Plasma from patients with seronegative myasthenia gravis inhibit nAChR responses in the TE671/RD cell line.

Myasthenia gravis (MG) is an autoimmune disorder in which anti-acetylcholine receptor (AChR) antibodies cause muscle weakness. In 10-15% of MG patients anti-AChR antibodies are undetectable (seronegative MG, SMG), though clinical and experimental evidence points to causative circulating factors. Using whole-cell patch-clamp techniques, we investigated the effects of heat-inactivated plasma from SMG patients (n = 7) on voltage-gated sodium [INa(V)] and ACh-induced nicotinic AChR (nAChR) currents in the human rhabdomyosarcoma cell line TE671/RD, comparing the results to those obtained with plasma from healthy individuals (HC, n = 6), patients with Guillain-Barré syndrome (GBS, n = 3) or those with other neurological diseases (OND, n = 3). None of the plasma samples inhibited INa(V). nAChR currents were rapidly (< 1 min) and significantly (P < 0.01) reduced by a 1:10 dilution of plasma from SMG patients compared with plasma from healthy controls and were not restored by washing. The inhibition appeared in some cases to be calcium dependent since for one of three plasmas it was prevented by 10 mM EGTA in the patch pipette. Currents were also reduced by two of three plasmas obtained from GBS patients at 1:3 dilution, but not by the three plasmas from patients with ONDs. The rapid action of plasma from SMG patients argues against an antibody-induced reduction in nAChR numbers; its calcium dependence in one case suggests action by a second messenger that might involve nAChR phosphorylation.