Similar to insulin, glucagon is first synthesized as part of a larger precursor protein. Glucagon secretion is regulated by many of the factors that also regulate insulin secretion. In most cases, however, these factors have the opposite effect on glucagon secretion.
Synthesis of Proglucagon. Glucagon is a simple 29-amino acid peptide. The initial gene product for glucagon, pre-proglucagon, is a much larger peptide. Like other peptide hormones, the "pre" piece is removed in the ER, and the prohormone is converted into a mature hormone as it is packaged and processed in secretory granules (see Chapter 31).
Secretion of Glucagon. The principal factors that influence glucagon secretion are listed in Table 35.2. With a few exceptions, this table is nearly a mirror image of Table 35.1, the factors that regulate insulin secretion. The primary regulator of glucagon secretion is blood glucose,- specifically, a decrease in blood glucose below about 100 mg/dL promotes glucagon secretion. As with insulin, amino acids, es pecially arginine, are potent stimulators of glucagon secretion. Somatostatin inhibits glucagon secretion, as it does insulin secretion.
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