Effect of divalent cations on potassium conductance of squid axons: determination of surface charge.
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.
D000818:Animals; D001369:Axons / Q000502:physiology*; D002118:Calcium / Q000494:pharmacology*; D003217:Conductometry; D004594:Electrophysiology; D008274:Magnesium / Q000494:pharmacology*; D008564:Membrane Potentials; D008974:Mollusca; D009994:Osmolar Concentration; D011188:Potassium / Q000378:metabolism*
D008274:Magnesium; D011188:Potassium; D002118:Calcium