NMR MEASUREMENTS OF NA+ AND CELLULAR-ENERGY IN ISCHEMIC RAT-HEART - ROLE OF NA+-H+ EXCHANGE

Interleaved Na-23- and P-31-nuclear magnetic resonance (NMR) spectra were continuously collected on perfused rat hearts subjected to low-flow ischemia (30 min, 10% flow) or zero-flow ischemia (21 min) followed by reperfusion. During untreated low-flow and zero-flow ischemia, intracellular Na+ (Na(i)+) increased by 53 +/- 11 (+/- SE) and 78 +/- 8%, respectively, and remained elevated for zero-flow hearts. However, during both low- and zero-flow ischemia, Na(i)+ did not increase in hearts treated with the Na+-H+ exchange inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA). The pH decreases during ischemia were unchanged. EIPA treatment reduced ATP depletion during ischemia. During reperfusion from zero-flow ischemia, EIPA-treated hearts displayed more rapid and extensive recoveries of phosphocreatine and ATP. Recovery of left ventricular developed pressure was improved for zero-flow hearts treated with EIPA during the ischemic period exclusively (104 +/- 13%) compared with untreated hearts (36 +/- 21%). These data indicate that Na+-H+ exchange is an important mechanism for Na(i)+ accumulation, but not for pH regulation, during myocardial ischemia. Additionally, Na+ homeostasis plays an important role in the postischemic recovery of cellular energy and ventricular function.