SELECTIVE-INHIBITION OF THE SLOW K-NERVE ENDING BY PLASMA FROM A MYASTHENIA-GRAVIS PATIENT( CURRENT AT MOTOR)

The effect of plasma from a myasthenia gravis (MG) patient, containing anti-presynaptic membrane receptor (PsmR) antibody on the membrane cu rrents of motor nerve ending was investigated in mouse intercostal ner ve triangularis sterni preparations by perineurial recording. After in hibition of both the fast K+ current and Ca2+-dependent K+ current by 30 mM Tetraethyl-ammonium (TEA) unmasked the voltage dependent fast Ca 2+ current and the ''Ca plateau'', which was contributed by the voltag e-dependent slow Ca2+ current and slow K+ current. Application of the MG plasma caused further prolongation and increase of the Ca plateau, due to blockage of the slow K+ current. This effect was observed immed iately after the application and could be partially reversed by washin g, whereas no change was found by addition of the plasma from healthy persons. When K+ current was completely blocked by 30 mM TEA and 300 m u M 3,4-diaminopyridine (3,4-DAP), the fast Ca2+. current and the slow Ca2+ current were revealed. Neither the fast nor the slow Ca2+ curren t could be affected by the MG plasma; It was also shown that the MG pl asma was devoid of noticeable effect on the voltage dependent Na+ curr ent, fast K+ current as well as the Ca2+-dependent K+ current. So the effect of the MG plasma with antibody to PsmR was concluded to inhibit the slow K+ current selectively. As we knew, the beta-bungarotoxin bi nding protein was a kind of K+ channel, these results further confirme d that the beta-bungarotoxin binding protein should be the target of t he antibody to PsmR found in the plasma of some patients suffering fro m MG.