The effects of myasthenic IgG on miniature end-plate currents in mouse diaphragm

In diaphragms from mice injected with purified IgG from a patient with myasthenia gravis, miniature end-plate currents (m.e.p.c.s) were reduced in size by about 50% and sensitivity to superperfused carbachol was also reduced. The ‘myasthenic’ m.e.p.c.s closely resembled m.e.p.c.s made small by α-bungarotoxin (α-BuTX) or (+)-tubocurarine with regard to alteration of height by poisoning of acetylcholinesterase and sensitivity to further receptor blockade. However, the time course of decay of the ‘myasthenic’ m.e.p.c.s was closer to normal than in m.e.p.c.s reduced in size by dTC or α-BuTX. It is postulated that myasthenic IgG acts to interfere with receptor function to open ionic channels more than it blocks binding of ACh to receptor.