How does insulin control key pathways?

Insulin is a powerful hormone released to reduce blood glucose concentrations. It acts to promote storage of glucose, proteins and fats and it has actions on the liver, muscle and adipose tissue.

Essential knowledge

• Insulin acts on carbohydrates by achieving a reduction in blood glucose by stimulating glucose uptake, storage of glucose as glycogen and metabolism of glucose to ATP (see How does insulin act?)
• Insulin acts on fats by increasing the synthesis of lipoproteins in the liver and reducing the rate of β-oxidation of lipids, both of which result in an increase in circulating lipids
• Insulin promotes building up of protein and inhibits the breakdown of protein

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In greater depth

Actions of insulin on carbohydrates

Insulin achieves a reduction in blood glucose by stimulating glucose uptake, storage of glucose as glycogen and metabolism of glucose to ATP.In addition to removing glucose from the blood stream, insulin also has a number of actions to reduce the amount of glucose re-entering the blood stream from the liver. It reduces the rate of release of glucose from the liver by:

1. Inhibiting glycogenolysis
2. Stimulating glycogen synthesis
3. Stimulating glucose uptake
4. Stimulating glycolysis
5. Indirectly inhibiting gluconeogenesis via inhibition of fatty acid mobilisation from adipose tissue

_{Figure: how insulin acts on carbohydrates. Full colour represents actions that are relevant to the effect of insulin on carbohydrates, whilst faded icons represent actions that do not relate to carbohydrates.}

Actions of insulin on fats

Insulin also has significant effects on lipid metabolism. In the liver, insulin increases the synthesis of lipoproteins. It also reduces the rate of β-oxidation of lipids, as the rate limiting step is inhibited by insulin.Both these actions result in an increase in circulating lipids. Insulin also reduces ketogenesis in the liver and lipolysis in the adipose tissue, as hormone sensitive lipase is inhibited by insulin. _{Figure: how insulin acts on fats. Full colour represents actions that are relevant to the effect of insulin on fats, whilst faded icons represent actions that do not relate to fats.}

Actions of insulin on protein

Insulin promotes building up protein and inhibits the breakdown of protein.Insulin has the following effects on protein metabolism:

• Stimulates transport of free amino acids across plasma membranes in liver and muscle
• Stimulates protein biosynthesis in muscle
• Reduces release of amino acid (proteolysis) from muscle

_{Figure: how insulin acts on protein. Full colour represents actions that are relevant to the effect of insulin on proteins, whilst faded icons represent actions that do not relate to proteins.}