Dantrolene and the neuromuscular junction: evidence for intracellular calcium stores.

Dantrolene sodium (DaNa) markedly depressed the frequency of spontaneous miniature potentials at the frog neuromuscular junction. Its action is still observed at low [Ca2+]o and is little affected by temperature; its effect is not readily reversible. Theophylline antagonises the action of DaNa. DaNa prevents the stimulatory effect of the ionophore A23187 at 25 degrees C, but is ineffective in antagonising the action of X537A. DaNa and theophylline do not affect the functioning and integrity of mitochondria in vitro and it is suggested that DaNa acts at intracellular Ca2+ stores (other than the mitochondria) in the presynaptic terminals.