Session 7

Question 1

What is the structure of insulin?

Answer 1

Insulin is an unusual polypeptide hormone as it contains two polypeptide chains (A and B chains) linked covalently by two disulphide bonds. In addition to the two inter-chain disulphide bonds that link A and B chains together, there is a third intra-chain disulphide bond within the A chain. The presence of two polypeptide chains and three disulphide bridges affect the way the molecule is synthesised as the
disulphide bridges have to connect the correct cysteine residues to ensure the biological activity of the molecule.

Question 2

How is insulin synthesised?

Answer 2

Insulin is synthesised as pre-proinsulin (a single-chain polypeptide of 109 amino acids) on ribosomes associated with the rough endoplasmic reticulum. The pre-part (23 amino acids) is a signal peptide that ensures the newly synthesised protein enters the cisternal space of the endoplasmic reticulum. The signal peptide is removed once the molecule enters the endoplasmic reticulum. The remaining proinsulin (86 amino acid, single-chain polypeptide) folds to ensure that there is correct alignment of the cysteine residues and the correct disulphide bonds form. Proinsulin is transported from the endoplasmic reticulum to the trans-Golgi apparatus and packaged into storage vesicles. Proteolysis in the storage vesicles removes a connecting peptide (C-peptide) of 31 amino acids together with four basic amino acids (3 arginine and 1 lysine) from near the middle of the chain. This breaks the single chain into two chains that are held together by disulphide bridges i.e. the mature insulin molecule. The storage vesicles contain the products of proteolysis i.e. insulin and C-peptide in equimolar amounts and a small amount of unchanged proinsulin. The entire contents of the storage granules are released during secretion.

Question 3

How can plasma C-peptide levels be used to look at endogenous insulin levels.

Answer 3

As C-peptide is released with insulin in equimolar amounts, its level in plasma is a useful marker of endogenous insulin release. Measurement of plasma C-peptide levels in patients receiving insulin can be used to monitor any endogenous insulin secretion.

Question 4

How is insulin stored and then transported?

Answer 4

Insulin is stored in the β-cell storage granules as a crystalline zinc-insulin complex. When released it dissolves in the plasma and circulates as a free hormone (i.e. not bound to a transport protein).

Question 5

What are the target tissues of insulin and how does it interact with them?

Answer 5

The major target tissues for insulin action are liver, skeletal muscle and adipose tissue. In addition, insulin is required for the normal growth and development of most tissues of the body. Insulin interacts with receptors on the surface of its target cells. The insulin receptor is a member of the tyrosine kinase receptor family.

Question 6

What are the major actions of insulin on carbohydrate, lipid and amino acid metabolism?

Answer 6

• Increase glucose transport into adipose tissue & skeletal muscle.
• Increase glycogenesis and decrease glycogenolysis in liver & muscle.
• Decrease gluconeogenesis in liver.
• Increase glycolysis in liver & adipose tissue.
• Decrease lipolysis in adipose tissue.
• Increase lipogenesis and esterification of fatty acids in liver & adipose tissue.
• Decrease ketogenesis in liver.
• Increase lipoprotein lipase activity in the capillary bed of tissues such as adipose tissue.
• Increase amino acid uptake and protein synthesis in liver, muscle & adipose tissue.
• Decrease proteolysis in liver, skeletal muscle & heart muscle.

Question 7

What is the timescale of the effects of insulin?

Answer 7

Insulin has a wide range of effects on its target tissues and affects carbohydrate, lipid and amino acid metabolism. These effects are largely anabolic and are related to insulin’s major function of clearing absorbed nutrients from the blood following a meal. Most of the effects occur rapidly (sec/hr) in response to an increase in the concentration of insulin in the circulation and are produced by changes in the activities of pre-existing functional proteins such as enzymes and transport molecules in target tissues. In addition, insulin has long-term (hr/days) effects on cell growth and division that relate to its ability to
stimulate the synthesis of new protein molecules and to stimulate DNA replication.

Question 8

How is insulin secretion controlled?

