Di-8-ANEPPS (4-{2-[6-(dibutylamino)-2-naphthalenyl]-ethenyl}-1-(3-sulfopropyl)pyridinium inner salt) has been used as a fast-response voltage-sensitive styrylpyridinium probe. However, little is known regarding the mechanism of di-8-ANEPPS actions on ion currents. In this study, the effects of this dye on ion currents were investigated in pituitary GH(3) cells. In whole-cell configuration, di-8-ANEPPS (10 microM) reversibly increased the amplitude of Ca(2+)-activated K(+) current. In inside-out configuration, di-8-ANEPPS (10 microM) applied to the intracellular surface of the membrane caused no change in single-channel conductance; however, it did enhance the activity of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels with an EC(50) value of 7.5 microM. This compound caused a left shift in the activation curve of BK(Ca) channels with no change in the gating charge of these channels. A decrease in mean closed time of the channels was seen in the presence of this dye. In the cell-attached mode, di-8-ANEPPS applied on the extracellular side of the membrane also activated BK(Ca) channels. However, neither voltage-gated K(+) nor ether-à-go-go-related gene (erg)-mediated K(+) currents in GH(3) cells were affected by di-8-APPNES. Under current-clamp configuration, di-8-ANEPPS (10 microM) decreased the firing of action potentials in GH(3) cells. In pancreatic betaTC-6 cells, di-8-APPNES (10 microM) also increased BK(Ca)-channel activity. Taken together, this study suggests that during the exposure to di-8-ANEPPS, the stimulatory effects on BK(Ca) channels could be one of potential mechanisms through which it may affect cell excitability.