Propranolol Hydrochloride is a synthetic beta-adrenergic receptor blocking agent chemically described as 2-Propanol, 1-[(1-methylethyl)amino]-3-(1-naphthalenyloxy)-, hydrochloride,(±)-. Its molecular and structural formula:β-agonist Powder
Propranolol Hydrochloride Oral Solution is available for oral administration containing either 20 mg per 5 mL or 40 mg per 5 mL of propranolol hydrochloride USP. The oral solution contains the following inactive ingredients: citric acid anhydrous, crème de menthe flavor, disodium edetate, methyl paraben, propylene glycol, propylparaben, purified water, saccharin sodium, sorbitol solution and strawberry flavor.
Propranolol is a nonselective beta-adrenergic receptor blocking agent possessing no other autonomic nervous system activity. It specifically competes with beta-adrenergic receptor agonist agents for available receptor sites. When access to beta-receptor sites is blocked by propranolol, the chronotropic, inotropic, and vasodilator responses to beta-adrenergic stimulation are decreased proportionately. 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. The significance of the membrane action in the treatment of arrhythmias is uncertain.
Mechanism of Action
The mechanism of the antihypertensive effect of propranolol has not been established. Factors that may contribute to the antihypertensive action include: (1) decreased cardiac output, (2) inhibition of renin release by the kidneys, and (3) diminution of tonic sympathetic nerve outflow from vasomotor centers in the brain. Although total peripheral resistance may increase initially, it readjusts to or below the pretreatment level with chronic use of propranolol. Effects of propranolol on plasma volume appear to be minor and somewhat variable.
In angina pectoris, propranolol generally reduces the oxygen requirement of the heart at any given level of effort by blocking the catecholamine-induced increases in the heart rate, systolic blood pressure, and the velocity and extent of myocardial contraction. Propranolol may increase oxygen requirements by increasing left ventricular fiber length, end diastolic pressure, and systolic ejection period. The net physiologic effect of beta-adrenergic blockade is usually advantageous and is manifested during exercise by delayed onset of pain and increased work capacity.
Propranolol exerts its antiarrhythmic effects in concentrations associated with beta-adrenergic blockade, and this appears to be its principal antiarrhythmic mechanism of action. In dosages greater than required for beta-blockade, propranolol also exerts a quinidine-like or anesthetic-like membrane action, which affects the cardiac action potential. The significance of the membrane action in the treatment of arrhythmias is uncertain.
The mechanism of the antimigraine effect of propranolol has not been established. Beta-adrenergic receptors have been demonstrated in the pial vessels of the brain.
The specific mechanism of propranolol’s antitremor effects has not
been established, but beta-2 (noncardiac) receptors may be involved. A
central effect is also possible. Clinical studies have demonstrated that
propranolol is of benefit in exaggerated physiological and essential
(familial) tremor
Propranolol is highly lipophilic and almost completely absorbed after
oral administration. However, it undergoes high first-pass metabolism by
the liver and on average, only about 25% of propranolol reaches the
systemic circulation. Peak plasma concentrations occur about 1 to 4
hours after an oral dose.
Administration of protein-rich foods increase the bioavailability of propranolol by about 50% with no change in time to peak concentration, plasma binding, half-life, or the amount of unchanged drug in the urine.
Distribution
Approximately 90% of circulating propranolol is bound to plasma proteins (albumin and alpha1 acid glycoprotein). The binding is enantiomer-selective. The S(-)-enantiomer is preferentially bound to alpha1 glycoprotein and the R(+)- enantiomer preferentially bound to albumin. The volume of distribution of propranolol is approximately 4 liters/kg.
