Actions of lead on transmitter release at mouse motor nerve terminals

The actions of lead (Pb²⁺) on transmitter release were studied at neuromuscular junctions in mouse diaphragm in vitro. The quantal content of end-plate potentials (EPPs) was reduced by Pb²⁺ in a dose-related manner consistent with inhibition of Ca²⁺ entry into nerve terminals, with a half-maximal effect at 1.4 M (in 0.5 mM Ca²⁺ and 2 mM Mg²⁺). Pb²⁺ also inhibited the increased frequency of MEPPs (f_MEPP where MEPPs denotes miniature EPPs) produced by Ba²⁺ in the presence of raised K⁺, blocking the calculated Ba²⁺ entry half-maximally at 170 M. However, at concentrations of 50–200 nM, Pb²⁺ often increased f_MEPP in 20 mM K⁺ in the presence of Ca²⁺ and acted to promote the irreversible effect of lanthanum (La³⁺) to raise f_MEPP. In nominally Ca²⁺-free solution with 20 mM K⁺, brief (1 min) application of Pb²⁺ (20–320 M) caused rapid dose-dependent reversible rises in f_MEPP. With prolonged exposure to Pb²⁺,f_MEPP rose and then slowly declined; after removal of Pb²⁺, once f_MEPP had fallen to low levels, f_MEPP responded nearly normally to Ca²⁺ or ethanol, but not to Pb²⁺ itself. In 5 mM K⁺, 0 mM Ca²⁺ and varied [Pb²⁺] (where [ ] denotes concentration), nerve stimulation caused no EPPs, but prolonged tetanic stimulation produced increases in f_MEPP graded with [Pb²⁺] that persisted as a tail; results were consistent with growth f_MEPP with the 4th power of intracellular Pb²⁺ and removal of intracellular Pb²⁺ with a time constant of about 30 s. These results suggest that Pb²⁺ acts to block the entry of Ca²⁺ and Ba²⁺ into the terminal via voltage-gated Ca²⁺ channels through which Pb²⁺, at higher concentrations, also penetrates and then acts as an agonist at intracellular sites that govern transmitter release.