ON THE CONDUCTANCE PATHWAY TRAVERSED BY STRONTIUM IN MEDIATING THE ASYNCHRONOUS RELEASE OF ACETYLCHOLINE BY MOTOR NERVE IMPULSES

A study was made to determine whether the Sr²⁺‐dependent asynchronous release of acetylcholine by nerve impulses is mediated by the conventional Ca²⁺ conductance channel or, as has been suggested recently, through an alternative ion pathway. Experiments were performed on the frog neuromuscular junction by the use of standard electrophysiological techniques. Repetitive nerve stimulation in Sr²⁺‐Ringer solutions caused a marked increase in miniature end‐plate potential (m.e.p.p.) frequency which was dependent on Sr²⁺ concentration and inhibited in a competitive fashion by the known Ca²⁺ antagonists, Co²⁺ and Mg²⁺. The equilibrium dissociation constants (K_ds) determined for both Co²⁺ (0.09 + 0.01 mm, mean ± s.e. mean, n = 5) and Mg²⁺ (3.7 ± 0.3 mm, mean ± s.e. mean, n = 4) were essentially the same as the reported values for these antagonists in blocking Ca²⁺‐mediated transmitter release by nerve impulses. These results suggest that Sr²⁺ mediates asynchronous evoked transmitter release through the conventional calcium conductance channel. 1978 British Pharmacological Society