NO Hyperpolarizes Pulmonary Artery Smooth Muscle Cells and Decreases the Intracellular Ca$^{2+}$ Concentration by Activating Voltage-Gated K$^{+}$ Channels

NO causes pulmonary vasodilation in patients with pulmonary hypertension. In pulmonary arterial smooth muscle cells, the activity of voltage-gated K$^{+}$ (K$_{\text{V}}$) channels controls resting membrane potential. In turn, membrane potential is an important regulator of the intracellular free calcium concentration ([Ca$^{2+}$]$_{\text{i}}$) and pulmonary vascular tone. We used patch clamp methods to determine whether the NO-induced pulmonary vasodilation is mediated by activation of K$_{\text{V}}$ channels. Quantitative fluorescence microscopy was employed to test the effect of NO on the depolarization-induced rise in [Ca$^{2+}$]$_{\text{i}}$. Blockade of K$_{\text{V}}$ channels by 4-aminopyridine (5 mM) depolarized pulmonary artery myocytes to threshold for initiation of Ca$^{2+}$ action potentials, and thereby increased [Ca$^{2+}$]$_{\text{i}}$. NO ($\approx $3 $\mu $M) and the NO-generating compound sodium nitroprusside (5-10 $\mu $M) opened K$_{\text{V}}$ channels in rat pulmonary artery smooth muscle cells. The enhanced K$^{+}$ currents then hyperpolarized the cells, and blocked Ca$^{2+}$-dependent action potentials, thereby preventing the evoked increases in [Ca$^{2+}$]$_{\text{i}}$. Nitroprusside also increased the probability of K$_{\text{V}}$ channel opening in excised, outside-out membrane patches. This raises the possibility that NO may act either directly on the channel protein or on a closely associated molecule rather than via soluble guanylate cyclase. In isolated pulmonary arteries, 4-aminopyridine significantly inhibited NO-induced relaxation. We conclude that NO promotes the opening of K$_{\text{V}}$ channels in pulmonary arterial smooth muscle cells. The resulting membrane hyperpolarization, which lowers [Ca$^{2+}$]$_{\text{i}}$, is apparently one of the mechanisms by which NO induces pulmonary vasodilation.