Differential contribution of nitric oxide to regulation of vascular tone in coronary conductance and resistance arteries: intravascular ultrasound studies.

We examined the role of nitric oxide in the maintenance of coronary vascular tone in 15 dogs. A 0.014 inch Doppler wire was introduced into the midsegment of the circumflex coronary artery and a 4.3F, 30 MHz two-dimensional ultrasound imaging catheter was introduced over the Doppler wire. Acetylcholine caused a dose-dependent vasodilation in both epicardial and resistance coronary arteries. However, N omega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthetase caused a dose-dependent vasoconstriction mainly in the epicardial coronary arteries, partially reversed by L-arginine. The vasodilator response to acetylcholine was inhibited by L-NAME only in the epicardial circulation. Thus using combined intracoronary two-dimensional and Doppler ultrasound, we have demonstrated both basal and acetylcholine-induced release of nitric oxide in epicardial coronary arteries. The failure of L-NAME to decrease basal and acetylcholine-induced increases in flow velocity suggests that endothelium-dependent relaxation in coronary resistance vessels may not be mediated by nitric oxide alone.