Novel technique to bioassay endocardium-derived nitric oxide from the beating heart

Nitric oxide is a potent vasodilator and antiplatelet substance released by the vascular endothelium. In the current study, isolated rabbit hearts were perfused retrograde in the aortic root with a balanced salt solution using a Langendorff technique. To perfuse the right cardiac chambers, an inflow cannula was placed in the superior vena cava and an outflow cannula in the right ventricular apex via the pulmonary artery. To detect endocardial vasodilator production, right heart perfusate was used to bathe a "bioassay" segment of canine coronary artery denuded of endothelium. Perfusate from unstimulated hearts did not alter smooth muscle tone in the bioassay tissue. Calcium inophore, a potent stimulus for endothelial nitric oxide production, produced relaxation of the bioassay smooth muscle when added to the cardiac perfusate but not when applied directly to the bioassay segment. Cardiac effluent vasodilator activity was abolished by removal of the endocardium or addition of nitric oxide synthesis inhibitors, but not by prostanoid inhibitors. These experiments describe a practical method to bioassay endocardial nitric oxide production in the beating heart.