Insulin is a protein hormone or a polypeptide hormone with 51 amino acid residues. Human insulin has a molecular weight of 5,734 daltons. It consists of two chains A and B, which are linked together by disulphide bridges formed between two cystine residues.
It decreases glucose level in the blood in three ways:
a.It increases conversion of glucose into glycogen and deposition of it in liver and muscles.
b.It increases the rate of oxidation of glucose in the tissues.
c. It increases the rate of conversion of glucose into fat and facilitates its storage in adipose tissue.
d. It also regulates the rate at which amino acids are catabolised into water and CO2.
e. Moderately, it also regulates the gluconeogenesis in the liver.
Thus, insulin reduces the glucose level in the blood (Hypoglycemia). If the insulin is not secreted sufficiently, the liver and the muscles are unable to convert the glucose into glycogen. As a result more glucose enters into the bloodstream raising the blood sugar level. This condition is called Hyperglycemia. The excess of glucose is eliminated along with the urine resulting in a disease calleddiabetes mellitus. A diabetic patient excretes large amount of urine (polyurea) and consumes excessive fluid (polydipsia). He always feels hungry and eats excessively (polyphagia). When insulin levels are low, fat catabolism is increased and fats are converted into glucose. This further increases blood glucose levels and results in the accumulation of ketone bodies (Ketosis).
The normal fasting blood glucose level is 70 to 110 mg/dl (dl = deciliter). This range is maintained under varying conditions of food intake, fasting or body exercise. After a carbohydrate meal the blood sugar may reach a peak level of about 140mg/dl. If such a high level is maintained for a prolonged time, the condition may be termed as hyperglycemia. Hyperglycemia over a long period may cause degenerative changes in several organs and systems leading to malfunctions and mortality. Elevated blood sugar level of 400 mg/dl or more, in a few days causes dehydration leading to coma and death.
It means a low plasma glucose concentration. This concentration can drop to very low values during fasting. It is termed as fasting hypoglycemia. It may result due to excess of insulin or other physiological factors. There is no fixed level of blood sugar at which hypoglycemia occurs.
Fasting hypoglycemia may show symptoms such as hunger, increased heart rate, tremulousness, weakness, nervousness and sweating. These are caused due to activities of the sympathetic nervous system. Other symptoms such as headache, confusion, uncoordination and slurred speech are due to availability of too little glucans to the brain. Serious brain defects such as convulsions (epilepsy) and coma can occur if the plasma glucose concentration goes low.
The name 'diabetes ' in Greek means ' syphon' or ' running through'. This term describes the enormity of urinary volume excreted by people suf-fering from this disease. A persistant hyperglycemia leads to diabetes mellitus. This disease can be due to a deficiency of insulin or to a hypo responsiveness to insulin.
In type I (insulin dependent) diabetes, the hormone is completely or almost completely absent from the islets of Langerhans and the plasma. In type II (insulin-independent) diabetes, the hormone is often present in plasma at near normal or even above normal levels.
The type I is less common. It is due to the total or near total destruction of the pancreatic b cells.
The type II is due to insulin resistance. The insulin target cells do not respond normally to the circulating insulin. This may result due to obesity, over-eating and lack of exercise. The insulin hyporesponsiveness can be corrected if the person reduces his or her caloric intake. Thus dietary control without any other therapy is frequently sufficient to eliminate the elevated blood glucose level of type II diabetics. An exercise programme is also useful, since it will help to increase the number of insulin receptors.