Glucagon has a short half-life and the bolus should be followed by a continuous infusion at 3-5 mg/hr. Giving the bolus more quickly increases risk for vomiting. We recommend giving glucagon as a 10 mg bolus over 10 minutes. Glucagon binding to these receptors results in increased adenylyl cyclase activity independent of β-receptor activity. Similar to β-receptors, cardiac glucagon receptors are coupled to Gs proteins. Glucagon, a polypeptide counterregulatory hormone secreted from the pancreatic α-cells, bypasses β-receptors to exert inotropic and chonotropic cardiac effects. β1-receptor antagonism results in decreased calcium entry and, subsequently, decreased inotropy and chronotropy.
These receptors normally act through a second messenger system (Gs proteins*) to activate adenyl cyclase (AC) and increase cyclic AMP (cAMP), which results in the influx of intracellular calcium through L-type calcium channels. Beta blockers competitively inhibit myocardial β1 receptors.
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