Figure 3.

Schematic showing potential sources of adenosine and adenine nucleotides in skeletal muscle during exercise Adenine nucleotides and adenosine might be released as such into interstitial fluid from skeletal muscle fibres, or formed after metabolism by ecto-phosphatases or ecto 5′nucleotidase (5′N). ATP may be released from motor and sympathetic neurons during their activation. ATP released from RBCs into plasma and ATP and adenosine released as such from endothelium, or generated by ectoenzymes, may diffuse into interstitial fluid. For further discussion see text.