Glucagon Secretion

A. Increased secretion
The α cell is responsive to a variety of stimuli that signal actual or potential hypoglycemia (Fig. 1). Specifically, glucagon secretion is increased by low blood glucose, amino acids, and catecholamines.

Figure 1: Regulation of glucagon release from pancreatic α cells. [Note: Amino acids increase release of insulin and glucagon, whereas glucose increases release of insulin and decreases release of glucagon.]
1. Low blood glucose: A decrease in plasma glucose concentration is the primary stimulus for glucagon release. During an overnight or prolonged fast, elevated glucagon levels prevent hypoglycemia .
2. Amino acids: Amino acids (for example, arginine) derived from a meal containing protein stimulate the release of glucagon. The glucagon effectively prevents the hypoglycemia that would otherwise occur as a result of the increased insulin secretion that also occurs after a protein meal.
3. Catecholamines: Elevated levels of circulating epinephrine (from the adrenal medulla), norepinephrine (from sympathetic innervation of the pancreas), or both stimulate the release of glucagon. Thus, during periods of physiologic stress, the elevated catecholamine levels can override the effect on the α cell of circulating substrates. In these situations, regardless of the concentration of blood glucose, glucagon levels are elevated in anticipation of increased glucose use. In contrast, insulin levels are depressed.
B. Decreased secretion
Glucagon secretion is significantly decreased by elevated blood glucose and by insulin. Both substances are increased following ingestion of glucose or a carbohydrate-rich meal . The regulation of glucagon secretion is summarized in Figure 1.