Biophysical Journal 9: 447-463 (1969)
© 1969 the Biophysical Society

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Effect of Divalent Cations on Potassium Conductance of Squid Axons

Determination of Surface Charge

ABSTRACT

Potassium conductance-voltage curves have been determined for a squid axon in high external potassium solution for a wide range of divalent cation concentrations. A decrease in divalent ion concentration shifts the conductance-voltage curve along the voltage axis in the direction of more hyperpolarized voltages by as much as 9 mv for an e-fold change in concentration. When the divalent ion concentration is less than about 5 mM, a further decrease does not cause a significant shift of the conductance-voltage curve. These results can be explained by assuming that on the outer surface of the membrane there is a negative fixed charge which can bind calcium ions, and that the axon is sensitive to the resulting double-layer potential. From our data, the best value for charge density was found to be one electronic charge per 120 square angstroms, and a lower limit to be one electronic charge per 280 square angstroms.

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