Propranolol Hydrochloride Extended-Release Capsules, USP, should not be considered a simple mg-for-mg substitute for conventional propranolol and the blood levels achieved do not match (are lower than) those of two to four times daily dosing with the same dose (see DOSAGE AND ADMINISTRATION). The significance of the membrane action in the treatment of arrhythmias is uncertain. At dosages greater than required for beta blockade, propranolol also exerts a quinidine-like or anesthetic-like membrane action, which affects the cardiac action potential. When access to beta-receptor sites is blocked by propranolol, the chronotropic, inotropic, and vasodilator responses to beta-adrenergic stimulation are decreased proportionately. It specifically competes with beta-adrenergic receptor-stimulating agents for available receptor sites. Propranolol is a nonselective, beta-adrenergic receptor-blocking agent possessing no other autonomic nervous system activity.
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