Effects of permeant monovalent cations on end-plate channels

Abstract

1. The time constant of decay (tau D) and peak amplitude of miniature end-plate currents (m.e.p.c.s) were recorded in voltage-clamped toad sartorius fibres. The conductance (gamma) and average lifetime (tau N) of end-plate channels activated by ionophoretically applied acetylcholine were calculated from records of current fluctuations and null potentials recorded in the same fibres. tau D was significantly greater than tau N measured at the same end-plate. 2. Substitution for LiCl for NaCl increased tau D and tau N but decreased gamma and the peak amplitude of m.e.p.c.s. In contrast substitution of CsCl for NaCl decreased tau D and tau N but increased gamma and the peak amplitude of m.e.p.c.s. 3. In normal (Na) solution and in solutions in which Na had been replaced with Li, Cs and K, the ratios of average decay time constants of m.e.p.c.s and average channel lifetimes followed the sequence (tau(Li) greater than tau(Na) greater than tau(Cs) greater than tau(K)). 4. Substitution of Li, Cs or K for Na had little effect on the acetylcholine null potential. Average null potentials in Li, Na and Cs solutions were -6.1, -3.2 and 0.1 mV at 20 degrees C, and -7.3, -5.3 and -0.1 mV at 8 degrees C, respectively. The average null potential in K solution measured at 8 degrees C was -2.4 mV. 5. Peak conductance during an m.e.p.c. (Gp) followed the sequence (Gp(K) greater than or equal to Gp(Cs) greater than Gp(Na) greater than Gp(Li)). Single channel conductance followed a similar sequence of gamma(K) greater than or equal to gamma(Cs) greater than gamma(Na) greater than gamma(Li). 6. The voltage sensitivity of the rate of decay of m.e.p.c.s and of average channel lifetime was affected by substituting monovalent cations for Na, being greater in Li solution and less in Cs or K solutions. The total amount of charge moving across a single channel or across channels activated during an m.e.p.c. was largely unchanged in Li, Na, Cs and K solutions. 7. Single channel conductance and peak conductance during an m.e.p.c. varied with membrane potential in normal (Na) solution, decreasing with membrane hyperpolarization. This effect was more marked in Li solution but was less evident in Cs or K solutions.