receptor binding and activation. How many molecules bind to the receptor and what process occurs during activation?

binding

• 2 signal molecules/ligands bind to the receptor dimer (one for each monomer

activation

• receptor is activated

• autophosphorylation occurs inside cell on Tyrosine

signal

• intracellular signalling proteins bind receptor

• intracellular signalling from receptor

What are the effects of insulin over time? Rapid, Intermediate and Delayed

rapid (seconds)

• increased membrane tranport (glucose, AA) in insulin sensitive cells

intermediate (minutes)

• activation of inhibition of enzymes

• anabolic actions: stim protein synth, lipogenesis, glycogenosis

delayed (hours)

• increase mRNAs for lipogenic and other enzymes

• promotion of cell growth (hours to days)

What is the role of GLUT 4 in regards to insulin and glucose

insulin absence: sequestered/stored in cells

insulin triggers exocytosis of GLUT4 → moves to cell membrane

GLUT4 transports glucose into cell

GLUT4 is then re-internalised

Descibe how insulin activates glycogenesis. Which enzymes does insulin inactivate or activate

stimulates cells in liver and muscle to store glucose as glycogen

insulin inhibits glycogen breakdown by stimulating dephosphorylation and inactivation of glycogen phosphorylase

insulin stimulates glycogen synth by stimulating desphophorylation and activation of glycogen synthase

Describe the opposing actions of insulin and glucagon. Which occurs during high and low blood sugar levels

high blood sugar/glucose stimulates insulin release

• insulin binds to membrane receptor

• glucose transport GLUT4 moves to cell membrane

• increases permeability to glucose (stim glucose uptake into tissues)

• also stimulates formation of glycogen in liver (activates glycogen synthase)

low blood sugar/glucose stimulates glucagon release

• glucagon binds to membrane receptor

• activates adenylate cyclase

• increases cAMP

  • activates cAMP dependent kinase and inhibition of glycogen synthase

  • activates glycogen phosphorylase - incresae glycogen breakdown

• releases glucose into blood

Describe insulin stimulation of protein synthesis in the liver and muscle. Insulin increases uptake and synthesis of ______ whilst decreasing _____ and release of ______

• increases AA uptake

• increases net protein synth

• decreases protein catabolism

• decreases release of gluconeogenic AA

Insulin increases or decreases LPL activity and in where?

increases LPL activity and mRNA levels in adipocytes and muscle

LPL hydroses TAG → non esterifed FFA

Insulin increases hepatic VLDL prod for what use?

VLDLs produced by uptake of chylomicron or VLDL remnants or FFAs in plasma or by de novo FA synth

insulin stimulates VLDL synthesis for short term storage

liver has critical role in homeostasis

• conversion of XS glucose into TAGs

• storage of XS TGs from meals that are later released as VLDLs

What is diabetes mellitus

group of metabolic disorders characterised by hyperglycaemia resulting from defects in insulin secretion and/or action

What is the prevalence of DM worldwide

1/10 adults has DM

½ adults undiagnosed

11.5% of global health cost

1/7 births affected by gestational DM

75% of ppl with DM live in low to middle income countries

almost 700,000 children have T1DM

1 person with DM dies every 9 seconds

Describe T1DM.T1DM is a ______. What is the prevalence and what are some risk factors

failure to produce active insulin

• autoimmune destruction of insulin producing pancreatic beta islet cells

• 1% aus prevalence

• risk factors: history, genetics, ethnicity/geography (further away from equator), age

Describe T2DM. T2DM is a _______. What is the prevalence and what are some risk factors

insulin resistance condition w inadequate insulin secretion

• 8% aus prevalence (4% overt)

• risk factors: fam history, >55yo, overweight/high BP, aboriginal/torres strait islander, indian, chinese, have PCOS, have pancreatitis, certain medications

describe gestational DM. what is the prevalence and what are some risk factors?

insulin resistant condition w inadequate insulin secretion

• 18% pregnant women aus prevalence

• risk factors: overweight, have PCOS, previous case

test: OGTT, FGB

What is the clinical impact of DM. What diseases does it increase the risk of____

• 2-4 fold increase CVS mortaility

• leading cause of new cases of end stage renal diseaese, blindness, nontraumatic lower extremity amputations

Compare and contrast T1 and T2DM. Discuss the onset, symptoms, weight, ketosis, insulin levels, C peptide and auto antibodies in each

	T1DM	T2DM
Onset	<30 years Sudden	>20 years Gradual
Symptoms	Severe	May be no symptoms
Weight	Thin	Obese
Ketosis	Spontaneous ketosis cells starving - cant use glucose must metabolise something else → fats	Not ketotic
Insulin levels	Low or absent	Low, N or High
C-peptide	Absent	Detectable
Islet cell auto-antibodies	Yes	No

T2DM is characterised by what two mechanisms?

