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Perturbation of metabolism in insulin-dependent diabetes
1. Describe the effect of insulin deficiency on the regulation of the pathways of
glycolysis and gluconeogenesis and the uptake of glucose into muscle and adipose
tissue.
In humans, blood glucose is tightly regulated by homeostatic mechanisms and
maintained within a narrow range. A balance is preserved between the entry of
glucose into the circulation from the liver, supplemented by intestinal absorption after
meals, and glucose uptake by peripheral tissues, particularly skeletal muscle.
Insulin is an anabolic hormone with profound affects on the metabolism of
carbohydrate, fats and protein. It is secreted from pancreatic beta-cells in response to
a rise in blood glucose. Insulin’s main function is to lower blood glucose and it does
this through a number of mechanisms which are as follows;
1. Increased uptake of glucose by cells in muscle and liver
2. Increased glucose catabolism ie glycolysis
3. Increased storage of glycogen in muscle and liver ie glycogenesis (therefore
inhibiting glycogenolysis)
4. Increased Fatty Acid and fat (triglyceride) synthesis
5. Decreased Triglyceride hydrolysis ie Lipolysis
6. Increased amino acid uptake and protein synthesis in liver
7. Decreased gluconeogenesis and protein catabolism
Glycolysis is the process of breaking down glucose to produce pyruvate which can be
used in the production of ATP. Insulin stimulates glycolysis to lower blood glucose
levels after a meal and to signal energy availability to all organs. Thus a deficiency
will mean reduced glycolysis.
Insulin is also important in the transport of glucose into cells. Glucose enters cells via
glucose transporters which are described as either insulin-sensitive or insulininsensitive. Cardiac and Skeletal Muscle, as well as adipose tissue contain GLUT4
glucose transporters which are insulin-sensitive. This means without insulin, glucose
uptake is not possible, and the processes of Glycolysis, Lipogenesis and Glycogenesis
will not occur.
Insulin also decreases gluconeogenesis mainly by inhibiting the actions of other
regulatory hormones, such as glucagon and epinephrine. These hormone break down
compounds to provide sources that can be used to produce glucose ie
gluconeogenesis. Lipolysis is stimulated via catecholamines to produce fatty acids
that can be oxidised for energy in many tissues.
2. Explain how the lack of insulin in DKA leads to overproduction of glucagon
with stimulation of gluconeogenesis and lipolysis
The balance between insulin and glucagon levels in the blood is the principal control
of metabolism. Both are synthesised as ‘pre-pro’ and ‘prohormones’ which are
cleaved to produce their active forms. When blood glucose levels are greater than 5
mmol/l, β-cells increase their output of insulin, thereby lowering BGL. A fall under
about 4 mmol/l leads to a decrease in insulin secretion, and activation of pancreatic βcells which secrete glucagon into the blood..
Apart from having endocrine effects, these hormones also have paracrine effects, that
is, effects on cells in the neighbouring vicinity. The cells of the Pancreatic Islets are
tightly packed, resulting in high concentrations of each hormone within the organelle.
An increase in insulin levels inhibits Glucagon release from α-cells. This is the basic
element of insulin’s control of hepatic gluconeogenesis and lipolysis in adipose tissue.
When blood sugar levels are diminished, insulin secretion decreases and suppression
of α-cells finishes and glucagon is released. Glucagon however does not affect the
insulin secreting cells.
Secretion of hormones is co-ordinated. Insulin actions include stimulation of glucose
and amino acid uptake from the blood to the various tissues. This process is coupled
with stimulation of anabolic (synthetic) processes – glycogenolysis, lipogenesis and
protein synthesis. Glucagon has opposing effects causing a release of glucose from
glycogen, breakdown of fats (lipolysis) and stimulation of gluconeogenesis. Hence
without insulin, there is unrestricted production of glucagon and therefore
uncontrolled gluconeogenesis and lipolysis. This further elevates the BGL. The
breakdown of fats (Triglycerides) also yields fatty acids which are metabolised in the
liver to form ketones. As there is an excess of circulating fatty acids, ketone
production exceeds their utilisation by tissues resulting in a build up. Ketones are
essentially acids, meaning a build up leads to what is known as Diabetic Ketoacidosis
(DKA).
Insulin is also required for uptake of glucose into cells. Both muscle and adipose
tissue cells contain GLUT4 glucose transport proteins that are insulin-sensitive
meaning that without insulin they will not work. Hence, glucose is unable to enter
cells and be metabolised to provide energy, thus the body senses a hypoglycaemic
state and encourages gluconeogenesis and lipolysis.
3. Indicate the mechanisms by which beta keto-acid production is increased in
response to oversupply of fatty acid to the liver, including the steps of the
pathways of fatty acid metabolism that are up regulated
One of the primary actions of insulin is to control storage and release of fatty acids in
and out of lipid depots. It does this through two mechanisms;
- regulation of several lipase enzymes
- activation of glucose transport into the fat cell via recruitment of glucosetransport protein 4 (GLUT4).
