letion of the major adenylate kinase AK1 isoform, which catalyzes adenine nucleotide exchange, disrupts cellular energetic economy and compromises metabolic signal transduction. However, the consequences of deleting the AK1 gene on cardiac energetic dynamics and performance in the setting of ischemia-reperfusion have not been determined. Here, at the onset of ischemia, AK1 knockout mice hearts displayed accelerated loss of contractile force compared with wild-type controls, indicating reduced tolerance to ischemic stress. On reperfusion, AK1 knockout hearts demonstrated reduced nucleotide salvage, resulting in lower ATP, GTP, ADP, and GDP levels and an altered metabolic steady state associated with diminished ATP-to-P i and creatine phosphate-to-Pi ratios. Postischemic AK1 knockout hearts maintained ϳ40% of -phosphoryl turnover, suggesting increased phosphotransfer flux through remaining adenylate kinase isoforms. This was associated with sustained creatine kinase flux and elevated cellular glucose-6-phosphate levels as the cellular energetic system adapted to deletion of AK1. Such metabolic rearrangements, along with sustained ATP-to-ADP ratio and total ATP turnover rate, maintained postischemic contractile recovery of AK1 knockout hearts at wild-type levels. Thus deletion of the AK1 gene reveals that adenylate kinase phosphotransfer supports myocardial function on initiation of ischemic stress and safeguards intracellular nucleotide pools in postischemic recovery. energy metabolism; adenine nucleotides; glycolysis; phosphotransfer; oxygen-18 phosphoryl labeling; phosphorus-31 nuclear magnetic resonance MAINTENANCE OF OPTIMAL CARDIAC function requires precise control of cellular nucleotide ratios and high-energy phosphoryl fluxes (11,22,30,32,33,36,40). Within the cellular energetic infrastructure, adenylate kinase has been recognized as an important phosphotransfer enzyme that catalyzes adenine nucleotide exchange (ATP ϩ AMP ª 2ADP) and facilitates transfer of both -and ␥-phosphoryls in ATP (9,15,24,25,43). In this way, adenylate kinase doubles the energetic potential of ATP as a high-energy-phosphoryl carrying molecule and provides an additional energy source under conditions of increased demand and/or compromised metabolic state (13-15, 31, 35, 42). By regulating adenine nucleotide processing, adenylate kinase has been implicated in metabolic signal transduction (12,15,27). Indeed, phosphoryl flux through adenylate kinase has been shown to correlate with functional recovery in the metabolically compromised heart (30) and to facilitate intracellular energetic communication (3,9,13