Intra-and extracellular sodium in guinea pig erythrocytes was evaluated with sodium-23 nuclear magnetic resonance (
23Na-NMR) by the use of a shift reagent, Dy (TTHA)
3-or Dy (PPPi)
27-. The test medium contained erythrocytes at 40% hematocrit level and NMR buffer (145 mM NaCl, 10 mM Dy (T-THA)
3-, 10% D
2O, adjusted to pH 7. 4 with tris, at 35°C). NMR spectra were obtained with a JEOL GSX 400 spectrometer operating at the Fourier transform mode of resonance signals, and the accumulated signals provoked by radio-frequency pulses of 90° were recorded on paper. Quantitative Na determination was performed by measuring the area under the peak of intracellular sodium (Na
i) NMR signals. Ouabain (Oua : 0.3 mM) and asebotoxin-III (ATX-III : 0.3 mM) produced an increase in Na
iNMR signals to a level of 188. 1% and 138. 1% of the control, respectively. Combined use of Oua (0.15 mM) and ATX-III (0.15 mM) produced an elevation of Na
i concentration to a high level of 219.0% of the control in a superadditive manner. Mechanisms of the Na
i elevation with Oua and ATX-III can be interpreted by assuming two different actions : ATX-III increases net Na
+-influx via Na+ channels, while Oua inhibits the pumping out of Na
+from the cell.
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