Answer 8

Factors include metabolites (glucose, amino acids, fatty acids), GI tract hormones (gastrin, secretin, cholecystokinin) and neurotransmitters (adrenaline, noradrenaline, acetyl choline). The metabolic signals, GI tract hormones and acetyl choline all stimulate secretion, while adrenaline and noradrenaline inhibit secretion

Question 9

What is the structure of glucagon and how is it synthesised?

Answer 9

Glucagon is a single chain, 29 amino acid peptide hormone. The molecule lacks disulphide bonds and has a flexible 3D structure that takes up its active conformation on binding to its receptor on the surface of target cells. It is synthesised by pancreatic alpha cells as a larger precursor molecule (pre-proglucagon) that undergoes posttranslational processing to produce the biologically active molecule

Question 10

What are the major actions of glucagon?

Answer 10

The major actions of glucagon are:  
• Increase glycogenolysis and decrease glycogenesis in liver. 
• Increase gluconeogenesis in liver.  
• Increase ketogenesis in liver. 
• Increase lipolysis in adipose tissue.

Question 11

What is the mechanism of action for glucagon?

Answer 11

Glucagon binds to a specific glucagon receptor in the cell membrane, which is a type of receptor termed a G protein-coupled receptor (GPCR). Binding to the receptor activates the enzyme adenylate cyclase, which increases cyclic AMP (cAMP) intracellularly. High levels of cAMP activate protein kinase A (PKA), which phosphorylates and thereby activates a number of important enzymes in target cells.

Question 12

What influences glucagon secretion?

Answer 12

The major factor that increases the rate of glucagon secretion is a decrease in the blood glucose concentration. Secretion is inhibited by insulin and an increase in blood glucose concentration.

Question 13

What is diabetes mellitus?

Answer 13

Diabetes mellitus is a group of metabolic disorders characterised by chronic hyperglycaemia (elevated blood glucose concentration), due to insulin deficiency, insulin resistance, or both. There are two major types of the disease, clearly distinguished by their epidemiology and probable causation, but not always so easily separated clinically.

Question 14

What are the differences between type 1 and type 2 diabetes mellitus?

Answer 14

Type 1 diabetes: 
• Commonest type in the young. 
• Characterised by the progressive loss of all or most of the pancreatic β-cells.  
• Is rapidly fatal if not treated. 
• Must be treated with insulin. 

Type 2 diabetes:
• Affects a large number of usually older individuals.
• Characterised by the slow progressive loss of β-cells along with disorders of insulin secretion and tissue resistance to insulin.
• May be present for a long time before diagnosis.
• May not initially need treatment with insulin but sufferers usually progress to a state where they eventually do.

Question 15

Explain the staging of diabetes mellitus?

Answer 15

Both type 1 and type 2 diabetes can be staged in terms of progression of the disease. In type 1 diabetes, people can be found with the relevant human leucocyte antigen (HLA) markers and auto-antibodies but without glucose or insulin abnormalities. Subsequently they may
develop impaired glucose tolerance, then diabetes (sometimes initially amenable to dietary control), before finally becoming totally insulin dependent. In type 2 diabetes, people can be found with insulin resistance, then as insulin production fails they develop impaired glucose tolerance. Finally they will develop diabetes that will be initially managed with diet, then tablets, and then insulin; if the process continues long enough for them to lose all insulin production.

Question 16

How common is diabetes?

Answer 16

Approximately 3.9 million people in the UK have diabetes (Diabetes Facts and Stats, Diabetes UK, 2015), the majority (~90%) with type 2 disease. A further 0.59 million are believed to be undiagnosed and numbers are predicted to increase to 5 million by 2025.

Question 17

How prevalent is type 1 diabetes?

Answer 17

About 15 people per 100,000 of the population each year are diagnosed with Type 1 diabetes. More present in the teenage years but the age related rate is otherwise similar up to old age. There are substantially different rates between different countries

Question 18

How do genetics influence type 1 diabetes?

Answer 18

It is likely that a genetic predisposition to the disease interacts with an environmental trigger to produce immune activation. This leads to the production of killer lymphocytes and macrophages and antibodies that attack and progressively destroy β-cells (an auto-immune process). The genetic predisposition is associated with the genetic markers HLA DR3 and HLA DR4. There is a strong seasonal variation, suggesting a link with a viral infection acting as a trigger to a rapid deterioration.

