Abstract

The ionic mechanism of actions of ruthenium red was examined in rat anterior pituitary GH₃ cells. In whole-cell recording experiments, ruthenium red reversibly caused an inhibition of Ca²⁺-activated K⁺ current [I_K(Ca)] in a dose-dependent manner. The IC₅₀value of ruthenium red-induced inhibition of I_K(Ca) was 15 μM. Neither carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 10 μM), 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 μM) into the bath medium did not change single-channel conductance but significantly suppressed the activity of large-conductance Ca²⁺-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 Ca²⁺ 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 μM) did not affect the amplitude of voltage-dependent K⁺ current but produced a significant reduction of voltage-dependent L-type Ca²⁺ current. These results indicate that ruthenium red can directly suppress the activity of BK_Ca channels in GH₃ cells. This effect is independent on the inhibition of Ca²⁺ release from internal stores or mitochondria.