We have investigated the effect of NADH and NAD on the gating of large conductance Ca(2+)-activated K(KCa) channels in arterial smooth muscle cells isolated from small pulmonary artery (outer diameter < 300 microns) and ear artery, using the patch clamp technique. In the inside-out configuration, intracellularly applied 2 mM NADH inhibited the activity of KCa channels in pulmonary arterial smooth muscle cells, while it had no significant effect on ear arterial smooth muscle cells. On the other hand, 2 mM NAD increased the opening of KCa channels in pulmonary arterial smooth muscle cells. The effects of another intracellular redox couple, glutathione(GSH) and glutathione disulfide(GSSG) were also dependent on their redox potentials. GSH(5 mM) inhibited KCa channels activity, while GSSG(5 mM) increased the activity of pulmonary arterial smooth muscle cells. It could be concluded that the modulation of KCa channels by intracellular redox state contributes, at least in part, to the hypoxic suppression of outward current in pulmonary arterial smooth muscle cells.