Calcium-dependent potassium channel inParamecium studied under patch clamp

Summary

We have studied a class of Ca ²⁺ _i -dependent K channels in inside-out excised membrane patches fromParamecium under patch clamp. Single channels had a conductance of 72 ±9.0 pS in a solution containing 100mM K⁺. The channels were selective for K⁺ over Rb⁺ with the permeability ratio of 1∶ 0.56. and over Na⁺, Cs⁺ or NH ⁺₄ with a ratio 1∶<0.1. The channel activity was dependent on Ca ²⁺ _i , which was applied to the cytoplasmic side; the Ca ²⁺ _i concentration for the half maximal activation was 2 μm. The Hill coefficient for the Ca ²⁺ _i dependence of the channel activity was 2.58, indicating that more than two Ca ²⁺ _i bindings are necessary for full activation. Unlike most Ca ²⁺ _i -dependent K channels in other organisms, the channels inParamecium were slightly more active upon hyperpolarization than upon depolarization. The voltage dependence was fitted to a Boltzmann curve with 41.2 mV pere-fold change in channel activity. While a high Ca ²⁺ _i concentration activated the channels, it also irreversibly reduced the channel activity over time. The decay of channel activity occurred faster at higher Ca ²⁺ _i concentrations. Quaternary ammonium ions suppressed ion passage through the channel; more highly alkylated quaternary ammonium ions were more efficient in blocking. Ba ²⁺ _i and Ca ²⁺ _i were relatively ineffective in blockage. It was concluded that these Ca ²⁺ _i -dependent K channels inParamecium are different from the previously described Ca ²⁺ _i -dependent K channels, and are perhaps of a novel class.