Abstract
The anionic dye erythrosin B increases quantal transmitter release from frog neuromuscular synapses. Experiments were performed to determine the role of ions and light in this presynaptic effect. In Ca-free saline containing 1 mM-EGTA [ethylene glycol bis [.beta.-aminoethyl ether]-N,N,N'',N''-tetraacetic acid] erythrosin B increased miniature end-plate potential (mepp) frequency at a more rapid rate than in normal saline. The dye''s effect was influenced by extracellular Ca ions in a complex manner. Dye-induced release was minimal in Ringer solution containing 0.1 mM-Ca, and higher in Ca concentrations above or below 0.1 mM. Erythrosin B-induced spontaneous release also occurred in saline which contained 1 mM-EDTA and was free of both Ca and Mg ions. Temporary removal of external Na ions did not alter the progressive increase in mepp frequency produced by the dye. Elevation of the K concentration of the external medium (from 2-20 mM), which presumably depolarized nerve terminals and increased their Ca permeability, but did not change the rate of increase of dye-induced release when preparations were in a reversed (outward) electrochemical gradient for Ca ions. A reduction in light intensity of at least 6 orders of magnitude reduced the effect of erythrosin B by 50%, suggesting that photoactivation is not the primary basis for the dye''s action. Apparently, erythrosin B is not acting solely by altering the ionic permeability of the presynaptic nerve terminal to Ca, Mg or Na ions, or by altering the Ca metabolism of the terminal. The enhanced effect of the dye in Ca-free saline suggests that it may be competing with Ca at a common site, while the ehancement of its effect in elevated external Ca suggests that the dye may also increase the permeability of the nerve terminal to Ca ions.