Heterogeneity of calcium channel autoantibodies detected using a small‐cell lung cancer line derived from a Lambert‐Eaton myasthenic syndrome patient

Abstract

We investigated the heterogeneity of anti-voltage-gated calcium channel (VGCC) antibodies in the Lambert-Eaton myasthenic syndrome (LEMS) using a small-cell lung carcinoma line (MB) derived from an LEMS patient. Four of 13 LEMS patients had raised titers of anti-¹²⁵I-ω-conotoxin-labeled (N-type) VGCCs, measured by radioimmunoassay using line MB as the source of antigen. Antagonists for L-type (nitrendipine and nifedipine) and retype (ω-conotoxin) VGCCs inhibited K⁺-stimulated (voltage-dependent) Ca²⁺ flux into this line-by 22% for L-type and 2% for N-type at maximum concentration. Inhibition by the LEMS IgGs, by contrast, ranged from 46 to 78% at a concentration of 2 mg/ml. These differing effects on Ca²⁺ flux inhibition by LEMS IgGs on the one hand and by L- and N-type channel antagonists on the other, taken together with the observation that many of the sera failed to react with ω-conotoxin-labeled (N-type) channels in the immunoprecipitation assay, suggest that in many LEMS patients the autoantibodies target other VGCC subtypes besides L- or N-types, and that these are important in inducing the myasthenic disorder.