Ouabain, Na(+)-free and K(+)-free solutions and relaxations to nitric oxide and nitrovasodilators

1. Experiments were designed to determine the effect of inhibition of Na+, K+ ATPase by ouabain, Na(+)-free and K(+)-free solutions on the relaxations induced by nitric oxide, nitroglycerin and sodium nitroprusside in coronary smooth muscle. 2. Rings without endothelium of canine left anterior descending coronary artery were suspended for isometric tension recording in modified Krebs-Ringer bicarbonate solution, in the presence of indomethacin, propranolol, and phenoxybenzamine. The rings were incubated with ouabain, Na(+)-free or K(+)-free solutions prior to contractions with prostaglandin F2 alpha. 3. Nitric oxide induced transient relaxations which were significantly depressed by ouabain but were not affected by Na(+)- or K(+)-free solutions. Thus, the inhibitory effect of ouabain on relaxations to nitric oxide is not due to inhibition of the Na+, K+ pump. 4. The relaxation induced by sodium nitroprusside was impaired both in the presence of ouabain and after incubation in Na(+)- or K(+)-free solution, suggesting that it may involve in part activation of Na+, K+ pumping. 5. Only high concentration of ouabain and Na(+)-free solution decreased the relaxation evoked by nitroglycerin, indicating that the mechanism of action of nitroglycerin on vascular smooth muscle does not involve the Na(+)-K+ pump.