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-mu-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-mu-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-mu-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 of 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.