Nuclear magnetic resonance studies of intracellular ions in perfused frog heart

Intracellular sodium, potassium, and lithium were observed in a perfused frog heart by nuclear magnetic resonance (NMR) spectroscopy. A perfusate buffer containing the shift reagent, dysprosium tripolyphosphate, was used in combination with mathematical filtering or presaturation of the extracellular resonance to separate the intra- and extracellular sodium NMR signals. Addition of 10 microM ouabain to the perfusate, perfusion with a zero potassium, low-calcium buffer, and replacement of 66% of the perfusate sodium with lithium resulted in the following percent changes in the intracellular sodium levels (mean +/- SD): ouabain + 460 +/- 60% (n = 6), zero potassium + 300 +/- 30% (n = 3), and lithium - 51 +/- 6% (n = 3). An increase of 45% in the intracellular sodium was observed when changing the pacing rate from 0 to 60 beats/min (with proportional changes for intermediate pacing rates). The ratio of intracellular potassium to sodium concentration was determined to be 2.3 by NMR, indicating that a substantial amount of the intracellular potassium is undetectable with these NMR methods. In addition, intracellular lithium was observed during perfusion with a lithium-containing perfusate.