Enhancement of transmission at the frog neuromuscular junction by adenosine deaminase: evidence for an inhibitory role of endogenous adenosine on neuromuscular transmission.

Adenosine deaminase reversibly increased the amplitude and the quantum content of the end-plate potentials (EPPs) recorded from superficial muscle fibers of frog sartorius preparations in which twitches have been prevented with high-magnesium solutions. Adenosine deaminase prevented the inhibitory effect of exogenously applied adenosine but not that of 2-chloroadenosine on the amplitude of EPPs. The effect of adenosine deaminase was abolished by erythro-9(2-hydroxy-3-nonyl)adenine (EHNA). The results suggest that endogenous adenosine exerts an inhibitory 'tone' over neuromuscular transmission.