The conductance of a channel to permeable ions depends on the number of ions near the mouth of the pore. Surface charge controls the local concentration, and impermeable cations can modify this charge. Correlating channel conductance with the concentration of impermeable cations therefore determines the local charge near the open pore. This paper presents data from cell-attached patches on embryonic chick ventricle cells, and it uses the conductance of inward-rectifier channels in the patch (in 100 mM K, with various concentrations of Na, Ca, Ba, and Mg) to estimate the local surface potential. The results indicate the presence of ionized residues near the mouth of the channel. Using the Boltzmann equation and the Gouy-Chapman relation, the surface potential due to these residues (in 100K/33Na/0Ca/0Ba/0Mg) is -40 mV, and the charge density is -0.25 e/nm2.

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