Importance of basal nitric oxide synthesis in regulation of myocardial blood flow

Study objective — The aim was to investigate whether basal coronary vascular tone and myocardial perfusion depend upon endothelial nitric oxide (NO) synthesis. Design — Myocardial blood flow and vascular resistance of the left and right ventricles were studied before and after intravenous infusions of either NG-nitro-L-arginine (L-NA), a specific inhibitor of NO synthase, or L-arginine, the precursor of NO synthesis. Radiolabelled microspheres were used to study myocardial blood flow in small tissue sections. Experimental material — 14 anaesthetised male cats, weight 2.1-3.5 kg, were used. Measurements and main results — Measurements were made before and 15 and 40 min after L-NA treatment (30 mg·kg−1 bolus followed by 1 mg·kg−1·min−1 infusion; n=8), and before and 15 min after L-arginine treatment (30 mg·kg−1 bolus followed by 10 mg·kg−1·min−1 infusion; n=6). L-NA significantly reduced coronary blood flow to the left and right ventricle, by 30(SEM 9) and 48(6)% respectively, after 15 min, but only to the right ventricle, by 45(8)%, after 40 min. Mean arterial pressure and myocardial vascular resistance were raised during the L-NA infusion. In contrast, L-arginine did not elicit any change in the variables studied. Conclusions — The conductance of the coronary vascular bed and the resting myocardial blood flow is regulated by L-arginine derived nitric oxide, and exogenous L-arginine availability is not a limiting factor in this NO generation.