Image via WikipediaBoth insulin and glucagon are hormones secreted from the Islets of Langerhans located in the pancreas. Insulin is released from the beta cells while glucagon is produced and released from the alpha cells. Although both these hormones are released from the pancreas, these two hormones act in opposite directions as far as biological action is concerned. Both hormones perform very important functions in the body. They are antagonists in every metabolic action. The question arises as to why we should have two hormones in conflict with each other. Actually, this antagonism plays a very important role in the survival of the organism.
Let us consider glucose metabolism. Glucose is an important fuel in the body used for the purpose of conservation of energy. Glucose is used by almost all the cells of the body and some are totally dependent on the presence of glucose for survival. The brain is the classical example of a tissue showing absolute dependence on glucose. Since glucose must be available for the tissues at all times, the only way it can be supplied to the tissues is through the circulation. Thus, the blood must always contain glucose.
Normally, the blood glucose levels never fall below 100mgs per deciliter. The tissues are drawing glucose from the circulation all the time. Even when we are sleeping, there are tissues which continue to be active. For example, the cardiac muscle is active and the respiratory muscles are also active. Part of the brain also must be active to ensure that we breathe and also to see to that the heart continues to beat. Therefore, the blood glucose levels should drop. But that does not happen. It is for this purpose that the liver stores glycogen. Thus, during night, when we are asleep and when we are not eating anything, the liver glycogen is broken down and is converted into glucose which is released into the circulation. This conversion of glycogen into glucose is promoted by glucagon. Insulin has no role to play here. Circulating insulin levels tend to be low at night.
In contrast, during day time, whenever we eat, the blood glucose levels rise and under these circumstances glucose in converted into glycogen in the liver and stored. Insulin promotes this transformation of glucose to glycogen in the liver. Thus, the presence of glucagon and insulin acting in opposite directions is easily explained. Both of them are not present at increased levels in the circulation at the same time. When the circumstances warrant an increase in glucagon, insulin falls and the reverse is true when insulin rises. Thus the presence of these two hormones which antagonize each other in metabolic processes is actually due to a finely tuned and well orchestrated homeostatic mechanism.
Dr.Vangeepuram Satakopan
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