Amiodarone. Biochemical aspects and haemodynamic effects.

Area of Science:

• Cardiology
• Pharmacology

Background:

• Amiodarone is an antiarrhythmic drug with known beta-adrenergic blocking effects.
• The precise mechanisms of amiodarone's beta-antagonism remain incompletely understood.

Purpose of the Study:

• To investigate the mechanisms of amiodarone's non-competitive beta-antagonism.
• To compare the hemodynamic responses to exercise of amiodarone versus propranolol in a canine model of myocardial infarction.

Main Methods:

• Radioligand binding assays and ex vivo studies in rats to assess amiodarone's effect on beta-adrenoceptors.
• In vitro studies on rat heart membranes to evaluate amiodarone's impact on adenylate cyclase activity.
• Hemodynamic monitoring in dogs with healed myocardial infarction during graded treadmill exercise after amiodarone or propranolol administration.

Main Results:

• Amiodarone did not compete for beta-adrenoceptor binding but decreased receptor density at higher concentrations.
• Amiodarone non-competitively inhibited adenylate cyclase activation by beta-agonists but had less effect on other stimulators.
• In dogs, amiodarone reduced heart rate and left ventricular end-diastolic pressure during exercise, while propranolol significantly reduced myocardial contractility.

Conclusions:

• Amiodarone's beta-antagonism is primarily mediated by a reduction in beta-adrenoceptor number, not a change in receptor affinity.
• Amiodarone's hemodynamic effects during exercise differ from propranolol, particularly concerning myocardial contractility.