Insulin Hormoneis a protein molecule of 51 amino acids arranged in one alpha chain (21 amino acids) and one beta chain (30 amino acids) held together with disulphide bonds. Hormone is synthesized and stored as proinsulin in beta cells and it is converted into active insulin just prior to its release in blood capillaries.
Secretion of insulin is stimulated by high blood glucose concentration that occurs immediately after digestion of food and absorption of glucose in small intestines. Thus insulin is released in variable amounts several times a day whenever food is consumed
Action of Insulin: All body cells are target cells of insulin hormone except nerve cells & RBCs. Action of insulin is to lower the blood glucose concentration and promotes its utilization by the target cells (chiefly liver, muscles & adipose tissue cells) in following manner:
Insulin increases uptake of glucose in cells of liver and muscles where it is largely stored in the form of glycogen. This process is glycogenesis. Surplus glucose is changed to proteins and lipids in liver cells and stored.
In adipose tissue, glucose is used for synthesis of fatty acids (Lipogenesis process) and stored as body fats.
Other body cells promote use of glucose for energy production through oxidation.
Due to lipogenesis and protein synthesis action of insulin, the hormone exerts a growth promoting effect on the body.
Diseases of insulin deficiency: A major disease called diabetes mellitus (associated with high blood glucose, presence of glucose in urine and more urine formation) is caused due to insulin deficiency in humans. It is of two kinds:
Insulin dependant diabetes or juvenile diabetes: The disease occurs in 12-15 year old children due to destruction of beta cells of islets of Langerhans. This destruction is caused by certain viral infections or auto immunity and it is mostly hereditary in nature. Due to loss of beta cells insulin is not produced and patient develops high blood glucose concentration ranging from 300-1200 mg/100 ml of blood (normal blood glucose being 80-100 mg/100 ml of blood). This is also termed as type I diabetes.
Insulin independent diabetes mellitus or adult diabetes: The disease usually occurs in patients above 40 years of age. Pancreas produces normal insulin or even more, but target cells do not respond to insulin, thus elevating blood glucose level (ranging between 150-300 mg/100 ml of blood). This is also termed as type II diabetes and can be controlled by regular exercise, diet control and some non-insulin drugs.
Glucagon Hormone: Glucagon is protein hormone containing 29 amino acids in single chain secreted by pancreatic islets. It acts opposite to insulin on carbohydrate metabolism. Its target organ is limited to liver where it promotes conversion of glycogen into glucose (process is called glycogenolysis). This hormone is required when blood glucose is lowered during fasting, between meals etc. Blood glucose concentration tends to rise by glucagon action.