Glucagon
Pharmacology. Glucagon is a polypeptide hormone that stimulates the formation of adenyl cyclase, which in turn increases the intracellular concentration of cyclic adenosine monophosphate (cAMP). This results in enhanced glycogenolysis and an elevated serum glucose concentration, vascular smooth-muscle relaxation, and positive inotropic, chronotropic, and dromotropic effects. These effects occur independently of beta-adrenergic stimulation (glucagon has a separate receptor on the myocardium) and seem to be most effective at increasing the heart rate. Glucagon may also increase arachidonic acid levels in cardiac tissue via an active metabolite, mini-glucagon. Arachidonic acid improves cardiac contractility owing to its effects on calcium. Glucagon is destroyed in the GI tract and must be given parenterally. After intravenous administration, effects are seen within 1–2 minutes and persist for 10–20 minutes. The serum half-life is about 3–10 minutes. Note: Glucagon usually is not considered first-line therapy for hypoglycemia because of its slow onset of action and reliance on glycogen stores. Instead, use glucose (See Glucose) if it is available.
Indications
Hypotension, bradycardia, or conduction impairment caused by beta-adrenergic blocker intoxication (See Beta-Adrenergic Blockers). Also consider in patients with hypotension associated with anaphylactic or anaphylactoid reactions who may be on beta-adrenergic–blocking agents.
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