Characterization of β-leptinotarsin-h and the effects of calcium flux antagonists on its activity

β-Leptinotarsin-h, purified from the hemolymph of the beetle Leptinotarsa haldemani, is a potent (∼1 nM) neuroactive protein that rapidly (few seconds) stimulates Ca2+ influx and neurotransmitter release. Our goals were to further characterize β-leptinotarsin-h and to test the hypothesis that it stimulates Ca2+ influx through presynaptic Ca2+ channels. Analysis of partial amino acid sequences revealed that β-leptinotarsin-h is a unique protein with significant similarity to only one other protein, the juvenile hormone esterase of Leptinotarsa decemlineata, commonly known as the Colorado potato beetle. We have examined the effect of β-leptinotarsin-h on Ca2+ current, Ca2+ uptake, Ca2+ levels, and neurotransmitter release in synaptosomes, cell lines, and neuronal systems. We found that its preferred site of action appears to be mammalian presynaptic nerve terminals. We tested antagonists of Ca2+ flux for their effects on β-leptinotarsin-h-stimulated Ca2+ uptake in rat brain synaptosomes. The non-selective Ca2+ channel blockers flunarizine, Ni2+, ruthenium red, high-concentration thapsigargin, and SKF 96365 inhibited β-leptinotarsin-h's activity, but none of the tested selective blockers of voltage-operated Ca2+ channels (ω-agatoxin IVA, ω-conotoxin GVIA, ω-conotoxin MVIIC, nicardipine, nifedipine, SNX-482) was inhibitory. Selective inhibitors of ligand-operated, store-operated, and transduction-operated channels were also not inhibitory. β-Leptinotarsin-h did not stimulate Na+ uptake, ruling out Na+ channels and many non-selective cation channels as targets. We conclude that β-leptinotarsin-h stimulated Ca2+ uptake through presynaptic Ca2+ channels; which channel is yet to be determined. β-Leptinotarsin-h may prove to be a useful tool with which to investigate calcium channels and calcium flux.