CORONARY EFFECTS OF NICORANDIL IN COMPARISON WITH NITROGLYCERIN IN CHRONIC CONSCIOUS DOGS

The effects of nicorandil (0.2 mg/kg, IV) and nitroglycerin (15-mu-g/kg, IV) on large and small coronary arteries were compared in conscious dogs instrumented with ultrasonic crystals and electromagnetic flow meters in the circumflex coronary artery. Nicorandil dilated the large coronary arteries to the same extent, but with a longer duration of action than nitroglycerin. The small coronary arteries dilated for a very short period of time with nitroglycerin, but dilated markedly with nicorandil. The dilatory action of nicorandil on large coronary arteries persisted after the action on the small coronary artery fell to the control value, indicating that the dilatory action on the large coronary arteries is due to the direct relaxing effect on smooth muscle and is not the result of the flow-dependent effect. The measurement of the plasma concentration of nicorandil after incremental infusions of the agent showed that the dilation of the small coronary artery took place at only a very high level (above 200 ng/ml). On the other hand, the large coronary arteries responded to nicorandil at a much lower concentration (about 100 ng/ml, the clinically effective plasma concentration of nicorandil) than the small coronary resistance arteries. In conclusion, whereas nicorandil possesses a dilatory action on both large and small coronary arteries, in a clinical setting, with a daily dosage of 15-30 mg, part of the beneficial effects of nicorandil may be the result of a dilation of the large coronary arteries and may be due to the fact that a coronary steal phenomenon does not occur after nicorandil administration.