[Repetitive firing in motor nerve endings: modulation by divalent cations, rhythmic activation and cholinergic agents].

The phenomenon of repetitive firing (RG) in motor nerve endings induced by 4-aminopyridine (4-AP) was studied in cut sartorius frog muscle under the voltage clamp conditions. In the presence of 4-AP (1.10(-4) mol/l) one stimulus applied to the nerve induced two end plate currents (EPC) in half of cells studied (n = 35). The elevation of calcium ion concentration up to 5.4 mmol/l or magnesium to 5-9 mmol/l or rhythmic activity (0.05 Hz and above) abolished RF. Substitution of calcium by strontium or barium enhanced RF (the number of EPC during the burst of RF). EPC with very slow time course were observed in some cells in the presence of barium, that disrupted to the burst of RF by repetitive nerve stimulation. Neostigmine, an anticholinesterase agent, increased the number of EPC in the burst of RF, but alpha-bungarotoxin exerted no effect on the RF. The role of calcium and calcium-activated potassium currents in generation and termination of RF in motor nerve endings is discussed.