Question 19

What is the classic clinical picture of someone with type 1 diabetes?

Answer 19

The classic picture of type 1diabetes is a lean, young person with a recent history of viral infection who presents a triad of symptoms:
• Polyuria - excess urine production. In the nephron of a healthy individual all of the glucose filtered from the blood is reabsorbed at the end of the proximal most section of the nephron, the proximal tubule. The reabsorption in this part of the kidney is isosmotic. In diabetes mellitus where large quantities of glucose in the blood are filtered by the kidney not all of this glucose is reabsorbed. The extra glucose remains in the nephron tubule. This places an extra osmotic load on the nephron, and means that less water is reabsorbed to maintain the isosmotic character of this section of the nephron. This extra water then remains with the glucose in the nephron tubule and is excreted as copious urine.
• Thirst (polydipsia drinking a lot) - due to excess water loss and the osmotic effects of glucose on the thirst centres.
• Weight loss as fat and protein are metabolised by tissues because insulin is absent.

Question 20

How is type 1 diabetes diagnosed?

Answer 20

Diabetes is easily diagnosed by measurement of plasma glucose levels. Blood glucose is elevated because of the lack of insulin. The lack of insulin causes decreased uptake of glucose into adipose tissue and skeletal muscle, decreased storage of glucose as glycogen in muscle and liver and increased gluconeogenesis in liver. The high blood glucose will lead to the appearance of glucose in the urine (glycosuria also called glucosuria).

Question 21

If not dealt with quickly what can type 1 diabetic patients have?

Answer 21

. If not dealt with urgently, these individuals will progress to a life-threatening crisis (diabetic ketoacidosis).

Question 22

Explain how ketosis develops in diabetic patients

Answer 22

The high rates of β-oxidation of fats in the liver coupled to the low insulin/anti-insulin ratio leads to the production of huge amounts of ketone bodies, such as acetoacetate, acetone and β-hydroxybutyrate. Acetone, which is volatile may be breathed out, and can be smelt on the patient’s breath. As this ketosis develops, the H+ associated with
the ketones produce a metabolic acidosis - keto-acidosis. The features of keto-acidosis are prostration, hyperventilation, nausea, vomiting, dehydration and abdominal pain. Keto-acidosis is a very dangerous condition. It is most important to test for ketones - conveniently in the urine - when assessing diabetes control.

Question 23

How prevalent is type 2 diabetes?

Answer 23

Unlike type 1, type 2 diabetes is relatively common in all populations enjoying an affluent life-style. The estimated prevalence in the UK is about 5%. Typically, the patients are older and often, though not invariably, overweight. The disease has often been present for some time, maybe years, before diagnosis.

Question 24

Is type 2 diabetes genetic?

Answer 24

. Whilst there is good evidence for a genetic predisposition to type 2 diabetes, there is recent evolving evidence of the involvement of the immune system. At diagnosis patients retain about 50% of their β-cells, however as the number of these cells falls, ultimately to none at all, patients develop disorders of insulin secretion or insulin resistance, so blood glucose is raised.

Question 25

How do type 2 diabetes patients present?

Answer 25

Patients may present with the classical triad of symptoms, but are more likely to present with a variety of symptoms such as lack of energy, persistent infections, particularly thrush infections of the genitalia, or infections of the feet, slow healing minor skin damage, or visual problems.

Question 26

How is type 2 diabetes diagnosed?

Answer 26

Diabetes is diagnosed, in the presence of symptoms (i.e. polyuria, polydipsia and unexplained weight loss) plus: • A random venous plasma glucose concentration ≥ 11.1 mmol/L or • A fasting plasma glucose concentration ≥ 7.0 mmol/L (whole blood ≥ 6.1 mmol/L) or • Plasma glucose concentration ≥ 11.1 mmol/L 2 hours after 75g anhydrous glucose in an oral glucose tolerance test (OGTT). With no symptoms diagnosis should not be based on a single glucose determination but requires confirmatory venous plasma glucose determination. At least one additional glucose test result on another day with a value in the diabetic range is essential, either fasting, from a random sample or from the two hour post glucose load. If the fasting or random values are not diagnostic the 2-hr value should be used.

