Depression of spontaneous and ionophore-induced transmitter release by ruthenium red at the neuromuscular junction

The effects on spontaneous and ionophore-induced transmitter release of the inorganic dye, ruthenium red (RuR), a known inhibitor of calcium binding sites, were observed at the frog sartorius neuromuscular junction using intracellular recording techniques. Both crude and purified RuR, at concentrations of 1 and 5 micron depressed or blocked spontaneous release of acetylcholine (ACh) and reduced postsynaptic sensitivity to ACh, the crude dye being more potent than the pure. Pretreatment of muscles with RuR prevented the catastrophic reaction of junctions to 100 micron X537A ionophore. Increased levels of Ca2+ restored spontaneous transmitter release to control levels after depression or blockade by RuR. It was concluded that RuR blocks a critical membrane-bound binding site for calcium which is necessary for quantal release of transmitter.