Anomalous permeabilities of the egg cell membrane of a starfish in K+-Tl+ mixtures.

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Abstract
Electrical properties of inward rectifying egg cell membranes of the starfish Mediastera aequalis were studied in the presence of K+-Tl+ mixtures. When the ratio of the external concentrations of these ions was changed while their sum was kept constant, both conductance and zero-current membrane potential went through a minimum, showing clear discrepancies from theoretical results based on conventional electrodiffusion models (e.g., Goldman''s equation). When the ratio of the 2 concentrations was fixed and their sum varied, the potential followed an ideal Nernst slope, consistent with Gdldman''s equation. The membrane conductance which, according to previous studies on similar membranes, was viewed as a function of the displacement of the membrane potential from its resting value .DELTA.V, showed marked differences between the cases in which K+ or Tl+ were the predominant ions: when K+ was the predominant permeant ion in solution, addition of small amounts of Tl+ inhibited the current, while corresponding blocking effects of K+ on the current were not observed when Tl+ was the predominant permeant ion. The time course of the conductance during voltage clamp was different in the 2 cases, being much faster in Tl+ than in K+ solution for comparable values of .DELTA.V. Most of these features were accounted for by a model in which it was assumed that the ionic channels had external binding sites for cations and that their permeability properties depended on the species of the cation bound (K+ or Tl+).