Propranolol competes with sympathomimetic neurotransmitters such as catecholamines for binding at beta(1)-adrenergic receptors in the heart, inhibiting sympathetic stimulation. This results in a reduction in resting heart rate, cardiac output, systolic and diastolic blood pressure, and reflex orthostatic hypotension.
Propranolol, the prototype of the beta-adrenergic receptor antagonists, is a competitive, nonselective beta-blocker similar to nadolol without intrinsic sympathomimetic activity. Propanolol is a racemic compound; the l-isomer is responsible for adrenergic blocking activity.
Symptoms of overdose include bradycardia, cardiac failure, hypotension, and brochospasm. LD50=565 mg/kg (orally in mice).