Nuclear Magnetic Resonance Studies on the Effects of Decreased External Sodium on Guinea Pig Cerebral Cortex Slices and the Permeabilities of Various Sodium Substitutes

Decreasing the external sodium concentration (Na+]e) to 10 mM in the presence of 280 mM sucrose had no significant effect on phosphocreatine (PCr) or on intracellular pH (pHi) as assessed using 31P nuclear magnetic resonance spectroscopy. Zero Na+]e in the presence of 300 mM sucrose caused a fall in PCr levels to 50% of control values, and the pHi fell to 6.85 from a control value of 7.30. 1H nuclear magnetic resonance spectroscopy confirmed that the sucrose had not entered the tissue. The decreases in PCr content and in pHi, known to occur on depolarization using 40 mM external potassium concentration (K+]e), were further decreased in the presence of 10 mM Na+]e), to 51.4 +/- 4.0 and 6.80 +/- 0.10% of control values, respectively. The free intracellular magnesium concentration was significantly increased from a control value of 0.37 +/- 0.10 mM to 0.66 +/- 0.13 mM (p less than 0.001), when Na+]e was decreased to 10 mM, but was not further affected by high K+]e or zero Na+. Membrane permeabilities of the sodium substitutes N-methyl-D-glucamine (NMG), tris(hydroxymethyl)aminomethane (Tris), tetramethylammonium (TMA), and choline were assessed using 1H nuclear magnetic resonance spectroscopy. In the presence of 10 mM Na+]e, NMG, TMA, and choline (all at 140 mM) were taken up and remained within the tissue for at least 2 h, but no uptake of Tris (140 mM) or sucrose (above) could be detected. Tissue lactate levels (from the lactate/N-acetyl aspartate ratio) increased in the presence of the substitutes that were taken up, although no change in pH was detected.