D600 as a direct blocker of Ca-dependent K currents in Helix neurons

The effects of D600 and inorganic Ca blockers on calcium and Ca-dependent potassium currents were investigated in Helix neurons under current and voltage-clamp conditions. At low concentrations (10-50 microM), D600 enlarged the Ca-dependent spike in U cells, which was blocked at higher concentrations. D600 130 and 17 microM reduced the Ca current ICa and the Ca-dependent K current IK(Ca) respectively by half. IK(Ca) blockade developed rapidly and was almost irreversible. ICa blockade had a slower time course and was easily reversible. Hyperpolarization unblocked part of the Ca channels but had no effect on the blocked K(Ca) channels. Both currents were simultaneously reduced by half with 22 microM cadmium. These data indicate that D600, verapamil and diltiazem, block K(Ca) channels directly whether or not they have a blocking effect on Ca channels. It is concluded on the basis of intracellular D600 injection that the site of action is located on the outer mouth of the K(Ca) channels.