On the use of ¹³³Cs as an NMR active probe of intracellular space in vivo

Data are presented from ¹³³Cs NMR studies on both excised and in situ tissues from rats fed a regular diet and administered i.p. CsCl in aqueous solution for 6 to 14 days. Cesium is an NMR‐active potassium analog which accumulates in the intracellular spaces of tissues [Davies et al., Biochemistry 27, 3547 (1988); Shehan, B.P. et al., Magn. Reson. Med. 30, 573 (1993)]. Chemical shifts, relaxation properties, sensitivity and detectability of cesium in tissues were investigated. Consistent with previous reports, two resonances (representing intra‐ and extracellular cesium) were detected in blood. Only one resonance was detected in brain, kidney, and muscle tissue. Efforts to resolve intra‐ and extracellular components by T₁ and T₂ relaxation discrimination were not successful. Following i.p. administration, cesium accumulates intracellularly with a brain‐to‐cerebrospinal fluid concentration (μmol/g) ratio of 9:1 and a thigh muscle‐to‐plasma concentration ratio of 40:1. Considering the small extracellular volume in these tissues (ca 18% and 10%, respectively), the net content differences between intra‐ and extracellular cesium are approximately 40‐fold in brain and 360‐fold in muscle. The concentration ratio of cesium in brain to cesium in cerebrosinal fluid decreased to 3:1 1 h after death, indicating a relatively slow rate of leakage of cesium from the intra‐ to extracellular space in the face of bioenergetic failure. These data suggest that the cesium signal is dominated by the intracellularly located cesium and, thus, cesium may be useful as a probe of the intracellular environment despite an inability to resolve and directly observe distinct resonances from intra‐ and extracellular spaces.