ADENOSINE AND ADENINE-NUCLEOTIDES ARE INDEPENDENTLY RELEASED FROM BOTH THE NERVE-TERMINALS AND THE MUSCLE-FIBERS UPON ELECTRICAL-STIMULATION OF THE INNERVATED SKELETAL-MUSCLE OF THE FROG

The independent release of adenosine and adenine nucleotides upon elec trical stimulation was studied in the innervated sartorius muscle of t he frog after blockade of the extracellular catabolism of adenosine mo nophosphate (AMP) through exo-AMP deaminase and ecto-5'-nucleotidase. Nerve stimulation (30 min, 0.2 Hz) induced the release of both adenosi ne (19 +/- 3 pmol) and adenine nucleotides (101 +/- 7 pmol). Experimen ts performed in the presence of tubocurarine (5 muM) to prevent purine release due to nerve-evoked muscle twitching, or under direct stimula tion of the muscle in low calcium solutions to prevent pre-synaptic re lease of purines, showed that there was an evoked release of adenosine and adenine nucleotides both from the nerve endings and from the twit ching muscle fibres. Removal of ecto-5'-nucleotidase inhibition shows that the catabolism of adenine nucleotides released during stimulation contributes in about 50% to the amount of endogenous extracellular ad enosine. When only one of the enzymes catabolizing AMP (ecto-5'-nucleo tidase or exo-AMP deaminase) was inhibited, the evoked release of aden ine nucleotides was undetectable, suggesting that each enzyme is able to catabolize all the AMP formed from adenine nucleotides released upo n stimulation. It is concluded that the concentration of endogenous ex tracellular adenosine is under the control of the relative activities of exo-AMP deaminase and ecto-5'-nucleotidase.