Conduction and blocking properties of a predominantly anion-selective channel from human platelet surface membrane...

We have investigated the basic properties of a predominantly anion-selective channel derived from highly purified human platelet surface membrane. Single channels have been reconstituted into planar phospholipid bilayers by fusion of membrane vesicles and recorded under voltage-clamp conditions. The channel is found to have the following properties: (i) Channel activity occurs in bursts of openings separated by long closed periods. (ii) The current-voltage relationship is nonlinear. Channel current is seen to rectify, with less current flowing at positive than at negative voltages. Rectification may be due to asymmetric block by HEPES/Tris buffers. In 450 mM KCl, 5 mM HEPES/Tris, pH 7.2, the single channel conductance at -40 mV is approximately 160 pS and at +40 mV is approximately 90 pS. (iii) The conductance-concentration relationship follows a simple saturation curve. Half maximal conductance is achieved at a concentration of approximately 1000 mM KCl, and the curve saturates at a conductance of approximately 500 pS. (iv) Reversal potentials interpreted in terms of the Goldman-Hodgkin-Katz equation indicate a Cl: K permeability ratio of 4:1. (v) The channel accepts all of the halides as well as a number of other anions. The following sequence of relative anion permeabilities (in the presence of K+) is obtained: F- less than acetate- less than gluconate- less than Cl- less than Br- less than I- less than NO3- less tha SCN-.(vi) Cations as large as TEA+ are permeant. (vii) Current through the channel is blocked in the presence of DIDS, SITS and ATP, but not by Zn2+.