Nitric Oxide Relaxes Canine Coronary Arterioles through Voltage‐Gated K+ Channels Sensitive to 4‐Aminopyridine

We demonstrated previously that sodium azide (NaN 3 ) increases blood flow in the canine coronary circulation; however, the underlying cellular and molecular mechanisms remain to be determined. Additional experiments revealed that NaN 3 relaxes coronary arteries contracted with thromboxane A 2 mimetic (1 μM U46619) but not those contracted with 60‐100 mM K + ; therefore, we tested the hypothesis that NaN 3 ‐induced coronary vasodilation depends upon the opening of K + channels. Specifically, we determined whether NaN 3 activates voltage‐dependent K + (K V ) channels sensitive to 4‐aminopyridine (4‐AP). To test the hypothesis, we measured coronary blood flow in vivo , assessed coronary artery and arteriole reactivity in vitro , and performed patch clamp experiments on freshly isolated smooth muscle cells. Bolus intracoronary injections of 0.1% NaN 3 increased blood flow (ED 50 = 1.2 ± 0.6 μl). NaN 3 relaxed precontracted artery rings (EC 50 = 0.3 ± 0.1 μM) and dilated pressurized arterioles (EC 50 = 0.2 ± 0.2 μM). The soluble guanylate cyclase inhibitor ODQ (1H‐[1,2,4]oxadiazole[4,3‐a]quinoxalin‐1‐one; 1 μM), the protein kinase G (PKG) antagonist Rp‐8‐p‐CPT‐cGMPS (8‐[4‐chlorophenylthio]‐guanosine 3′,5′‐cyclic monophosphorothioate; 3 μM), and 4‐AP (0.3 mM) blunted NaN 3 ‐induced vasodilation. NaN 3 hyperpolarized the membrane potential of smooth muscle cells (13 ± 1 mV) and increased 4‐AP‐sensitive K V current (17 ± 3% at 0 mV), while intracoronary infusion of 4‐AP ([plasma] 0.3 mM) attenuated NaN 3 ‐induced blood flow responses. Our data indicate that PKG signaling mediates vascular effects of NaN 3 and that K V channels, or proteins which regulate them, are likely PKG phosphorylation targets.