Lambert-Eaton syndrome antibodies inhibit acetylcholine release and P/Q-type Ca2+ channels in electric ray nerve endings

Abstract

The types of voltage-dependent calcium channels (VDCCs) present in the cholinergic terminals isolated from the electric organ of the ray, Narke japonica, were characterized on the basis of their pharmacological sensitivity to specific antagonists. Inhibition of these channel types by autoantibodies from patients with the Lambert-Eaton syndrome (LES) was then studied to determine the specificity of the pathogenic IgG.

In normal untreated synaptosomal preparations, maximal doses of N- and P and/or Q-type Ca2+ channel antagonists, -conotoxin GVIA and -agatoxin IVA, inhibited depolarization-evoked ACh release by 47 % and 43 %, respectively. Calciseptine, an L-type VDCC antagonist, caused a 20 % reduction in the release. This indicates that the exocytotic release process is predominantly mediated by N- and P/Q-type VDCCs.

LES IgG or sera caused an inhibition of ACh release by 39-45 % in comparison with the control antibody-treated preparations. The ionomycin-induced ACh release, however, was not altered by the antibodies. Additionally, the same LES antibodies inhibited whole-cell calcium currents (ICa) in bovine adrenal chromaffin cells. Thus, the pathogenic antibodies exert their action on VDCCs present in the synaptosomes.

The efficacy of three Ca2+ channel antagonists in blocking ACh release was determined in preparations pretreated with LES IgG. -Agatoxin IVA produced only an additional 3-5 % reduction in release beyond that obtained with LES antibodies. Despite the pretreatment with LES IgG, -conotoxin GVIA and calciseptine inhibited the release to nearly their control levels.

These results indicate that LES antibodies mainly downregulate P/Q-type Ca2+ channels which contribute to presynaptic transmitter release from the cholinergic nerve terminals of electric organ.

The present findings are consistent with the hypothesis that P/Q-type VDCCs at the neuromuscular junction are the target of LES antibodies and that their inhibition by the antibodies produces the characteristic neuromuscular defect in this disease.