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A method is presented to measure the absolute concentration of intracellular Na+ ([Na+]i) in vivo by using interleaved 23Na- and 31P-nuclear magnetic resonance (NMR) spectroscopy and TmDOTP5- as shift reagent and chemical marker of tissue extracellular space (ECS). The technique was used to determine [Na+]i and relative ECS in livers of control rats (21 +/- 3 and 0.11 +/- 0.02 mM, respectively) and in rats exposed to carbon tetrachloride (103 +/- 29 and 0.23 +/- 0.03 mM, respectively). The NMR measurements were confirmed independently on excised tissue samples by using atomic absorption spectroscopy. The results confirm that TmDOTP5- can be used as a combined cation shift reagent and ECS marker, thereby allowing quantitation of [Na+]i in vivo by NMR.

The positive inotropic effect of cardiac glycosides has been attributed to inhibition of the Na-K-ATPase, accumulation of intracellular sodium and enhanced calcium availability due to Na-Ca exchange. However, few measurements of intracellular sodium in the functioning left ventricle following ouabain exposure at therapeutic doses are available. Our experimental objective was to quantitate the relationship between contractile state and intracellular sodium measured by 23Na nuclear magnetic resonance spectroscopy or atomic absorption in the intact heart. Isolated guinea-pig hearts, perfused in the Langendorff mode, were paced and then exposed to ouabain (3x10(-7)m) for 30 min. Left-ventricular pressure was monitored continuously. Intracellular sodium was measured either at 1-min intervals throughout the perfusion by shift reagent-aided 23Na nuclear magnetic resonance spectroscopy in the beating heart or following 30 minutes of perfusion by atomic absorption in myocardial tissue. While treatment with ouabain was associated with almost a two-fold rise in developed pressure, there was no significant increase in intracellular sodium measured by either technique. Thus, the positive inotropic effect of ouabain in this model is not associated with significant changes in bulk intracellular sodium. However, these results do not exclude the possibility of shifts between intracellular pools which would not be detected in bulk measurements, or changes in NMR-invisible intracellular pools which are not detectable by single quantum spectroscopy techniques., (Copyright 1998 Academic Press Limited)

A combination of 31P and 23Na NMR spectroscopy has been used to quantify the concentration of intracellular sodium, [Na]IC in the isolated and perfused mouse liver. The 31P resonances of dimethyl methylphosphonate and LaDOTP5-, markers of total tissue space and extracellular space, respectively, were used to determine the intracellular liver volume. For a mean wet weight of 1.7 +/- 0.3 g, the intracellular liver volume as measured by 31P NMR averaged 1.2 +/- 0.2 ml. The amount of intracellular sodium was measured from the baseline-resolved intracellular 23Na resonance during perfusion of the shift reagent, TmDOTP5-. These two measurements resulted in an NMR-determined value for [Na]IC of 29.0 +/- 5.2 mM. Separate measurement of total tissue Tm and Na by atomic absorption spectroscopy on the same samples provided an AAS-determined value for [Na]IC of 32.1 +/- 7.4 mM. These results indicate that intracellular sodium in the isolated, perfused liver is 100% visible by 23Na NMR spectroscopy.