Vasodilators such as nitroglycerin have been reported to increase collateral blood flow to ischaemic myocardium. One proposed mechanism of action is dilatation of collateral vessels. The purpose of this study was to determine whether dilatation of vessels within the ischaemic area also contributes to the increase in flow. The left anterior descending coronary artery (LAD) was ligated in chloralose-anaesthetised dogs. Peripheral LAD pressure (PLAD) was measured distal to the occlusion. Myocardial blood flows were measured with 15 micrometers radioactive microspheres. Ischaemic bed vascular resistance was calculated as PLAD/LAD flow. After 1 h of ischaemia, nitroglycerin (2.3 micrograms . kg-1 . min-1) decreased ischaemic bed resistance by 10% when infused into the LAD, by 16% when infused into the circumflex coronary artery, and by 15% when infused via the right atrium. However, adenosine infusion (2 micrograms . kg-1 . min-1) into the LAD did not decrease ischaemic bed resistance. We conclude that nitroglycerin increases collateral flow in part by dilating large coronary arteries within an acutely ischaemic area.
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