Ca(2+)-activated K+ currents (IK(Ca)) and voltage-dependent Ca(2+)-insensitive K+ currents (IK(V)) were recorded using the patch clamp technique to study the pulmonary (PASMC) and ear arterial smooth muscle cells (EASMC) of the rabbit and the possible regulatory mechanisms related to hypoxia. When a hypoxic solution (1 mM Na2S2O4, gassed with 100% N2) was superfused, the activity of Ca(2+)-activated K+ channels (KCa channels) recorded at a pipette potential of -70 mV in cell-attached mode was decreased to 49 +/- 7% in PASMC, whereas EASMC KCa channels did not respond to hypoxia. In inside-out patches (bathed symmetrically in 150 mM KCl), reducing agents such as dithiothreitol (DTT; 5 mM), reduced glutathione (GSH; 5 mM) and NADH (2 mM) decreased KCa channel activity in PASMC, but they did not affect the EASMC KCa channel. However, oxidizing agents such as 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB; 1 mM), oxidized GSH (GSSG; 5 mM) and NAD (2 mM) increased KCa channel activity in both PASMC and EASMC. In the whole-cell configuration, using a pipette solution containing a high concentration of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 10 mM), PASMC IK(V) were activated by depolarizing step pulses to voltages more positive than -30 mV (holding potential, -80 mV). IK(V) was increased by application of a membrane-permeable oxidizing agent, 2,2'-dithio-bis(5-nitropyridine) (DTBNP; 200 microM), whereas it was decreased by application of DTT (5 mM). From these results, it could be suggested that hypoxic pulmonary vasoconstriction is attributable, at least in part, to a change of cellular redox state, which decreases outward K+ currents. This hypothesis is further supported by the observation that the basal redox state of EASMC KCa channels is more reduced than that of PASMC KCa channels. The distinct responses to hypoxia of pulmonary and systemic arterial smooth muscle could be explained by this difference.