Hormonal Control of Blood Sugar Levels

A number of mechanisms function to maintain blood glucose at a remarkably constant level (70-100 mg/dl) under fasting condition. Many hormones are involved in regulating blood sugar levels.

Insulin:

It is produced by the β-cells of the islets of Langerhans in the pancreas.It has been called the "feasting hormone" because its liberation is enhanced by a high glucose level in the blood.The mechanism by which insulin lowers blood glucose involves an increase in the rate of glucose utilisation for oxidation by increasing facilitated diffusion of glucose into muscle and adipose cells. Insulin increases glycogenesis and glucose is stored as glycogen in the liver and muscle cells.Insulin enhances lipogenesis by the uptake of glucose by adipose and liver cells for conversion into fat.

Glucagon:

It is produced by the alpha cells of the islets of Langerhans. It has an effect exactly opposite to that of insulin. It causes a rise in the amount of sugar in blood by increasing glycogenolysis by activating the enzyme phosphorylase.It enhances gluconeogenesis from amino acids and lactate.Insulin and glucagon may thus be considered antagonists and their opposing effects at least in part maintain carbohydrate metabolism in a steady state.

Epinephrine(adrenaline):

It is a hormone secreted by the chromaffin cells of the adrenal medulla.It favours the breakdown of liver and muscle glycogen to yield blood glucose (glycogenolysis) and decreases the release of insulin from the pancreas, thus raising the blood sugar.Secretion of epinephrine is increased during anger or fear,and the subsequent glucose formation provides extra energy for crisis  response.

Glucocorticoids:

These steroid hormones elaborated by the adrenal cortex also influence blood glucose levels by stimulating gluconeogenesis. These hormones reduce glucose utilisation in the extra hepatic tissue and also increase the rate at which protein is converted onto glucose, thus counteracting the action of insulin.

Thyroxine:

Severe lowering of blood glucose concentration increases thyroxine secretion.Thyroid hormones enhance the action of epinephrine. Hepatic glycogenolysis and gluconeogenesis are increased, leading to a rise in blood glucose concentration. Thyroxine also increases the rate of hexose absorption from the intestine.

Growth hormone :

This is elaborated by the anterior pituitary gland.This raises blood glucose level by increasing amino acid uptake and protein synthesis by all cells by diminishing cellular uptake and increasing the mobilisation of fat for energy.

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Nutrition Science by B. Srilakshmi

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