Antagonism of calcium currents and neurotransmitter release by barium ions at frog motor nerve endings

Abstract

1. The effects of Ba2+ (0.1-2 mM) on the component of the perineural voltage change associated with nerve terminal calcium currents (prejunctional Ca2+ currents) were compared with the effects of this ion to antagonize calcium-dependent acetylcholine (ACh) release. These experiments were made on isolated neuromuscular junctions of the frog. 2. In the presence of sufficient concentrations of K+ channel blockers to eliminate measurable prejunctional K+ currents, low concentrations of Ba2+ selectively antagonized prejunctional Ca2+ currents in normal Ca2+ solutions. Higher concentrations of Ba2+ also substantially reduced the Na+ component of the perineural waveform. 3. Ba2+ inhibited the prolonged prejunctional Ca2+ currents that developed in the presence of higher concentrations of K+ channel blockers. 4. Simultaneous measurements of the prejunctional Ca2+ currents and the electrophysiological correlates of ACh release (i.e. end-plate potentials, EPPs) were made under conditions of modest K+ channel blockade. Under these conditions, Ba2+ generally produced simultaneous decreases in both Ca2+ currents and EPP amplitudes. In some instances, a prolongation of prejunctional Ca2+ currents and a transient increase in EPP amplitudes preceded the decreases in both electrophysiological events. 5. These results suggest that Ba2+ ions can antagonize the entry of calcium into motor nerve endings and this effect is likely to be responsible for the inhibitory effects of Ba2+ on evoked ACh release.