Analysis of the action of 4-aminopyridine during repetitive stimulation at the neuromuscular junction.

4-Aminopyridine (4-AP) increased the quantal content (m) of end-plate potentials (e.p.p.s) evoked by continual stimulation (0.2--25 Hz) in frog end-plates depressed by Mg2+. The increase in m was due to an increase in the binomial parameter n. This was interpreted to mean that 4-AP increased the number of activated release sites. In junctions blocked by d-tubocurarine, 4-AP first increased and then decreased the amplitude of e.p.p.s. elicited during a train of stimuli of increasing frequency, indicating that 4-AP increased transmitter release more than mobilization.