Splitting of triglycerides produces free fatty acids and glycerol. One might expect
that the body would use this glycerol to aid in storage of fatty acids when required.
However, this does not occur as adipocytes lack glycerokinase which is necessary for
synthesis of α-glycerol phosphate from glycerol. The glycerol released by lipolysis
goes to the liver for further metabolism (ie use as a substrate for gluconeogenesis).
The α-glycerol phosphate backbone is produced from glucose delivered into the fat
cells. Storage of triglycerides after a meal is, therefore, dependent upon insulinstimulated glucose uptake and glycolysis (glycolysis produces the glycerol phosphate
molecule). Fat cells take up both fatty acids and glucose simultaneously. The fatty
acids come from the action of lipoprotein lipase at the capillary wall. Glucose uptake
is stimulated by insulin and occurs through the insulin-sensitive glucose transport
protein GLUT4.
Splitting of triglycerides back to glycerol and fatty acids follows the actions of several
lipases.
- Triglycerides are split to diglycerides by Adipose Triglyceride Lipase (ATGL)
-
Diglycerides are thereafter converted to monoglycerides by HormoneSensitive Lipase (HSL)
The monoglycerides are hydrolyzed by a cytosolic Monoglyceride Lipase
These three enzymes and their control elements are extensively phosphorylated by
Protein Kinase A (PKA). Glucagon, norepinephrine, and epinephrine bind to the G
protein-coupled receptor, which activates adenylate cyclase to produce cyclic AMP.
cAMP consequently activates PKA, which phosphorylates (and activates) hormonesensitive lipase. Insulin activates protein phosphatase 2A, which dephosphorylates
HSL, thereby inhibiting its activity. Insulin also activates the enzyme
phosphodiesterase, which break down cAMP and stops the re-phosphorylation effects
of protein kinase A. Thus lipase activity increases or decreases in response to cAMP
levels. In the case of Type I Diabetes Mellitus,
- the lack of insulin leads to an increase in glucagon production.
- Increased glucagon leads to increased levels of cAMP.
- Increased cAMP leasd to increased lipase activity, leading to more lipolysis
and increased levels of Fatty Acids
Once Fatty Acids (FA) have been cleaved from glycerol, they diffuse into the blood
and travel to the liver where they are further metabolised. In the liver FA are
enzymatically broken down in β-oxidation to form acetyl-CoA. Normally, acetyl-CoA
is further oxidized and its energy transferred as electrons to NADH, FADH2, and GTP
in the citric acid cycle (TCA cycle). However, if the amounts of acetyl-CoA generated
in fatty-acid β-oxidation challenge the processing capacity of the TCA cycle (as will
result from an over supply of FA from excessive lipolysis) or if activity in the TCA
cycle is low due to low amounts of intermediates such as oxaloacetate (levels low as
intermediates are used in gluconeogenic pathways), acetyl-CoA is then used instead in
biosynthesis of Ketones ie Ketogenesis. Ketones include Acetoacetate, βhydroxybutyrate and Acetone. Ketones are created at moderate levels in everyone's
bodies, such as during sleep and other times when no carbohydrates are available.
However, when ketogenesis is happening at higher than normal levels, the body is
said to be in a state of ketosis. Both Acetoacetate and β -hydroxybutyrate are acidic,
and, if levels of these ketone bodies are too high, the pH of the blood drops, resulting
in ketoacidosis.
4. Outline how hyperglycemia causes polyuria
Each kidney contains about a million functional units called nephrons. The first step
in the production of urine is a process called filtration (green arrow). In filtration,
there is bulk flow of water and small molecules from the plasma into Bowman’s
capsule (the first part of the nephron). Because of the non-specific nature of filtration,
useful small molecules such as glucose, amino acids, and certain ions end up in the
forming urine, which flows into the kidney tubules. To prevent the loss of these useful
substances from the body, the cells lining the kidney tubules transfer these substances
out of the forming urine and back into the extracellular fluid. This process is known as
reabsorption (purple arrows).
Under normal circumstances, 100% of the glucose that is filtered is reabsorbed.
Glucose reabsorption involves transport proteins that require specific binding. In a
diabetic that has hyperglycaemia, the amount of glucose that is filtered can exceed the
capacity of the kidney tubules to reabsorb glucose, because the transport proteins
become saturated. Glucose is a solute that draws water into the urine by osmosis.
Thus, hyperglycaemia causes a diabetic to produce a high volume of glucosecontaining urine. This then leads to a state of polyuria and diuresis, ultimately leading
to dehydration and loss of fluid volume. Important electrolytes are also lost in the
urine resulting in osmotic disturbances throughout the body.