Figure 3.

Purinergic neuromuscular transmission depicting the synthesis, storage, release and inactivation of ATP. ATP, stored in vesicles in nerve varicosities, is released by exocytosis to act on post-junctional receptors for ATP on smooth muscle. ATP is broken down extracellularly by ATPases and 5′-nucleotidase to adenosine, which is taken up by varicosities to be resynthesized and reincorporated into vesicles. Adenosine is broken down further by adenosine deaminase to inosine and hypoxanthine and removed by the circulation. [From Burnstock (1972), reproduced with permission from the American Society for Pharmacology and Experimental Therapeutics.]