Glucagon, which is secreted by the alpha cells of the islets, generally opposes the actions of insulin in the liver. Glucagon stimulates the breakdown of glycogen to glucose and stimulates gluconeogenesis, both of which serve to increase the release of glucose into the blood and raise plasma glucose levels. The secretion of glucagon is stimulated by low blood sugar levels as well as increased levels of plasma amino acids. Recall that increased levels of amino acids also stimulate insulin secretion. This may seem as somewhat of a paradox. However, the simultaneous release of glucagon and insulin helps to prevent hypoglycemia (low blood glucose levels), especially with low carbohydrate diets. Thus, the ratio of glucagon to insulin plays a very important role in glucose metabolism.

This diagram shows the homeostatic regulation of blood glucose levels. Blood glucose concentration is tightly maintained between 70 milligrams per deciliter and 110 milligrams per deciliter. If blood glucose concentration rises above this range (hyperglycemia), insulin is released from the pancreas. Insulin triggers body cells to take up glucose from the blood and utilize it in cellular respiration. Insulin also inhibits glycogenolysis, in that glucose is removed from the blood and stored as glycogen in the liver. Insulin also inhibits gluconeogenesis, in that amino acids and free glycerol are not converted to glucose in the ER. If blood glucose concentration drops below this range, glucagon is released, which stimulates body cells to release glucose into the blood. All of these actions cause blood glucose concentration to decrease. When blood glucose concentration is low (hypoglycemia), alpha cells of the pancreas release glucagon. Glucagon inhibits body cells from taking up glucose from the blood and utilizing it in cellular respiration. Glucagon also stimulates glycogenolysis, in that glycogen in the liver is broken down into glucose and released into the blood. Glucagon also stimulates glucogenogenesis, in that amino acids and free glycerol are converted to glucose in the ER and released into the blood. All of these actions cause blood glucose concentrations to increase.

Regulation of Blood Glucose Levels.
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