The effects of pentachlorophenol (PCP) at the toad neuromuscular junction.

1. Effects of PCP at the frog neuromuscular junction were studied in vitro in sciatic nerve sartorius muscle of the toad Pleurodema-thaul. 2. Within the concentration 0.003-0.1 mM, PCP caused a dose-time-dependent block of evoked transmitter release acompanied by an increase in the rate of spontaneous quantal release. 3. PCP induced an increase in miniature endplate potential (MEPP) frequency and it was not antagonized in a Ca2(+)-free medium, indicating that it does not depend upon Ca2+ influx from the external medium, but may act by releasing Ca2+ from intraterminal stores. 4. The present data, together with previous results concerning PCP at eighth sympathetic ganglia indicate that 3,4-diaminopyridine (3,4-DAP) counteracts the effects of PCP on synaptic transmission. This result suggests that PCP interfering Ca2+ influx occurs during depolarization of motor nerve terminals.