Nitroglycerin fails to dilate coronary collateral vessels during exercise

Area of Science:

• Cardiovascular Physiology
• Exercise Physiology
• Pharmacology

Background:

• Coronary collateral vessels can compensate for reduced blood flow in myocardial regions.
• Nitroglycerin is a vasodilator commonly used to treat angina.
• The effect of nitroglycerin on collateral blood flow during exercise is not fully understood.

Purpose of the Study:

• To investigate if nitroglycerin enhances blood flow to collateral-dependent myocardium during exercise.
• To assess the impact of nitroglycerin on coronary vascular resistance in the presence of collateral vessels.

Main Methods:

• Induction of coronary collateral vessel growth in dogs via intermittent occlusions of the left circumflex coronary artery (LCX).
• Permanent LCX occlusion followed by measurements at rest and during exercise.
• Administration of intravenous nitroglycerin and assessment of myocardial blood flow using radioactive microspheres.

Main Results:

• Exercise increased blood flow to both normal and collateral-dependent myocardial regions.
• Nitroglycerin administration did not further increase blood flow to these regions during exercise.
• Neither transcollateral resistance (TCR) nor small-vessel resistance (SVR) significantly changed after nitroglycerin.

Conclusions:

• Nitroglycerin failed to augment blood flow in collateral-dependent myocardium during exercise.
• The nitric oxide system appears to be maximally recruited in collateral vessels during exercise.
• These findings suggest limitations in nitroglycerin's efficacy for improving perfusion in exercise-induced myocardial ischemia with established collaterals.