Observations of 23Na in frog skin by NMR

The shift reagent dysprosium tripolyphosphate [Dy(PPPi)7-2] has been applied to the nuclear magnetic resonance (NMR) study of 23Na in frog skin. The anion complex produces slight increases in the transcellular short-circuit current and conductance of the tissue. However, the electrophysiological responsiveness of the tissue to vasopressin, amiloride, and transient removal of extracellular potassium appear unimpaired. A large signal of [31P]phosphocreatine was also observed, further documenting that relatively little damage was produced by the reagent. Three signals of 23Na were observed, reflecting the sodium pools in an external standard solution, the extracellular space, and a medium relatively inaccessible to Dy(PPPi)7-2. The size of the reagent-insensitive signal could be increased by ouabain, a selective inhibitor of sodium extrusion, and could be decreased by amiloride, a selective inhibitor of sodium entry. The results indicate that the anion complex-insensitive signal reflects, at least in part, intracellular sodium and suggest that 23Na may become a useful complementary tool for monitoring the intracellular sodium content of transporting epithelia.