Reduced insulin secretion

• mechanism unclear

• worsens with time

• beta cell exhaustion

Peripheral insulin resistance

• genetic + ethnicity

• obesity

• inactivity/low physical fitness

• intrauterine and childhood factors

• smoking and drugs

What are the characteristics of insulin resistance? What markers does it increase/decrease

• high BP

• microalbuminaemia

• abdominal obesity

• increased PAI-1, fibrinogen, factor 7

• decreased HDL, increased VLDL, TG, LDL

• hyperinsulinaemia, glucose intolerance, DM

What is the defining characteristics of metabolic syndrome?

abdominal obesity (waist circumference) = >102cm M >89cm F

increased FBG = 6.1-7.0 mmol/L

hypertension = >130/80 mmHg

increased TGs = >1.7 mmol/L

decreased HDL cholesterol = <1.04 mmol/L M <1.30 mmol/L F

What are the defining symptoms of DM

• unexplained weight loss

• blurred vision

• numbess

• slow wound healing

• fatigue

• excessive thirst - polydipsia

• frequent urination - polyuria

• recurrent infections

What tests are used to diagnose DM

1. fasting blood glucose

   • measures current steady-state plasma glucose conc

2. HbA1c

   • measures retrospective glucose modification of Hb over 120 days

3. Oral glucose tolerance test

   • measures response to insulin to glucose dose

Describe fasting blood glucose testing. What risk does it correlate well with? What are the normal and abnormal values. What happens when an abnormal result is obtained?

• simple reproducible test that correlates well with risk of microvascular complications

• N = <6.1 mmol/L

• impaired = 6.2-7.0 mmol/L

• overt = ≥ 7.0 mmol/L

if impaired → do OGTT to confirm

Describe HbA1c testing.What information does the HbA1c test give? What percentages are considered normal or abnormal?

5% HbA is glycated as HbA1c

• results from covalent attachment of glucose to N-terminal valine of HbA beta-chain by nonenzymic glycation

• is dependent of interaction between concentration of glucose during the preceding 8-12 weeks

• HbA1c provides index of “average” plasma glucose over preceding 2-3 months

• >7% = poor control of diabetes

Describe the oral glucose tolerance test. How is the test performed? What values are considered normal and abnormal

• dynamic function test

• tests response to bolus dose of glucose (75g)

• measure plasma glucose after 120 mins

• N = <7.8 mmol/L

• impaired = 7.8-11.0 mmol/L

• overt = >11.1 mmol/L

Describe coupled enzymic kinetic assay for glucose concentration. What happens when neither the substrate nor product absorbs light? what can be done to measure the intended marker instead

glucose is measured by enzymatic reactions

• glucose + ATP → glucose-6-P + ADP

  • catalysed by hexokinase

  • reaction limited by glucose conc

• G-6-P + NADP+ → 6 phosphogluconate + NADPH + H+

  • NADPH can be measured to determine glucose conc - 1 NADPH = 1 glucose molecule

  • absorbs at 340 nm

Explain the develpment of diabetic ketoacidosis (DKA). What are the three defining characteristics?

insulin deficiency can cause

• increased lipolysis

  • increased FFA → increased ketones → acidosis

  • → vomiting → dehydration

• decreased glucose uptake

  • hyperglycaemia → glycosuria → osmotic diuresis

  • → dehydration

• dehydration leads to hypotension and shock

  • → catecholamines, cortisol, GH, ADH

    • these all exacerbates lipolysis and decreased glucose uptake creating a vicious cycle

What are the macrovascular and microvascular complications of DM

macrovascular

• lipid changes:

  • increased TC, LDL, TG

  • low HDL

• glycation and peroxidation of:

  • lipoproteins

  • thrombotic/thrombolytic factors

microvascular

• hyperglycaemia

  • stimulates adol reductase enzymes

  • metabolise glucose to sorbitol (polyol sugar)

  • tissue accum of sorbitol

• non enzymatic glycation of proteins

Describe diabetic retinopathy. What % of patients show retinopathy and what are some risk factors?

• leading cause of blindess in working eyes

• 95% of diabetics show retinopathy 15 years after onset

• risk factors: age, DM duration, high blood glucose level, BP, genetics

• can progress rapidly during pregnancy

Describe diabetic nephropathy. What % of patients show nephropathy and what are some risk factors? What symptoms do these patients often show?

• affects 25% of DM patients

• risk factors: age, DM duration, high blood glucose level, BP, genetics

• progressive condition leading to renal failure

• characterised by proteinuria and high BP

• can lead to:

  • loss of feet sensation

  • foot ulceration

  • erectile dysfunction

  • gastroparesis and vomiting

  • postural hypotension

What is the treatment for T1DM

• requires insulin therapy and glucose monitoring + DM self-managment education and support

• also anti-hyperlipidemic therapy (statins), and anti-hypertensive therapy + stop smoking

What is the treatment for T2DM. What medications can be used?

lifestyle modification: diet and exercise

metformin

• lowers blood glucose by decrease liver glucose production

• diminish intestinal absorption and enhance insulin sensitivity

semaglutide

• enhances glucose dependent insulin secretion

What are the effects of insulin on glycogenesis, protein synthesis and lipogenesis

glycogenesis

• stimulates glycogenesis

• inactivates glycogen phosphorylase → inhbit glycogen breakdown

• activate glycogen synthase → stimulate glycogen synth

protein synthesis

• increase AA uptake and protein synth

• decrease catabolism and release of gluconeogenic AA

Lipogenesis

• increases LPL activity and mRNA levels

• stimulate VLDL synthesis