A cation channel for K+ and Ca2+ from Tetrahymena cilia in planar lipid bilayers.

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Abstract
The single-channel properties of monovalent and divalent cations of a voltage-independent cation channel from Tetrahymena cilia were studied in planar lipid bilayers. The single-cannel conductance reached a maximum 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+ .gtoreq. K+ .gtoreq. 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 50 mM Ca-gluconate. In the experiments with mixed solutions of K+ and Ca2+, a maximum conductance of Ca2+ (.gamma.maxCa) and an apparent Michaelis constant of Ca2+ (KmCa) were obtained by assuming a simple competitive relation between the cations. .gamma.maxCa and KmCa were 14.0 pS and 0.160 mM, respectively. Single-channel conductances in mixed solutions were well-fitted to this competitive model supporting that this cation channel behaves as a single-ion channel. This channel had relatively high-affinity Ca2+-binding sites.