We investigated the electrical responses of Ca-activated K (KCa) currents induced by hypoxia and reduction or oxidation of the channel protein in pulmonary (PASMC) and ear (EASMC) arterial smooth muscle cells using the patch-clamp technique. In cell-attached patches, in the presence of a high K solution (containing 0.316 microM Ca2+), the activity of KCa channels from PASMC was decreased (by 49 +/- 7% compared to control, pipette potential = -70 mV) by changing to a hypoxic solution (1 mM Na2S2O4, aeration with 100% N2 gas). EASMC channels did not respond to hypoxia. In order to investigate the possible mechanisms involved, using inside-out patches bathed symmetrically in 150 mM KCl, we applied redox couples to the intracellular side. Reducing agents, such as dithiothreitol (DDT, 5 mM), reduced glutathione, (GSH, 5 mM), and nicotinamide adenine dinucleotide reduced (NADH, 2 mM) decreased PASMC, but not EASMC, KCa channel activity. However, oxidizing agents such as 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB, 1 mM), oxidized glutathione (GSSG, 5 mM) and NAD (2 mM) increased KCa channel activity in both PASMC and EASMC. The increased activity due to oxidizing agents was restored by applying reducing agents. From these results, we could suggest that the basal redox state of the EASMC KCa channel is more reduced than that of the PASMC channel, since the response of KCa channels of the EASMC to intracellular reducing agents differs from that of the PASMC. This difference may be related to the different responses of PASMC and EASMC KCa channels to hypoxia.