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Altered reactivity of coronary arteries located distal to a chronic coronary occlusion
J A Rapps1, M Sturek, A W Jones
1Department of Physiology and Dalton Cardiovascular Research Center, University of Missouri, Columbia 65211, USA.
Insights
Collateral-dependent coronary arteries show increased alpha-adrenergic vasoconstriction and reduced relaxation to adenosine. This altered reactivity is due to enhanced smooth muscle alpha1-adrenergic response and decreased nitric oxide synthesis.
Area of Science:
- Cardiovascular Physiology
- Vascular Biology
- Pharmacology
Background:
- Chronic coronary artery occlusion leads to collateral-dependent vasculature.
- Collateral-dependent arteries display altered responses to vasoactive agents.
Purpose of the Study:
- To investigate the effects of chronic coronary artery occlusion on vasomotor responsiveness in collateral-dependent arteries.
- To compare the reactivity of large and small collateral-dependent arteries with normal arteries.
Main Methods:
- Used a canine model with Ameroid occlusion of the left circumflex (LCX) coronary artery.
- Isolated and compared in vitro responses of LCX and left anterior descending (LAD) coronary arteries.
- Measured contractile responses and myoplasmic free Ca2+ to various agonists and antagonists.
Main Results:
- Enhanced alpha-adrenergic contractile responses to norepinephrine and phenylephrine in large LCX arteries.
- Alpha1-adrenergic receptor antagonism abolished norepinephrine-induced contractions.
- Impaired relaxation to adenosine in both large and small LCX arteries compared to LAD arteries.
- Norepinephrine-induced increases in myoplasmic free Ca2+ and contractile tension were enhanced in LCX arteries.
Conclusions:
- Collateral-dependent coronary arteries exhibit enhanced alpha-adrenergic vasoconstriction and impaired adenosine-induced vasorelaxation.
- Enhanced alpha-adrenergic responsiveness is mediated by increased smooth muscle alpha1-adrenergic reactivity and reduced nitric oxide synthesis.
Abstract:
The coronary vasculature located distal to a chronic occlusion (collateral-dependent) has been shown to exhibit altered reactivity to vasoactive agonists. Thus we evaluated effects of chronic coronary artery occlusion on vasomotor responsiveness of collateral-dependent arteries isolated from a canine model of Ameroid occlusion of the left circumflex (LCX) coronary artery. We compared in vitro responses of large (approximately 1.3- to 1.4-mm-ID) and small (approximately 0.6-mm-ID) LCX arteries located distal to an occlusion with responses of similar-sized segments of the unoccluded left anterior descending (LAD) coronary artery. Alpha-adrenergic receptor-mediated contractile responses to norepinephrine (10(-9)-10(-4) M) and phenylephine (10(-9)-10(-4) M) in the presence of propranolol were markedly enhanced in large LCX arteries compared with LAD arteries (P < 0.001). Prazosin (1 microM), an alpha1-adrenergic receptor antagonist, abolished contractile responses of LCX and LAD arteries to norepinephrine. Inhibition of nitric oxide synthesis with N(omega)-nitro-L-arginine methyl ester (100 microM) enhanced norepinephrine-induced contractions of LAD arteries to a greater extent than contractions of LCX arteries. We simultaneously measured myoplasmic free Ca2+ (fura 2 fluorescence ratio) and contractile responses in LCX and LAD arteries denuded of endothelium; norepinephrine-induced increases in myoplasmic free Ca2+ and contractile tension were significantly enhanced in LCX arteries compared with LAD arteries. In addition, large and small LCX arteries exhibited impaired relaxation in response to adenosine (10(-8)-10(-3) M) compared with LAD arteries (P < 0.05). In contrast, relaxation in response to the beta-adrenergic agonist isoproterenol (10(-9)-10(-4) M) and sodium nitroprusside (10(-10)-10(-4) M) was not significantly different in LCX and LAD arteries. Thus collateral-dependent coronary arteries exhibit enhanced alpha-adrenergic vasoconstriction and impaired vasorelaxation in response to adenosine. The enhanced alpha-adrenergic contractile responsiveness involves at least two mechanisms: 1) enhanced alpha1-adrenergic reactivity of smooth muscle and 2) decreased alpha-adrenergic-induced synthesis of nitric oxide by the endothelium.