The ionic mechanism of actions of ruthenium red was examined in rat anterior pituitary GH(3) cells. In whole-cell recording experiments, ruthenium red reversibly caused an inhibition of Ca(2+)-activated K(+) current [I(K(Ca))] in a dose-dependent manner. The IC(50) value of ruthenium red-induced inhibition of I(K(Ca)) was 15 microM. Neither carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 10 microM), an uncoupler of oxidative phosphorylation in mitochondria, nor cyclosporin A (200 nM), an inhibitor of the mitochondrial permeability transition pore, affected the amplitude of I(K(Ca)). In inside-out configuration, application of ruthenium red (50 microM) into the bath medium did not change single-channel conductance but significantly suppressed the activity of large-conductance Ca(2+)-activated K(+) channel (BK(Ca)) channels. The ruthenium red-induced decrease in the channel activity of BK(Ca) channels was reversed by an increase in intracellular Ca(2+) concentration. Ruthenium red also shifted the activation curve of BK(Ca) channels to positive membrane potentials. The change in the kinetic behavior of BK(Ca) channels caused by ruthenium red in these cells is due to a decrease in mean open time and an increase in mean closed time. Ruthenium red (50 microM) did not affect the amplitude of voltage-dependent K(+) current but produced a significant reduction of voltage-dependent L-type Ca(2+) current. These results indicate that ruthenium red can directly suppress the activity of BK(Ca) channels in GH(3) cells. This effect is independent on the inhibition of Ca(2+) release from internal stores or mitochondria.