Question 27

How is type 1 diabetes managed?

Answer 27

Type 1 diabetes cannot be cured, and must be managed for the rest of the patient's life. Insulin is used to treat Type 1 diabetics and some cases of Type 2 diabetes. It must be injected. It comes in a variety of forms with different time courses of action, and may be injected by a number of devices. Patients must be educated to treat themselves at appropriate times with appropriate doses and formulations so as to mimic as closely as possible the behaviour of pancreatic islets in controlling blood glucose. On occasion, particularly if the patient has an infection, or has suffered some trauma, insulin dosage needs to be increased, or there is a risk of diabetic ketoacidosis. The social and psychological implications are huge, and the degree of control achieved by patients can be very variable. Appropriate dietary management and regular exercise are vital components of the treatment regime. The management of blood glucose requires frequent blood glucose measurement. A small amount of blood from a finger prick is sufficient to measure blood glucose using a BM stick and reader (BM stands for Boehringer Mannheim, the German pharmaceutical company (now called Roche) who first made these kits). These devices are only accurate if used carefully. There is always a risk that blood glucose will fall too low ("hypoglycaemia"). Both patients and their associates need to be aware of the signs and symptoms of hypoglycaemia, which can occasionally be fatal unless treated with glucose, either by mouth or by infusion.

Question 28

How is type 2 diabetes managed?

Answer 28

Type 2 diabetes can sometimes be managed by diet or by "oral hypoglycaemic" drugs such as sulphonylureas that increase insulin release from the remaining β-cells, and reduce insulin resistance and particularly metformin that reduces gluconeogenesis. As with type 1 diabetes appropriate dietary management and regular exercise are vital components of the treatment regime

Question 29

Can type 2 diabetes be prevented?

Answer 29

Studies in the USA, Finland and China have shown that control of diet and exercise can significantly reduce the chances of an individual getting type 2 diabetes. Participants in the USA studies who were randomly assigned to lifestyle intervention (diet and exercise) reduced their risk of getting type 2 diabetes by 58% compared to the control group. In participants who were aged 60 years and over, the reduction was 71%.

Question 30

What are the metabolic consequences of persistent hyperglycaemia?

Answer 30

Persistent hyperglycaemia is associated, in some tissues, with the abnormal metabolism of glucose to products that may be harmful to cells. This is because the uptake of glucose into cells of tissues such as peripheral nerves, the eye and the kidney does not require insulin and is determined by the extracellular glucose concentration. Thus, during hyperglycaemia the intracellular concentration of glucose in these tissues increases and glucose is metabolised via the enzyme aldose reductase which catalyses the reaction: Glucose + NADPH + H+ → Sorbitol + NADP+ This reaction depletes cellular NADPH and leads to increased disulphide bond formation in cellular proteins, altering their structure and function. In addition, the accumulation of sorbitol causes osmotic damage to cells. Persistent hyperglycaemia is also associated with the increased glycation (non-enzymatic glycosylation) of plasma proteins (e.g. lipoproteins) that leads to disturbances in their function. In this process glucose reacts with free amino groups in proteins to form stable covalent linkages. The extent of glycation depends on the glucose concentration and the half-life of the protein. Glycation changes the net charge and 3-D structure of the protein and therefore affects protein function

Question 31

What is HbA1c?

