Isoproterenol [eye-soe-proe-TER-e-nole] is a direct-acting synthetic catecholamine that predominantly stimulates both β1and β2-adrenergic receptors. Its nonselectivity is one of its drawbacks and the reason why it is rarely used therapeutically. Its action on α receptors is insignificant

1. Actions:

 a. Cardiovascular: Isoproterenol produces intense stimulation of the heart to increase its rate and force of contraction, causing increased cardiac output 

Figure : Cardiovascular effects of intravenous infusion of isoproterenol.

 It is as active as epinephrine in this action and, therefore, is useful in the treatment of atrioventricular block or cardiac arrest. Isoproterenol also dilates the arterioles of skeletal muscle (β2 effect), resulting in decreased peripheral resistance. Because of its cardiac stimulatory action, it may increase systolic blood pressure slightly, but it greatly reduces mean arterial and diastolic blood pressure .

b. Pulmonary: A profound and rapid bronchodilation is produced by the drug (β2 action).

Figure : Clinically important actions of isoproterenol and dopamine.

 Isoproterenol is as active as epinephrine and rapidly alleviates an acute attack of asthma when taken by inhalation (which is the recommended route). This action lasts about 1 hour and may be repeated by subsequent doses

c. Other effects: Other actions on β receptors, such as increased blood sugar and increased lipolysis, can be demonstrated but are not clinically significant. 

2. Therapeutic uses:
Isoproterenol is now rarely used as a broncho-dilator in asthma. It can be employed to stimulate the heart in emergency situations.

3. Pharmacokinetics: Isoproterenol can be absorbed systemically by the sublingual mucosa but is more reliably absorbed when given parenterally or as an inhaled aerosol. It is a marginal substrate for COMT and is stable to MAO action.

4. Adverse effects: The adverse effects of isoproterenol are similar to those of epinephrine.

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