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Enhanced alpha-adrenergic responsiveness in ischemic myocardium: role of alpha-adrenergic blockade
Insights
Alpha-adrenergic blockade effectively reduces dangerous heart rhythms like premature ventricular complexes and ventricular fibrillation during heart attacks. This protection is linked to increased alpha-adrenergic receptors in the heart muscle.
Area of Science:
- Cardiology
- Pharmacology
- Physiology
Background:
- Premature ventricular complexes and ventricular fibrillation are life-threatening arrhythmias often occurring during myocardial ischemia and reperfusion.
- The role of adrenergic receptors in mediating these cardiac dysrhythmias is complex and requires further elucidation.
Purpose of the Study:
- To investigate the specific roles of alpha- and beta-adrenergic blockade in preventing ventricular arrhythmias induced by coronary artery ligation and reperfusion.
- To determine the impact of adrenergic blockade on heart rate regulation during ischemia and reperfusion.
- To assess changes in alpha-adrenergic receptor density in ischemic and reperfused myocardium.
Main Methods:
- Utilized a feline model of coronary artery ligation and reperfusion to induce ventricular arrhythmias.
- Administered alpha-adrenergic blockers (phentolamine, prazosin) and a beta-adrenergic blocker (propranolol).
- Measured regional coronary flow, systemic hemodynamics, and idioventricular rate (IVR).
- Quantified alpha-adrenergic receptor density using radioligand binding assays (3H-prazosin) in myocardial tissue.
Main Results:
- Alpha-adrenergic blockade significantly reduced premature ventricular complexes and abolished ventricular fibrillation.
- Beta-adrenergic blockade did not provide similar protection against these arrhythmias.
- Efferent sympathetic nerve stimulation increased IVR before ischemia (blocked by propranolol) and during reperfusion (blocked by alpha-blockade).
- Alpha-adrenergic receptor density in the myocardium increased significantly during ischemia and early reperfusion.
Conclusions:
- Enhanced alpha-adrenergic receptor activity and density play a critical role in the development of malignant ventricular dysrhythmias during myocardial ischemia and reperfusion.
- Alpha-adrenergic blockade emerges as a potential therapeutic strategy for managing these life-threatening arrhythmias.
Abstract:
alpha-Adrenergic blockade with phentolamine or prazosin but not beta-adrenergic blockade reduces premature ventricular complexes and abolished ventricular fibrillation induced by coronary artery ligation or reperfusion in cats. The protective influences were independent of regional coronary flow or systemic hemodynamics. Efferent sympathetic nerve stimulation increased the idioventricular rate (IVR) prior to myocardial ischemia, a response blocked by propranolol, whereas during reperfusion the increased IVR was abolished only by alpha-blockade. Enhanced alpha-adrenergic responsiveness during reperfusion was also apparent with the alpha-agonist methoxamine. More recently we have demonstrated that alpha-adrenergic receptors, assessed by ligand binding with 3H-prazosin, increased nearly twofold in ischemic myocardium by 30 minutes (Bmax = 14 + 2 to 27 + 3 fmol/mg prot) and remain elevated during early reperfusion (12 + 1 to 18 + 1) before returning to control values by 15 minutes after reperfusion. 3H-DHA binding or Na+- -K+ adenosine triphosphatase activity was not altered at any time, indicating the specificity of the alteration. Thus enhanced alpha-adrenergic receptors and suggests the potential use of alpha-adrenergic blockade as one intervention to alleviate these malignant dysrhythmias.