Enhancement of Ca2+-dependent outward current in sheep bladder myocytes by Evans blue dye

 Whole-cell and inside-out patch-clamp techniques were used to assess the action of a well-known dye, Evans blue, on membrane currents in bladder isolated smooth muscle cells from sheep. In whole cells Evans blue dose-dependently increased the outward current by up to fivefold. In contrast, Evans blue had no effect on inward Ca2+ current. The effect on outward current was abolished or reduced if the cells were bathed in Ca2+-free solution, iberiotoxin (5 × 10–8 M), or charybdotoxin (5 × 10–8 M), but was unaffected by externally applied caffeine (5 mM) or in cells exposed to heparin (1 mg/ml) via the patch pipette. In inside-out patches bathed in a Ca2+ concentration of 5 × 10–7 M, Evans blue (10–4 M) increased the open probability of large-conductance (298-pS) Ca2+-dependent K+ channels (BK channels), shifting the half maximal-activation voltage by –70 mV. We conclude that Evans blue dye acts as an opener of BK channels.