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Department of Pharmacology, Royal Free Hospital Medical School, London.

1. The effect of Lambert-Eaton myasthenic syndrome (LEMS) immunoglobulin G (IgG) on Ca2+ channels in undifferentiated mouse neuroblastoma x rat glioma hybrid cells (NG 108 15) was studied using the whole-cell patch clamp technique. 2. Sustained inward Ca2+ channel currents were evoked by depolarizing pulses from holding potentials of -80 and -40 mV, and were blocked by 5 microM-nitrendipine (L-type currents). Transient inward Ca2+ channel currents were activated from a holding potential of -80 mV by small depolarizing steps (T-type currents). Noradrenaline (10 microM) was without effect on transient currents. 3. LEMS IgG selectively reduced sustained (L-type) Ca2+ channel current amplitudes evoked from either holding potential used. In the presence of nitrendipine (5 microM), there was no significant effect of LEMS IgG on the remaining transient (T-type) Ca2+ channel current amplitudes. 4. Studies of the potential for maximal inward current indicated that voltage sensitivities of both L- and T-type Ca2+ channel current amplitudes were unaffected by LEMS IgG, whether recorded in the presence or absence of nitrendipine. LEMS IgG had no significant effect on the time-to-peak or decay of Ca2+ channel currents. 5. It is concluded that LEMS IgG acts selectively to cause functional loss of L-type, but not T-type, Ca2+ channels in NG 108 15 cells. Any effect of LEMS IgG on N-type channels (not present in these undifferentiated cells) was not studied here. LEMS IgG also acts at motor nerve terminal Ca2+ channels leading to muscle weakness. Thus antigenic similarities must exist between L-type channels in NG 108 15 cells and Ca2+ channels at motor nerve terminals.

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