Purine effects at the neuromuscular junction and their modification by theophylline, imidazole and verapamil.

Interactions between some substances (theophylline, noradrenaline, imidazole, ouabain and verapamil) and adenosine or adenosine triphosphate (ATP) were examined by recording the twitch tension of partially magnesium blocked phrenic-rat diaphragm preparations stimulated indirectly. Theophylline (an inhibitor of phosphodieterases) prevented and reversed the neuromuscular depression induced either by adenosine or ATP, and these substances antagonized the neuromuscular facilitation caused by imidazole (an activator of phosphodiesterases); noradrenaline and ouabain did not modify and verapamil increased that depression. These results indicate that the putative purine presynaptic receptor is not the ATPase, that it does not appear to operate by implication of cyclic AMP, but that it could mediate a process involved in the reduction of transmitter release by regulating the entry of calcium that follows the depolorization of the motor nerve endings.