Extracellular metabolism of adenine nucleotides and adenosine in the innervated skeletal muscle of the frog

Adenosine and adenine nucleotides are independently released from both the nerve terminals and the muscle fibres upon electrical stimulation of the innervated skeletal muscle of the frog

The independent release of adenosine and adenine nucleotides upon electrical stimulation was studied in the innervated sartorius muscle of the frog after blockade of the extracellular catabolism of adenosine monophosphate (AMP) through exo-AMP deaminase and ecto-5'-nucleotidase. Nerve stimulation (30 min, 0.2Hz) induced the release of both adenosine (19 +/- 3 pmol) and adenine nucleotides (101 +/- 7 pmol). Experiments performed in the presence of tubocurarine (5 microM) to prevent purine release due to nerve-evoked muscle twitching, or under direct stimulation of the muscle in low calcium solutions to prevent pre-synaptic release of purines, showed that there was an evoked release of adenosine and adenine nucleotides both from the nerve endings and from the twitching 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 adenosine. When only one of the enzymes catabolizing AMP (ecto-5'-nucleotidase or exo-AMP deaminase) was inhibited, the evoked release of adenine nucleotides was undetectable, suggesting that each enzyme is able to catabolize all the AMP formed from adenine nucleotides released upon stimulation. It is concluded that the concentration of endogenous extracellular adenosine is under the control of the relative activities of exo-AMP deaminase and ecto-5'-nucleotidase.

Production of adenosine from extracellular ATP at the striatal cholinergic synapse

The components of the ectonucleotidase pathway at the immunoaffinity-purified striatal cholinergic synapse have been studied. The ecto-ATPase (EC 3.6.1.15) had a Km of 131 microM, whereas the ecto-ADPase (EC 3.6.1.6) had a Km of 58 microM, was Ca(2+)-dependent, and was inhibited by the ATP analogue 5'-adenylylimidodiphosphate (AMPPNP). The ecto-5'-nucleotidase (EC 3.1.3.5) had a Km of 21 microM, was inhibited by AMPPNP and alpha,beta-methylene ADP, and by a specific antiserum. The Vmax values of the ATPase, ADPase, and 5'-nucleotidase enzymes present at this synapse were in a ratio of 30:14:1. Very little ecto-adenylate kinase activity was detected on these purified synapses. The intraterminal 5'-nucleotidase enzyme, which amounted to 40% of the total 5'-nucleotidase activity, was inhibited by AMPPNP, alpha,beta-methylene ADP, and the antiserum, and also had the same kinetic properties as the ectoenzyme. The time course of ATP degradation to adenosine outside the nerve terminals showed a delay, followed by a period of sustained adenosine production. The delay in adenosine production was proportional to the initial ATP concentration, was a consequence of feedforward inhibition of the ADPase and 5'-nucleotidase, and was inversely proportional to the ecto-5'-nucleotidase activity. The function and characteristics of this pathway and the central role of 5'-nucleotidase in the regulation of extraterminal adenosine concentrations are discussed.

Adenine nucleotide analogues, including gamma-phosphate-substituted analogues, are metabolised extracellularly in innervated frog sartorius muscle

The metabolism of adenine nucleotides and of their analogues by ecto-enzymes in the innervated frog sartorius muscle was investigated with HPLC. The breakdown of beta, gamma-methylene-ATP was also evaluated by studying the ability of the adenosine uptake inhibitor, dipyridamole, and of the adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), to modify the effect of beta, gamma-methylene-ATP on nerve-evoked twitches. ATP-gamma-S at low (10 microM) but not at high (> or = 100 microM) concentrations was quickly metabolised into a compound with a higher negative charge. L-ATP, homo-ATP and 2-methylthio-ATP were metabolised into compounds with a lower negative charge. Beta-gamma-Imido-ATP and gamma-anilino-ATP were only metabolised slightly. As determined by HPLC, beta, gamma-methylene-ATP was not metabolised. In contrast, this ATP analogue inhibited nerve-evoked twitch responses, an effect which was potentiated by dipyridamole and antagonised by DPCPX. Alpha, beta-Methylene-ATP was dephosphorylated into alpha, beta-methylene-ADP, which was virtually resistant to metabolism in the absence of ATP. In the presence of ATP, alpha, beta-methylene-ADP was transiently phosphorylated into alpha, beta-methylene-ATP. Formation of ATP from ADP was observed even in the absence of an exogenous phosphate donor, and was prevented by the adenylate kinase inhibitor, P1P5-di-(adenosine-5')pentaphosphate (AP5A). AP5A caused only partial inhibition of AMP formation from ADP. The results suggest that some ATP analogues with substitutions in the gamma-phosphate, such as ATP-gamma-S and beta, gamma-methylene-ATP, are metabolised in the innervated frog sartorius muscle. The ADP analogue, alpha, beta-methylene-ADP, might be a substrate for an ecto-nucleoside diphosphate kinase. ADP, besides being dephosphorylated, is also a substrate for an ecto-adenylate kinase in innervated frog sartorius muscle.

Ecto-5'-nucleotidase is associated with cholinergic nerve terminals in the hippocampus but not in the cerebral cortex of the rat

The extracellular catabolism of exogenously added AMP was studied in immunopurified cholinergic nerve terminals and in slices of the hippocampus and cerebral cortex of the rat. AMP (10 microM) was catabolized into adenosine and inosine in hippocampal cholinergic nerve terminals and in hippocampal slices, as well as in cortical slices. IMP formation from extracellular AMP was not detected. alpha, beta-Methylene ADP (100 microM) inhibited almost completely the extracellular catabolism of AMP in these preparations. The relative rate of catabolism of AMP was greater in hippocampal slices than in cortical slices. AMP was virtually not catabolized when added to immunopurified cortical cholinergic nerve terminals, although ATP could be catabolized extracellularly under identical conditions. The comparison of the relative rates of catabolism of exogenously added AMP, calculated from the amount of AMP catabolized after 5 min, in hippocampal cholinergic nerve terminals and in hippocampal slices revealed a nearly 50-fold enrichment in the specific activity of ecto-5'-nucleotidase upon immunopurification of the cholinergic nerve terminals from the hippocampus. The results suggest that there is a regional variation in the subcellular distribution of ecto-5'-nucleotidase activity in the rat brain, the ecto-5'-nucleotidase in the hippocampus being closely associated with the cholinergic nerve terminals, whereas in the cerebral cortex ecto-5'-nucleotidase activity seems to be located preferentially outside the cholinergic nerve terminals.

5'-Nucleotidase: molecular structure and functional aspects

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