Increased hypoxic stress decreases AMP hydrolysis in rabbit heart

Objective: AMP conversion to adenosine by cytosolic 5'-nucleotidase (5NT) o r to IMP by AMP deaminase determines the degree of nucleotide degradation, and thus ATP resynthesis, during reoxygenation. To elucidate the regulation of AMP hydrolysis during ischemia, data from P-31 NMR spectroscopy and bio chemical analyses were integrated via a mathematical model. Since 5NT is do wnregulated during severe underperfusion (5% flow), we tested 5NT regulatio n during less severe underperfusion (10% flow) and then made the perfusate hypoxic to see if the greater stress reactivated 5NT. Methods: P-31 NMR spe ctra and coronary venous effluents were obtained from Langendorff-perfused rabbit hearts subjected to two 30-min periods of underperfusion (10% flow); the second period with or without additional hypoxia (30% O-2). Data were analyzed with a mathematical model describing the kinetics of myocardial en ergetics and metabolism. Results: A single 30-min period of 10% flow causes downregulation of AMP hydrolysis and the data from the second period of un derperfusion are best described by lower 5NT activity, even in the presence of extra hypoxia. Thirty percent less purines appear in the venous effluen t than predicted by the phosphoenergetics (PCr and ATP) when IMP is not all owed to accumulate by the model, however the model indicates that a constan t accumulation of IMP via AMP deaminase could explain the discrepancy betwe en expected and measured purines in the venous effluent. Conclusions: While AMP hydrolysis to adenosine is prominent in early ischemia and acts to pre serve cellular energy potential, during a second ischemic period, nucleotid es are conserved by the stable inhibition of AMP hydrolysis. Furthermore, d uring 10% how conditions, nucleotides are conserved, possibly via an IMP-ac cumulatory pathway. (C) 1999 Elsevier Science B.V. All rights reserved.