Multiple quantum filtered ²³Na NMR spectroscopy of the isolated, perfused rat liver

Isolated, perfused rat livers were examined by single-quantum (SQ) and double-quantum-filtered (DQ-filtered) ²³Na spectroscopy during prolonged global ischemia and during perfusion with ouabain, low-buffer potassium, or lithium-enriched buffer. Baseline separation of the intracellular (Na(i)⁺) and extracellular (Na(e)⁺) sodium resonances using TmDOTP^5- allowed a direct comparison of temporal changes in SQ versus DQ-filtered Na(i)⁺. The SQ Na(i)⁺ signal increased ~150% during the first 15 min of global ischemia and then remained relatively constant over the next 45 min, while the DQ- filtered signal steadily increased ~400% over the same 60 min period. In similar experiments in which all perfusate sodium was replaced by lithium, the DQ-filtered Na(i)⁺ signal increased ~180% over a similar period of ischemia. Exposure of livers to ouabain also resulted in larger increases in DQ-filtered versus SQ signal of Na(i)⁺. The ~290% increase in DQ-filtered sodium observed during perfusion of livers with a hypokalemic buffer (1.2 mM K⁺) could be completely reversed by continued perfusion with a buffer containing normal levels of K⁺ (4.7 mM). These data suggest that the DQ- filtered Na(i)⁺ signal of liver does not simply report an increase in [Na(i)⁺], but may be exquisitely sensitive to other intracellular events initiated by altered physiology.