Purine salvage at the cholinergic nerve endings of the Torpedo electric organ: The central role of adenosine

Abstract

The uptake and metabolism of adenosine, adenine, inosine and hypoxanthine were studied at the cholinergic nerve endings of the Torpedo electric organ. In isolated synaptosomes there is a linear uptake (measured up to 60 min) for adenosine and adenine at concentrations of 0.3 μM Uptake of adenosine exceeds that of adenine by a factor of 10. Adenosine is transported into synaptosomes via a saturable uptake system ( K m, 2 μM; V max, ∼- 30 pmols/min/mg protein). 2′-Deoxyadenosine is a competitive inhibitor of synaptosomal adenosine uptake. The nerve terminal possesses anabolic pathways for the formation of adenosine 5′-triphosphate from both adenosine and adenine. Adenosine becomes phosphorylated rapidly after entry into synaptosomes to form adenosine 5′-monophosphate; adenosine 5′-diphosphate and adenosine 5′-triphosphate were also major metabolites (70%). Adenine, inosine and hypoxanthine first accumulate in the synaptosomes. However, adenine leads to major formation of nucleotides (41% adenosine 5′-triphosphate after 60 min). Only traces of adenosine-3′:5′ cyclic monophosphate are formed from both adenosine and adenine. If adenosine 5′-triphosphate is added to a suspension of intact synaptosomes it becomes degraded to adenosine. We conclude that cholinergic nerve endings in the Torpedo electric organ possess an effective purine salvage system. Adenosine 5′-triphosphate released from either a pre- or a postsynaptic source would become degraded to adenosine in the extra-cellular medium and be re-used via an uptake system for renewed synthesis of adenosine 5′-triphosphate in nerve terminals.