Effects of lead on neuromuscular transmission in the frog

The acute effects of Pb2+ on synaptic transmission at the frog neuromuscular junction were measured using conventional microelectrode techniques. Experiments were performed on preparations bathed in high magnesium/low calcium Ringer solution in order to record subthreshold endplate potentials (EPPs). The effects of Pb2+ on the muscle membrane and postsynaptic membrane were minimal since relatively high doses of Pb2+ caused no significant change in the input resistance of the muscle fiber and in the amplitude of the acetylcholine (ACh) iontophoteric potential when the ACh micropipette was highly localized. However, when the ACh micropipette was moved away from the receptors, the resulting ACh potential was reduced significantly by Pb2+. Pb2+ is a potent blocker of the EPP. Extracellular recordings from motor nerve terminals showed that endplate currents (EPCs) were reduced by Pb2+ while the nerve terminal potentials were unaffected. Therefore, Pb2+ blocks evoked transmitter release at a step following the depolarization of the nerve terminal. The blocking effect on the EPP was overcome when [Ca2+]o was raised. The log-log relationship between [Ca2+]o (abscissa) and EPP amplitude was shifted to the right in the presence of 1 microM Pb2+; the mean +/- S.E. slopes were 4.16 +/- 0.12 (control) and 4.05 +/- 0.13 (Pb2+). Reciprocal plots relating [Ca2+]o-1 to (EPP)-1/5 confirmed that Pb2+ competitively antagonized the action of Ca2+. The dissociation constant between Pb2+ and the Ca2+ receptor site was found to be 0.99 microM. Pb2+ is about 3 X 10(3) times more potent than is Mg2+, about 150 times more potent than is either Mn2+ or Co2+, and about 3 times more potent than Cd2+ is in blocking evoked release of ACh. After Pb2+ decreased the EPP, the MEPP frequency began to increase; this was probably the result of intracellular Pb2+ disrupting the Ca2+ sequestering activity of mitochondria and/or other intraterminal organelles. [Ca2+]i was thereby increased and an increase in MEPP frequency followed. Decreased MEPP amplitudes were observed when the MEPP frequency had been increased by Pb2+. Pb2+ may affect most chemical synapses in a manner which is similar to its effects on the neuromuscular junction and that this may be one of its important neurotoxic effects.