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High-resolution 23Na and 39K nuclear magnetic resonance (NMR) spectra of perfused, beating rat hearts have been obtained in the absence and presence of the downfield shift reagent Dy(TTHA)3- in the perfusing medium. Evidence indicates that Dy(TTHA)3- enters essentially all extracellular spaces but does not enter intracellular spaces. It can thus be used to discriminate the resonances of the ions in these spaces. Experiments supporting this conclusion include interventions that inhibit the Na+/K+ pump such as the inclusion of ouabain in and the exclusion of K+ from the perfusing medium. In each of these experiments, a peak corresponding to intracellular sodium increased in intensity. In the latter experiment, the increase was reversed when the concentration of K+ in the perfusing medium was returned to normal. When the concentration of Ca2+ in the perfusing medium was also returned to normal, the previously quiescent heart resumed beating. In the beating heart where the Na+/K+ pump was not inhibited, the intensity of the intracellular Na+ resonance was less than 20% of that expected. Although the data are more sparse, the NMR visibility of the intracellular K+ signal appears to be no more than 20%.

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A slight error is present in the gage as described and constructed, due to lack of parallelism of the two arms of the tube containing mercury; it is probably less than the errors of calibration and construction. For the most accurate work this could be eliminated by making the gage in the somewhat more inconvenient form of a U-tube with parallel limbs.It is obvious that the longer the radius of the scale is, and the shorter AB, the greater will be the reading accuracy, and the less will be the range of the scale of the instrument. The maximum range of the gage is from h = 0 to hd, and of the scale S, 90°.When designed with a volume ratio V:v suitable for vacua too high to permit the use of rubber tubing, an oil-sealed metal union or a mercurysealed, ground-glass joint can be used as the pivot.Thanks are due Professor R. S. Tour for suggestions as to the graphical calibration of the scale.

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