Nitric oxide mediates flow-dependent epicardial coronary vasodilation to changes in pulse frequency but not mean flow in conscious dogs.

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Abstract
BACKGROUND Although epicardial coronary arteries dilate in response to changes in flow, the mechanisms responsible for this and the mechanical stimuli that are sensed by the endothelium are not completely defined. We performed the present study to determine the importance of nitric oxide in eliciting epicardial dilation to sustained changes in mean flow and pulse frequency in the coronary circulation of conscious dogs. METHODS AND RESULTS Dogs were chronically instrumented with a circumflex coronary occluder, piezoelectric crystals to measure epicardial diameter, and a coronary artery catheter placed distal to the crystals for intracoronary drug infusion. Studies were conducted in dogs in the conscious state. We inhibited nitric oxide production by administering the arginine analog N omega-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg IV), which attenuated the epicardial artery diameter changes to left atrial infusions of acetylcholine (10 micrograms/min) from 140 +/- 23 (+/- SEM) to 46 +/- 20 microns (P < .05). Epicardial dilation to sustained increases in mean coronary artery at a constant heart rate. Intracoronary adenosine increased mean flow to the same extent (180 +/- 21 versus 177 +/- 24 mL/min after L-NAME, P = NS), but inhibiting nitric oxide production had no effect on flow-mediated epicardial dilation, with coronary diameter increasing by 264 +/- 36 microns under control conditions and 294 +/- 67 microns after L-NAME (P = NS). In contrast, when pulse frequency was increased by pacing to a rate of 200 beats per minute, mean coronary flow increased to a similar level (78 +/- 9 versus 75 +/- 9 mL/min after L-NAME), but the epicardial diameter change to pacing was attenuated from 170 +/- 29 microns under control conditions to 54 +/- 23 microns after L-NAME (P < .01). CONCLUSIONS These results demonstrate that in vivo, nitric oxide production is primarily responsible for eliciting epicardial coronary vasodilation to endothelium-dependent agonists and changes in coronary flow pulse frequency. The failure of L-NAME to affect epicardial vasodilation during sustained increases in mean flow when pulse frequency is held constant suggests that additional mechanisms are involved in flow-mediated vasodilation of epicardial coronary arteries.

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