α-Latrotoxin Actions in the Absence of Extracellular Ca2+ Require Release of Stored Ca2.

α-Latrotoxin (αLTX) causes exhaustive release of neurotransmitters from nerve terminals in the absence of extracellular Ca2+ (Ca2+e). To investigate the mechanisms underlying this effect, we loaded mouse neuromuscular junctions with BAPTA-AM. This membrane-permeable Ca2+-chelator demonstrates that Ca2+e-independent effects of αLTX require an increase in cytosolic Ca2+ (Ca2+cyt). We also show that thapsigargin, which depletes Ca2+ stores, induces neurotransmitter release, but inhibits the effect of αLTX. We then studied αLTX's effects on Ca2+cyt using neuroblastoma cells expressing signaling-capable or signaling-incapable variants of latrophilin-1, a G protein-coupled receptor of αLTX. Our results demonstrate that αLTX acts as a cation ionophore and a latrophilin agonist. In model cells at 0 Ca2+e, αLTX forms membrane pores and allows the influx of Na+; this reverses the Na+-Ca2+ exchanger, leading to the release of stored Ca2+ and inhibition of its extrusion. Concurrently, αLTX stimulates latrophilin signaling, which depletes a Ca2+ store and induces transient opening of Ca2+ channels in the plasmalemma that are sensitive to inhibitors of store-operated Ca2+ entry. These results indicate that Ca2+ release from intracellular stores and that Ca2+ influx through latrophilin-activated store-operated Ca2+ channels contributes to αLTX actions and may be involved in physiological control of neurotransmitter release at nerve terminals.