Measurement of tissue potassium in vivo using 39K nuclear magnetic resonance

Abstract

39K nuclear magnetic resonance (NMR) spectra were readily obtained, in vivo, from rat muscle, kidney, and brain in 5-10 min with signal-to-noise ratios of approximately 20:1. Quantitation of the K+ signal was achieved by reference to an external standard of KCl/dysprosium nitrate as well as by reference to the proton signal from tissue water. In vitro NMR studies of isolated tissue showed a K+ visibility (NMR K+/total tissue K+) of 96%, 62 +/- 8%, 47 +/- 1.9%, 45 +/- 3.5%, and 43 +/- 2.5% for blood, brain, muscle, kidney, and liver, respectively. Absolute tissue K+ was determined by flame photometry of acid-digested tissue. Changes in tissue K+ status by chronic K+ depletion or acute K+ loading produced changes of 39K NMR signal intensity that were equal to changes of absolute tissue K+. Acidosis, alkalosis, mannitol, or RbCl infusion did not significantly change the NMR K+ signal. These results indicate that the changes in K+ detected by NMR were specifically and accurately detected. To investigate the factors that affect the 39K NMR signal, the effects of liver homogenate on 39K NMR signal intensity were studied. Addition of homogenate produced a 60% loss of signal intensity, suggesting that a large portion of cell K+ may be only 40% visible. Addition of RbCl to undiluted homogenate increased the NMR K+ signal by 11 +/- 2 mumol/g. Addition of H2O or NaCl had no effect, suggesting that Rb+ was replacing K+ in sites of low (less than 40%) NMR visibility. These results demonstrate that 39K NMR experiments can be performed using intact organs. To explain the lack of detectable K+ and changes in K+ NMR visibility, a three compartment model is proposed.

Factors affecting 39K NMR detectability in rat tissue

“…In our initial studies (reported herein) of in vitro rat thigh muscle carried out in a spectrometer having a 7.0 T magnetic field strength, we observed higher detectability (57 t 5%) than had been previously reported ( 4 ) .…”

“…Measurements of the 39K NMR signal in biological tissues (1)(2)(3)(4) have shown that not all the potassium present is detectable when compared to chemical analysis methods such as atomic emission or absorption spectroscopy. Many of these studies showed the relative amount of potassium detected, commonly termed NMR visibility, to be around 40%, although there was some variability between tissues (1)(2)(3)(4).…”

“…Many of these studies showed the relative amount of potassium detected, commonly termed NMR visibility, to be around 40%, although there was some variability between tissues (1)(2)(3)(4). The 40% visibility figure has been rationalized as consistent with certain spectral characteristics arising from the quadrupolar relaxation of this spin-; nucleus.…”

NMR-Active Nuclei for Biological and Biomedical Applications

“…Similarly, a TmDOTP shift reagent was used to assist 39 K NMR measurement of intracellular potassium during ischemia in the perfused guinea pig heart [234]. A number of studies have demonstrated the usefulness of 39 K NMR for in vivo measurement of potassium levels and its transport in a range of different rat tissues including blood, brain, muscle, kidney, liver, testes and mandibular salivary gland [235][236][237][238]. A number of feasibility studies have also been performed for 39 K-MRI analysis of human muscle ( Figure 17) and brain [239][240][241].…”

In vivo 39K, 23Na and 1H MR imaging using a triple resonant RF coil setup

“…Several efforts have been made to assess the detectability and the physical properties of these nuclei which are important for their quantification in different tissues by means of NMR [8][9][10][11][12][13][14][15].…”