Effects of divalent cations on toad end-plate channels

Abstract

Miniature end-plate currents (MEPCs) and acetylcholine-induced current fluctuations were recorded in voltageclamped, glycerol-treated toad sartorius muscle fibers in control solution and in solutions with added divalent cations. In isosmotic solutions containing 20mm Ca or Mg, MEPCs had time constants of decay (τ _D ) which were about 30% slower than normal. In isotonic Ca solutions (Na-free), greater increases in both τ _D and channel lifetime were seen; the null potential was −34 mV, and single-channel conductance decreased to approximately 5 pS. Zn or Ni, at concentrations of 0.1–5mm, were much more effective in increasing τ _D than Ca or Mg, although they did not greatly affect channel conductance. The normal temperature and voltage sensitivity of τ was not significantly altered by any of the added divalent cations. Surface potential shifts arising from screening of membrane fixed charge by divalent cations cannot entirely explain the observed increases in τ, especially when taken together with changes in channel conductance.