Figure 1.

Prejunctional Ca²⁺ currents in normal Ca²⁺ current  Ringer and its antagonism by Ba²⁺ (0.1 mM). The averaged control response shown (a) is characterized by a downward (inward) tetrodotoxin-sensitive Na⁺ current (Na⁺) and an upward (outward) deflection that largely reflects the prejunctional calcium current (Ca²⁺) that mediates ACh release. For the control record, the peak of the Ca²⁺ component ranged from 1.9–2.2 mV (mean±1 s.e.mean=2.1±0.04 mV, n=5 stimuli). For details of the repetitive firing, see text. After 15 min of superfusion with ringer containing 0.1 mM Ba²⁺, the peak of the prejunctional Ca²⁺ current was decreased without a change in the Na⁺ component. The peak of the Ca²⁺ component in (b) ranged from 1.3–1.6 mV (mean±1 s.e.mean=1.5±0.06 mV, n=5 stimuli). The decrease was highly statistically significant (Mann Whitney rank sum test, P<<0.01). Note also the elimination of repetitive firing (also indicative of a reduction in Ca²⁺ entry). Each trace is the averaged response to five stimuli delivered at 0.3 Hz.