A Cation Channel for K⁺ and Ca²⁺ from Tetrahymena Cilia in Planar Lipid Bilayers

Abstract

The single-channel properties for monovalent and divalent cations of a voltage-independent cation channel from Tetrahymena cilia were studied in planar lipid bilayers. The single-channel conductance reached a maxi-mum value as the K⁺ concentration was increased in symmetrical solutions of K⁺. The concentration dependence of the conductance was approximated to a simple saturation curve (a single-ion channel model) with an apparent Michaelis constant of 16.3 mM and a maximum conductance of 354 pS. Divalent cations (Ca²⁺, Ba²⁺, Sr²⁺, and Mg²⁺) also permeated this channel. The sequence of permeability determined by zero current potentials at high ionic concentrations was Ba²⁺ ?? K⁺ ?? Sr²⁺>Mg²⁺>Ca²⁺.
Single-channel conductances for Ca2+ were nearly constant (13. 9 pS-20. 5 pS) in the concentrations between 0. 5 mM and 50mM Ca-gluconate. In the ex-periments with mixed solutions of K⁺ and Ca²⁺, a maximum conductance of Ca²⁺ (γ^Ca_max) and an apparent Michaelis constant of Ca²⁺ (K^Ca_m) were obtained by assuming a simple competitive relation between the cations. γ^Ca_max and K^Ca_m were 14. 0 pS and 0. 160 mM, respectively. Single-channel conductances in mixed solu-tions were well-fitted to this competitive model supporting that this cation chan-nel behaves as a single-ion channel. This channel had relatively high-affinity Ca²⁺-binding sites.