Nitric oxide (NO) stimulates Ca²⁺-activated K⁺ (K_Ca) channels and causes relaxation via cGMP-dependent pathways in human and rat pulmonary arteries

NO has been shown to cause vasorelaxation in pulmonary arteries and stimulate K_Ca channels in rat pulmonary arterial smooth muscle cells (PASMCs). The effects of NO are thought to be mediated by production of cGMP by soluble guanylyl cyclase; however, it has recently been reported that NO can also activate K_Ca channels in the absence of cGMP (Bolotina et al., Nature 368:850-853, 1994). In this study, we determined whether NO activates K_Ca channels in human PASMCs and, if so, whether this action involves cGMP-independent mechanisms Human PASMCs were isolated from intrapulmonary arteries 6-14 mm in diameter. PASMCs were superfused with modified Tyrode's solution and whole-cell voltage-clamped. Pipette solution contained (in mM): 35 KCl, 90 K-gluconate, 10 NaCl, 5 MgATP, 0.5 GTP, 10 HEPES, 10 EGTA, 3 CaCl₂, pH 7.2, free [Ca²⁺] ≈65 nM. Outward K⁺ currents were activated by depolarizing pulses from -70 mV to potentials between -50 to +60 mV. The NO-donor, 3-morpholinosydnonimine (SIN-1, 10^-6 M), caused large increases in outward K currents, which were completely blocked by charybdotoxin (100 nM), suggesting that NO activated K_Ca channels. The effect of SIN-1 on K_Ca currents was completely obliterated by the selective soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxidiazolol[4,3-a]quinoxalin-1-one (ODQ, 3 μM). In rings of the same arteries preconstricted with phenylephrine, SIN-1 (10^-7-10^-5 M) caused concentration-dependent relaxation, which was also abolished by ODQ (3 μM). Similar results were obtained in rat pulmonary arteries. These data suggest that NO stimulates K_Ca channels and causes vasorelaxation in human and rat pulmonary arteries through activation of soluble guanylyl cyclase rather than a cGMP-independent pathway.