Single calcium-dependent potassium channels in clonal anterior pituitary cells

Single Ca2+-dependent K+-channel currents were recorded in intact and excised inside-out membrane patches of the anterior pituitary clone AtT-20/D16-16. The frequency of channel openings and lifetimes depends both on membrane potential and on the Ca2+ concentrations at the inner membrane surface. The curve of the open-state probability of the channel as a function of membrane potential appears to translate along the voltage axis with changes in internal Ca2+ concentration. For Ca2+ concentrations between 10(-7) and 10(-6) M, the shift is consistent with the hypothesis that three Ca2+ ions are required to open a Ca2+-dependent K+ channel. Single channel conductances are estimated to be 124 pS in patches with normal external K+ (5.4 mM) and 208 pS in excised patches with symmetrical K+ (145 mM) across the membrane. Tetraethylammonium (20 mM) added to the cytoplasmic surface reversibly blocks the Ca2+-dependent K+ channel.