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

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.