A CORRELATION BETWEEN QUANTAL CONTENT AND DECAY TIME OF END-PLATE CURRENTS IN FROG MUSCLES WITH INTACT CHOLINESTERASE

1. The relationship between quantal content and prolongation of endpla te currents (EPC) was studied in the frog sartorius with intact synapt ic acetylcholinesterase. 2. The prolongation of EPC was more pronounce d in endplates with a higher quantal content both before and after pot entiation of quantal release by 4-aminopyridine (4-AP). When the quant al content of EPC was lowered, either by high Mg2+ or repetitive stimu lation, the EPC decay constant was reduced. 3. A certain critical valu e of about 120 quanta per nerve impulse was found, at which point the decay of EPC remained constant even through the quantal content was re duced further. 4. The reduction in both density and number of postsyna ptic receptors, produced by alpha-bungarotoxin and (+)-tubocurarine le d to a profound reduction in EPC decay during the progressive fall in EPC amplitude in both 4-AP-treated and -untreated endplates. Both drug s are known to produce a shortening of EPC in anti-cholinesterase (ant i-ChE)-treated muscles, due to a decrease in receptor density and less frequent repetitive binding of ACh. 5. It is assumed that the prolong ation of multiquantal EPC is caused by an increased ACh concentration near the receptors, which may provide the opportunity for repetitive b inding even with full cholinesterase activity. The critical quantum co ntent of about 120 might be the number of quanta at which the probabil ity of multiple release at single active zones is increased above zero .