ATTENUATION OF EXERCISE-INDUCED MYOCARDIAL-ISCHEMIA IN DOGS WITH RECRUITMENT OF CORONARY VASODILATOR RESERVE BY NIFEDIPINE

There is now evidence that under resting conditions coronary vasodilator reserve exists even in the presence of myocardial ischemia. Therefore, we tested the hypothesis that a vasodilator reserve may exist during exercise so that during exercise-induced ischemia a reduction in coronary constrictor tone can be produced that attenuates the decreases in regional myocardial blood flow and function distal to a severe coronary stenosis without changing the determinants of myocardial oxygen demand. Nine dogs were instrumented with an ameroid constrictor on the left circumflex coronary artery and were studied 2 to 3 weeks later. During a control treadmill run, heart rate increased from 119 .+-. 20 to 225 .+-. 20 beats/min and peak left ventricular pressure increased from 144 .+-. 17 to 163 .+-. 28 mm Hg. Poststenotic subendocardial blood flow (measured by a microsphere technique) fell from 1.19 .+-. 0.36 to 0.51 .+-. 0.30 ml/min .cntdot. g and systolic wall thickening (by sonomicrometry) decreased from 24.3 .+-. 5.8% to 6.0 .+-. 6.1%. During an identical run after nifedipine (10 .mu.g/kg iv), systemic hemodynamics were not significantly altered. However, subendocardial blood flow was increased to 0.85 .+-. 0.51 ml/min .cntdot. g (p < .05) and systolic wall thickening to 11.4 .+-. 7.8% (p < .01). We conclude that in this study the amelioration of exercise-induced myocardial ischemia was due to the recruitment by nifedipine of coronary vasodilator reserve.