Possible screening of surface charges on crayfish axons by polyvalent metal ions

1. The effect of different polyvalent metal ions in the external solution upon the threshold membrane potential for spike initiation in crayfish axons has been studied by means of intracellular micro-electrodes. The metal ions tested included six divalent cations (Ca(2+), Mg(2+), Sr(2+), Ba(2+), Co(2+), and Ni(2+)) and three trivalent cations (La(3+), Y(3+) and Eu(3+)).2. Identical extracellular concentrations of different cations with the same valence had essentially the same effect on threshold membrane potential. However, a very low concentration of trivalent cations (about 225 muM) was found to be equivalent to a much higher divalent cation concentration (13.5 mM) as measured by their effects on threshold potential.3. Upon a tenfold increase in concentration, the threshold potential for spike initiation was shifted in a positive direction by 30.6 mV with divalent cations and by 20.8 mV with trivalent cations.4. It is shown that a hypothesis involving screening of negative charges at the axonal membrane surface, based on Gouy-Chapman theory, predicts these various experimental results rather closely.5. It is concluded that a high negative charge density, sufficient to render a screening mechanism possible, exists at the surface of crayfish axons in the region of the sodium ;gates'.6. The density of surface charges is calculated to be approximately 1e(-)/43 A(2). This calculation is discussed in connexion with the possible molecular identity of the sodium ;gates' in crayfish axons and other excitable systems.