Glucagon is a polypeptide hormone consisting of 29 amino acids. It is produced by alpha cells in the islets of Langerhans in the pancreas.
Many organs respond to the influence of glucagon. In the liver primarily glycogenolysis but also gluconeogenesis are stimulated, resulting in an increase in blood sugar. While fasting, glucose from the liver is initially derived almost exclusively from hepatic glycogen. However, since only about 70 g of glycogen are stored in the human liver, glycogenolysis ordinarily can sustain the plasma glucose only for 8 to 10 h. Exercise and the stress of severe illness may shorten this protective period. If glycogen stores have been depleted by fasting or previous stimulated glycogenolysis then the ability of glucagon to raise blood sugar is severely impaired.
Smooth muscle is relaxed in the gastrointestinal tract and bronchial tree. Glucagon increases the force of contraction of the heart, with little or no effect on the heart rate.
The initial response after intravenous injection occurs in about 1 minute and after intramuscular injection 4 to 10 minutes. Peak response in the treatment of hypoglycaemia after intravenous injection is 5 to 20 minutes, after subcutaneous or intramuscular injection 5 to 20 minutes. Duration of action of a single intramuscular dose is 12 to 32 minutes and after an intravenous dose 9 to 25 minutes. About one third of the dose is absorbed after intramuscular injection. The plasma elimination half-life is 8 to 18 minutes. Half-life is 3 to 4 times longer following intramuscular administration because of continuing absorption.
Glucagon is a short acting measure to increase blood sugar. It may have little effect in prolonged hyperinsulinism, whether caused by an insulin overdose or an oral hypoglycaemic, if the insulin levels have been high for longer than several hours because of depletion of the glycogen stores on which the hormone relies for its main effect. If the patient were fasted prior to initiation of insulin release, this time would be even shorter. Glucagon has no direct effect on insulin release and does not significantly alter the levels of insulin or C-peptide.
The dose for the treatment of hypoglycaemia in an adult is 1 mg SC, IM or IV.