Role of Insulin (and Other Hormones) in “Switching” Between Carbohydrate and Lipid Metabolism
From the preceding discussions, it should be clear that insulin promotes the utilization of carbohydrates for energy, whereas it depresses the utilization of fats. Conversely, lack of insulin causes fat utilization mainly to the exclusion of glucose utilization, except by brain tissue. Furthermore, the signal that controls this switching mechanism is principally the blood glucose concentration. When the glucose concentration is low, insulin secretion is suppressed and fat is used almost exclusively for energy everywhere except in the brain. When the glucose concentration is high, insulin secre-tion is stimulated and carbohydrate is used instead of fat, and the excess blood glucose is stored in the form of liver glycogen, liver fat, and muscle glycogen. There-fore, one of the most important functional roles of insulin in the body is to control which of these two foods from moment to moment will be used by the cells for energy.
At least four other known hormones also play important roles in this switching mechanism: growthhormone from the anterior pituitary gland, cortisol from the adrenal cortex, epinephrine from the adrenal medulla, and glucagon from the alpha cells of the islets of Langerhans in the pancreas. Glucagon is discussed in the next section. Both growth hormone and cortisol are secreted in response to hypoglycemia, and both inhibit cellular utilization of glucose while promoting fat utilization. However, the effects of both of these hormones develop slowly, usually requiring many hours for maximal expression.
Epinephrine is especially important in increasing plasma glucose concentration during periods of stress when the sympathetic nervous system is excited. However, epinephrine acts differently from the other hormones in that it increases the plasma fatty acid concentration at the same time. The reasons for these effects are as follows: (1) epinephrine has the potent effect of causing glycogenolysis in the liver, thus releasing within minutes large quantities of glucose into the blood; (2) it also has a direct lipolytic effect on the adipose cells because it activates adipose tissue hormone-sensitive lipase, thus greatly enhancing the blood concentration of fatty acids as well. Quantita-tively, the enhancement of fatty acids is far greater than the enhancement of blood glucose. Therefore, epinephrine especially enhances the utilization of fat in such stressful states as exercise, circulatory shock, and anxiety.
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