Effects of intracoronary and intravenous nitroglycerin on coronary collateral function

Previous studies have demonstrated that nitroglycerin is capable of enhancing coronary collateral function. In the present investigation, the effects of intravenous and intracoronary nitroglycerin were compared to determine whether nitroglycerin augments collateral function by a direct action on the coronary circulation or only by indirect mechanisms. Dogs were studied 2 to 4 weeks after implantation of an ameroid constrictor around the left anterior descending coronary artery. Retrograde flow and peripheral coronary pressure were measured from side arms of a cannula inserted in the left anterior descending coronary artery distal to the ameroid constrictor. Systemic arterial pressure was held constant at approximately 100 mm Hg. Intracoronary nitroglycerin, 0.3 to 100 mug/min, produced dose-dependent increases in retrograde flow but no changes in peripheral coronary pressure. Intravenous nitroglycerin, 10 to 300 mug/min, produced dose-dependent increases in both retrograde flow and pheripheral coronary pressure. Decreasing heart rate from 180 to 60 beats/min increased both retrograde flow and peripheral coronary pressure, but the effect of nitroglycerin on these parameters was not influenced by heart rate. The results indicate that nitroglycerin enhances coronary collateral function by a direct coronary effect and that this direct effect is further augmented by indirect mechanisms. Decreasing heart rate also augments collateral flow, but the effect of nitroglycerin on coronary collateral flow is independent of heart rate.