title: Possible screening of surface charges on crayfish axons by polyvalent metal ions.
abstract: 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.
mesh_terms: D000200:Action Potentials; D000818:Animals; D003400:Astacoidea / Q000502:physiology*; D001369:Axons / Q000187:drug effects / Q000502:physiology*; D001466:Barium Sulfate / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D005063:Europium / Q000494:pharmacology; D066298:In Vitro Techniques; D007811:Lanthanum / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects*; D009532:Nickel / Q000494:pharmacology; D013324:Strontium / Q000494:pharmacology; D015019:Yttrium / Q000494:pharmacology
chemical_list: D001466:Barium Sulfate; D003035:Cobalt; D005063:Europium; D015019:Yttrium; D007811:Lanthanum; D009532:Nickel; D008274:Magnesium; D002118:Calcium; D013324:Strontium