Nuclear magnetic resonance of tissue 23Na. Correlation time.

Shporer and Civan (Biochim. Biophys. Acta (1974) 354, 291--304) reported the effect of magnetic-field strength on the NMR relaxation times of 23Na in frog skeletal muscle. From these data, they estimated the correlation time c for bound 23Na whose tumbling is severely restricted, and they suggested that the fraction of bound 23Na does not exceed some few percent of the total 23Na population. However, a step in their theoretical approach seems oversimplified. With an improved approach, we obtained an effectivec of 4--9 ns for bound 23Na. This value is some 10 times shorter than the corresponding value estimated by them from the same data. On the other hand, their conclusion concerning the amount of bound 23Na seems to remain valid. The origin of the observed difference between the two transverse relaxation times of tissue 23Na is also discussed.