The relationship between constitutive ATP release and its extracellular metabolism in isolated rat kidney glomeruli

ATP and adenosine are important extracellular regulators of glomerular functions. In this study, ATP release from glomeruli suspension and its extracellular metabolism were investigated. Basal extraglomerular ATP concentration (1nM) increased several fold during inhibition of ecto-ATPase activity, reflecting the basal ATP release rate. Mechanical perturbation increased the amounts of ATP released from glomeruli. ATP added to glomeruli was almost completely degraded within 20 minutes. In that time, AMP was the main product of extracellular ATP metabolism. Significant accumulation of AMP was observed after 5 min (194 +/-16 microM) and 20 min (271 +/-11 microM), whereas at the same time concentration of adenosine was only 10 muM. A competitive inhibitor of ecto-5-nucleotidase alpha-beta-methylene-ADP (AOPCP), decreased extraglomerular ATP and adenosine concentration by 80% and 50%, respectively. Similarly, AMP (100 microM) also markedly reduced extraglomerular ATP accumulation, whereas IMP, its deamination product, was not effective. P1, P5-diadenosine pentaphosphate (Ap5A) - an inhibitor of ecto-adenylate kinase prevented significantly the disappearance of ATP from extraglomerular media caused by AMP. These findings demonstrate that the decrease in extracellular ATP concentration observed after addition of AOPCP or AMP is caused by the presence of ecto-adenylate kinase activity in the glomeruli. The enzyme catalyses reversible reaction 2ADP<->ATP+AMP, and a rise in the AMP concentration can lead to fall in ATP level. The present study provides evidence the extraglomerular accumulation of ATP reflects both release of ATP from glomeruli cells and its metabolism by ecto-enzymes. Our data suggest that AMP, produced from ATP in the Bowman's capsular space, might plays a dual role as a substrate for ecto-adenylate kinase and ecto-nucleotidase reactions being responsible for the regulation of intracapsular ATP and adenosine concentration. We conclude that AMP degrading and converting ecto-enzymes effectively determine the balance between ATP and adenosine concentration and thus the activation of P2 and/or adenosine receptors.