Adenosine formation and energy metabolism: a 31P-NMR study in isolated rat heart

Temporal and quantitative relations between cytosolic energy metabolism, adenosine efflux, and coronary flow were examined during 10 min of isoproterenol (ISO) infusion (60 nM) or hypoxia (5% O2) in isolated isovolumic rat heart. Myocardial metabolism was monitored using 31P-nuclear magnetic resonance spectroscopy, and venous effluent was collected and assayed for adenosine. During ISO infusion, coronary flow increased to approximately 170%, and [ATP]/[ADP] [Pi] (cytosolic phosphorylation potential) declined to less than 25% of preinfusion levels, respectively (P less than 0.001). During hypoxia, coronary flow increased to 190%, and [ATP]/[ADP] [Pi] declined to less than 25% of normoxic levels (P less than 0.001). Release of adenosine into the coronary venous effluent increased greater than 10-fold and displayed significant inverse linear correlations with log[ATP]/[ADP] [Pi] and positive linear correlations with free cytosolic [AMP] and coronary flow during ISO infusion and hypoxia. Adenosine deaminase (ADA) treatment reduced coronary vasodilation by approximately 30% during ISO infusion and 40% during hypoxia (P less than 0.001) and augmented chronotropic and inotropic responses to ISO infusion (P less than 0.01). Infusion of ADA potentiated changes in [ATP]/[ADP] [Pi] and [AMP] observed during ISO infusion and hypoxia (P less than 0.05). These results indicate that 1) endogenous adenosine mediates metabolic vasodilation in the heart, 2) adenosine modulates the response of isolated myocardium to catecholamines, 3) myocardial adenosine formation appears to be linked to cytosolic metabolism via changes in [ATP]/[ADP] [Pi] and [AMP], and 4) endogenous adenosine provides a significant, metabolically beneficial action in isolated hearts during hypoxia and inotropic stimulation.