Transmitter mobilization at the frog neuromuscular junction.

During frequency facilitation of frog neuromuscular junctions depressed by Mg++, the relationship between quantal content (m) and frequency of stimulation (0.5 to 8 Hz) is exponential. The slope of the relationship (k) reflects transmitter mobilization and the zero-frequency intercept (mo) reflects the basic release process. The catecholamines, tetraethylammonium, guanidine and raised [Ca++]o increased mo but had no effect on k. At junctions where release ranged from 200 to 500 quanta sec-1 during steady-state conditions, this result was interpreted to mean that the drugs increased both transmitter release and mobilization. An analog of hemicholinium-3, DMAE, depressed k, reflecting the ability of DMAE to depress transmitter mobilization. The alternative possibility that the frequency facilitation relationship was altered by effects of the drugs on the number of activated release sites was also considered.