EFFECTS OF PERMEANT MONO-VALENT CATIONS ON ENDPLATE CHANNELS

Abstract

The time constant of decay (.tau.D) and peak amplitude of miniature end-plate currents (mepc) were recorded in voltage-clamped toad [Bufo marinus] sartorius fibers. 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 fibers. .tau.D was significantly greater than .tau.N measured at the same end-plate. Substitution for LiCl for NaCl increased .tau.D and .tau.N but decreased .gamma. and the peak amplitude of mepc. In contrast, substitution of CsCl for NaCl decreased .tau.D and .tau.N but increased .gamma. and the peak amplitude of mepc. 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 mepc and average channel lifetimes followed the sequence (.tau.(Li) > .tau.(Na) > .tau.(Cs) > .tau.(K)). 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.degree. C, and -7.3, -5.3 and -0.1 mV at 8.degree. C, respectively. The average null potential in K solution measured at 8.degree. C was -2.4 mV. Peak conductance during a mepc (Gp) followed the sequence (Gp(K) .gtoreq. Gp(Cs) > Gp(Na) > Gp(Li)). Single channel conductance followed a similar sequence of .gamma.(K) .gtoreq. .gamma.(Cs) > .gamma.(Na) > .gamma.(Li). The voltage sensitivity of the rate of decay of mepc 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 a mepc was largely unchanged in Li, Na, Cs and K solutions. Single channel conductance and peak conductance during a mepc 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.