The use of dietary loading of 133Cs as a potassium substitute in NMR studies of tissues

¹³³Cs NMR chemical shifts and relaxation times have been measured for tissue samples in vitro and in vivo from rats which have been fed on a high cesium, low potassium diet, which leads to a predominantly intracellular distribution of this ion, similar to that of K ⁺. The high sensitivity, large chemical shift range, and narrow linewidths of ¹³³Cs, compared with ³⁹K, allow chemical shift differences to be observed between tissues, and in subcellular organelles such as mitochondria. For example, in vitro tissue chemical shifts, relative to 150 mM CsCl, are 1.06 ± 0.11 ppm for liver, 0.02 ± 0.05 ppm for brain, 1.76 ± 0.20 ppm for erythrocytes, and ?0.13 ± 0.02 ppm for plasma. T₁ and spin-echo T₂ values range from 1.26 ± 0.05 s (T₂), and 0.028 ± 0.006 s (T₂) for liver, to 6.49 ± 0.19 s and 1.12 ± 0.03 s for plasma. ¹³³Cs relaxation times show the same relative trends between tissues as are observed in ³⁹K tissue Studies.