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We studied the changes caused by intravenous injection of nitroglycerin (0.3 to 0.6 mg) and dipyridamole (5 to 10 mg) on the resistance of large and small coronary arteries of dogs. The direction of change in resistance of large conductive vessels (R c ) was estimated by dividing the pressure drop from aorta to a superficial branch by the flow in that vessel. The total resistance (R t ) in the same vessel was estimated by relating aortic pressure to the same flow. The effects of both drugs on aortic pressure were comparable 3 to 7 minutes after injection. Nitroglycerin during this period caused a reduction in R c , whereas dipyridamole caused a significant reduction in R t for 12 minutes but no consistent changes in R c . In the same preparation, small intracoronary doses of the drugs, which hardly affected the aortic pressure, caused comparable changes in resistance. This was also evident in a preparation in which the flow in the coronary vessel was held constant. These two drugs therefore differ in their site of action on the coronary vascular bed, a difference which may explain their contrasting therapeutic effects in chronic coronary disease in man.
Fam et al. (Wed,) studied this question.