Answer 31

``` Glycated haemoglobin (HbA1c). Glucose in the blood will react with the terminal valine of the haemoglobin molecule to produce glycated haemoglobin (HbA1c). The percentage of haemoglobin that is glycated is a good indicator of how effective blood glucose control has been. As red blood cells normally spend about 3 months in the circulation the % HbA1c is related to the average blood glucose concentration over the preceding 2-3 months. In normal healthy individuals 4-6% of haemoglobin is glycated and in poorly controlled diabetics this value can increase above 10%. ```

Question 32

What are the macrovascular complications of diabetes?

Answer 32

Macrovascular complications include: • Increased risk of stroke. • Increased risk of myocardial infarction. • Poor circulation to the periphery - particularly the feet.

Question 33

What are the microvascular complications of diabetes?

Answer 33

Diabetic eye disease Diabetic kidney disease (neuropathy) Diabetic neuropathy Diabetic feet

Question 34

What is diabetic eye disease?

Answer 34

Visual problems can arise from changes in the lens due to osmotic effects of glucose (glaucoma), and possibly cataracts, but the more important problem is diabetic retinopathy - damage to blood vessels in the retina which can lead to blindness. Damaged blood vessels may leak and form protein exudates on the retina or they can rupture and cause bleeding in to the eye. In addition, new vessels may form (proliferative retinopathy). These vessels are very weak and can easily bleed.

Question 35

What is diabetic kidney disease?

Answer 35

The kidney is affected by damage to the glomeruli, poor blood supply because of changes in kidney blood vessels, or damage from infections of the urinary tract which are more common in diabetics. An early sign of nephropathy is an increase in the amount of protein in the urine (microalbuminuria).

Question 36

What is diabetic neuropathy?

Answer 36

Diabetic neuropathy: Diabetes damages peripheral nerves in a number of ways producing a variety of effects, including changes of or a loss of sensation, and changes due to alteration in the function of the autonomic nervous system.

Question 37

What is diabetic feet?

Answer 37

Diabetic feet: Poor blood supply, damage to nerves, and increased risk of infection all conspire to make the feet of diabetics particularly vulnerable. In the past loss of feet through gangrene was not uncommon. Care is needed to keep feet in good condition.

Question 38

What is metabolic syndrome?

Answer 38

The concept of Metabolic syndrome arose from the observation of a common pattern of symptoms in obese people. The metabolic syndrome is defined as a group of symptoms including insulin resistance, dyslipidaemia, glucose intolerance and hypertension associated with central adiposity. The co-occurrence in the same individual of a number of cardiovascular risk factors such as dyslipidaemia and hypertension, usually in association with overweight or obesity and a sedentary life style is known as the ‘metabolic syndrome’ (BMJ 2003;327:61-2). These abnormalities may be related to insulin resistance as they are commonly found in type 2 diabetics. The major factors that predispose to insulin resistance are obesity and a sedentary life-style. Insulin resistance in turn, is associated with the development of a dyslipidaemic profile (↑ VLDL, ↑LDL & ↓HDL) that is highly atherogenic. It is also associated with an increased risk of hypertension. The WHO criteria for metabolic syndrome are central obesity with a waist: hip ratio above 0.9 for men and 0.85 for women and/or a body mass index (BMI) above 30 kg/m2, blood pressure above 140/90 mmHg, triglycerides above 1.7 mmol/L, HDL cholesterol <0.9 mmol/L in men and <1 mmol/L in women, glucose fasting or 2 h after a glucose load above 7.8 mmol/L and glucose uptake during hyperinsulinaemic euglycaemic clamp in lowest quartile for population

Question 39

Where is metabolic syndrome prevalent?

Answer 39

increasingly prevalent in developed countries Metabolic syndrome is present in approximately 80% of people with type 2 diabetes Metabolic syndrome increases the risk of type 2 diabetes 5 fold and heart disease by approximately 3 fold. The syndrome is increasing in prevalence worldwide as a consequence of increasing obesity prevalence

Question 40

How well is obesity and metabolic syndrome linked?

Answer 40

Metabolic syndrome is strongly associated with obesity, particularly if fat is stored around the abdomen. The influence of obesity on the incidence of the metabolic syndrome has also been observed in children and obesity is now an important cause of type 2 diabetes occurring in children.

Question 41

How does ethnicity link with obesity?

Answer 41

Some ethnic groups have a higher predisposition to central obesity than others, for example, the amount of central obesity is greater among South Asians than Europeans and is greater among Europeans than African-Caribbeans. Central fat accumulation is strongly associated with insulin resistance and so diabetes..

Question 42

What is the issue with using BMI?

Answer 42

In diagnosing metabolic syndrome, BMI does not allow for large muscle bulk and it is known that central fat around the abdomen not just total body fat collection is associated with insulin resistance, fatty liver and other symptoms in metabolic syndrome. Therefore, BMI is not an ideal measurement of central fat.