Segmental alpha-receptor-mediated vasoconstriction in the canine coronary circulation

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Regulation

Background:

• Sympathetic alpha-receptors mediate coronary vasoconstriction, potentially competing with metabolic vasodilation.
• Previous in vitro studies suggested dominant alpha-receptor activity in large epicardial coronary vessels.

Purpose of the Study:

• To test the hypothesis that alpha-receptor-mediated vasoconstriction occurs predominantly in large epicardial coronary vessels.
• To investigate the distribution of adrenergic alpha-receptor activity within the coronary vasculature in vivo.

Main Methods:

• Anesthetized, open-chest dogs were used with beta-receptor blockade (propranolol).
• The circumflex coronary artery was pump-perfused at constant pressure to isolate large vessel resistance.
• Alpha-receptor activation was induced via intracoronary norepinephrine and stellate ganglion stimulation.

Main Results:

• Alpha-receptor stimulation increased total coronary vascular resistance.
• The relative increase in large coronary vessel resistance was approximately 60% of the increase in the total coronary vascular bed.
• Nitroglycerin confirmed large vessel vasomotor responsiveness.

Conclusions:

• Contrary to the hypothesis, alpha-receptor-mediated vasoconstriction is not proportionally greater in large coronary vessels compared to the entire coronary vascular bed.
• These findings indicate a more distributed role for alpha-adrenergic activity in regulating coronary blood flow.