Effects of Pb2+ and Cd2+ on Acetylcholine Release and Ca2+ Movements in Synaptosomes and Subcellular Fractions from Rat Brain and Torpedo Electric Organ

In this work we examined the effects of Pb2+ and Cd2+ on (a) [3H]ACh release and voltage-sensitive Ca2+ channels in rat brain synaptosomes, and (b) 45Ca2+ binding to isolated brain mitochondria and microsomes, and synaptic vesicles isolated from Torpedo electric organs. Pb2+ (Ki approximately 1.1 microM) and Cd2+ (Ki approximately 2.2) competitively block the K+-evoked influx of 45Ca2+ through the 'fast' calcium channels in synaptosomes. The Kis obtained with synaptosomes are in good agreement with the Ki values obtained from electrophysiological experiments at the frog neuromuscular junction (KPb:0.99 microM, KCd: 1.7 microM)7. The Ki for the inhibition of ACh release from synaptosomes by Cd2+ is 4.5 microM. Pb2+ is a less effective inhibitor of transmitter release (Ki approximately 16 microM) because it secondarily augments spontaneous transmitter efflux. Cd2+ has no effect on spontaneous release at concentrations less than or equal to 100 microM. The enhancing effect of Pb2+ on spontaneous release is (a) not abolished by omission of Ca2+ from the bathing medium, (b) is delayed by 1-2 min after the beginning of Pb2+ exposure, (c) is reversed upon the removal of Pb2+. In the presence of physiological concentrations of ATP (1 mM), Mg2+ (1 mM) and Pi (2 mM), 1-10 microM Pb2+ inhibits calcium uptake but Pb2+ greater than 10 microM causes a several-fold stimulation of passive binding of calcium to the organelles. This effect is associated with Pb2+-induced enhancement of Pi uptake. Cd2+ inhibits Ca2+ binding at all concentrations tested (1-50 microM) and reduces the Pb2+-induced Ca2+-binding to organelles. Neither Pb2+ nor Cd2+ have any discernible effects on spontaneous loss of calcium from mitochondria or microsomes preloaded with 45Ca. In summary, these data are consistent with the notion that Pb2+ and Cd2+ are potent blockers of presynaptic voltage-sensitive Ca2+ channels and the evoked release of transmitter which is contingent on Ca2+ influx through these channels. Our results are not consistent with the hypothesis that Pb2+ augments spontaneous release by interfering with intraterminal Ca2+-buffering by mitochondria, endoplasmic reticulum, or synaptic vesicles.