Photodynamic Action of Methylene Blue on the Paramecium Membrane

An electrophysiological study of photodynamic action on the Paramecium membrane was carried out. In the presence of methylene blue (MB), light‐spot stimulation of an anterior and a posterior part induced a depolarization and a hyperpolarization of the membrane, respectively. Under voltage‐clamping, the anterior stimulation induced an inward current, while the posterior stimulation induced an outward current. The amplitudes of these currents were dependent on the membrane potential. When K + channels were blocked with Cs + and tetraethylammonium (TEA + ), the posterior outward current was inhibited, while the anterior inward current was not inhibited. Intracellular application of the Ca 2+ chelator, 1,2 ‐bis (2‐aminophenoxy) ethane‐ N,N,N',N' ‐tetraacetic acid (BAPTA) also inhibited the posterior outward current, but the anterior inward current was unaffected. These results suggest that photodynamic action on the Paramecium membrane primarily opens the Ca 2+ channels and the following influx of Ca 2+ activates the Ca 2+ ‐dependent K + channels localized mainly on the posterior part of the membrane.