On the action of ruthenium red and neuraminidase at the frog neuromuscular junction.

1. The actions of the polyvalent cationic dye Ruthenium Red and the enzyme neuraminidase were studied at the frog neuromuscular junction. 2. Both Ruthenium Red (0.5-40 microM) and neuraminidase (1, 2 and 4 u.) reduced the miniature end-plate potential (m.e.p.p.) amplitude irreversibly with little change in frequency. This effect could be attributed to the depressant action of these agents on the amplitude of extracellularly recorded miniature end-plate currents (m.e.p.c.s) coupled with their shortening effect on the duration of these m.e.p.c.s. For example, 10 microM-Ruthenium Red reduced the time constant of decay (tau D) of extracellular m.e.p.c.s to 35% of the control value. 3. Ruthenium Red (5, 10 and 15 microM) produced a pronounced reduction in the post-synaptic end-plate sensitivity to microiontophoretically applied acetylcholine. In some cases this effect was fully reversible on wash-out of the dye. 4. Amplitude histograms of m.e.p.c.s recorded under voltage clamp showed a similar shift of the modal value to lower levels in the presence of Ruthenium Red. Over the voltage range -70 to -110 mV the action of Ruthenium Red on the tau D was not voltage dependent. 5. The actions of Ruthenium Red on tau D were not affected by anaesthetic agents, such as the short-chain alcohols which increased tau D, or ketamine which decreased tau D. 6. Some possible modes of action of Ruthenium Red and neuraminidase are briefly discussed.