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

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

The NMR 'split peak effect' in cell suspensions: Historical perspective, explanation and applications

The physicochemical environment inside cells is distinctly different from that immediately outside. The selective exchange of ions, water and other molecules across the cell membrane, mediated by integral, membrane-embedded proteins is a hallmark of living systems. There are various methodologies available to measure the selectivity and rates (kinetics) of such exchange processes, including several that take advantage of the non-invasive nature of NMR spectroscopy. A number of solutes, including particular inorganic ions, show distinctive NMR behaviour, in which separate resonances arise from the intra- and extracellular solute populations, without the addition of shift reagents, differences in pH, or selective binding partners. This 'split peak effect/phenomenon', discovered in 1984, has become a valuable tool, used in many NMR studies of cellular behaviour and function. The explanation for the phenomenon, based on the differential hydrogen bonding of the reporter solutes to water, and the various ways in which this phenomenon has been used to investigate aspects of cellular biochemistry and physiology, are the topics of this review.

NMR of (133)Cs(+) in stretched hydrogels: One-dimensional, z- and NOESY spectra, and probing the ion's environment in erythrocytes

(133)Cs nuclear magnetic resonance (NMR) spectroscopy was conducted on (133)Cs(+) in gelatin hydrogels that were either relaxed or stretched. Stretching generated a septet from this spin-7/2 nucleus, and its nuclear magnetic relaxation was studied via z-spectra, and two-dimensional nuclear Overhauser (NOESY) spectroscopy. Various spectral features were well simulated by using Mathematica and the software package SpinDynamica. Spectra of CsCl in suspensions of human erythrocytes embedded in gelatin gel showed separation of the resonances from the cation inside and outside the cells. Upon stretching the sample, the extracellular (133)Cs(+) signal split into a septet, while the intracellular peak was unchanged, revealing different alignment/ordering properties of the environment inside and around the cells. Differential interference contrast light microscopy confirmed that the cells were stretched when the overall sample was elongated. Analysis of the various spectral features of (133)Cs(+) reported here opens up applications of this K(+) congener for studies of cation-handling by metabolically-active cells and tissues in aligned states.

The interaction of sterically stabilized magnetic nanoparticles with fresh human red blood cells

Sterically stabilized superparamagnetic iron oxide nanoparticles (SPIONs) were incubated with fresh human erythrocytes (red blood cells [RBCs]) to explore their potential application as magnetic resonance imaging contrast agents. The chemical shift and linewidth of (133)Cs(+) resonances from inside and outside the RBCs in (133)Cs nuclear magnetic resonance spectra were monitored as a function of time. Thus, we investigated whether SPIONs of two different core sizes and with three different types of polymeric stabilizers entered metabolically active RBCs, consuming glucose at 37°C. The SPIONs broadened the extracellular (133)Cs(+) nuclear magnetic resonance, and brought about a small change in its chemical shift to a higher frequency; while the intracellular resonance remained unchanged in both amplitude and chemical shift. This situation pertained over incubation times of up to 90 minutes. If the SPIONs had entered the RBCs, the intracellular resonance would have become broader and possibly even shifted. Therefore, we concluded that our SPIONs did not enter the RBCs. In addition, the T 2 relaxivity of the small and large particles was 368 and 953 mM(-1) s(-1), respectively (three and nine times that of the most effective commercially available samples). This suggests that these new SPIONs will provide a superior performance to any others reported thus far as magnetic resonance imaging contrast agents.

Cs + ADC in rat brain decreases markedly at death

Spectroscopic resolution of intracellular and extracellular compartments can be used to probe the kinetic environment of those spaces and the compartment-specific changes that occur following injury. This is important for understanding the biophysical mechanisms that underlie the remarkable diffusion-weighted MRI contrast of injured central nervous system (CNS) tissue. Cesium-133 is a physiologic analog of potassium that is actively taken up by cells and resides primarily in the intracellular space. The (133)Cs(+) signal can, thus, be exploited to probe the kinetic environment of the intracellular space. Two principal (133)Cs(+) resonances were observed at 11.74 T. These resonances arise separately from (133)Cs(+) in brain and temporalis muscle. The apparent diffusion coefficient (ADC) of Cs(+) in brain decreased from 1.0 +/- 0.2 microm(2)/ms in healthy tissue to 0.24 +/- 0.04 microm(2)/ms following global ischemia (average ADC +/- average uncertainty), while there was no significant change in the ADC of Cs(+) in temporalis muscle after injury. This finding underscores the tissue-specific nature of the decrease in ADC that accompanies brain injury. Further, as the Cs(+) ADC should reflect water ADC in the intracellular space, these results strongly support the hypothesis that the decrease in water ADC associated with CNS injury arises largely from kinetic changes taking place in the intracellular space.

Monitoring of brain potassium with rubidium flame photometry and MRI

An animal model was developed to monitor [K(+)] in the brain using partial K(+) replacement with Rb(+) and (87)Rb MRI. Fifty-one rats were given 0-80 mM of RbCl in the drinking water for up to 90 days. Focal cerebral ischemia was produced in 15 of the animals. Na, K, and Rb content in precision-guided submilligram samples of cortical brain were determined by emission flame photometry. Multinuclear (87)Rb/(23)Na/(1)H MRI was performed on phantoms and rats at 3T using a twisted projection imaging (TPI) scheme for (87)Rb/(23)Na, and custom-built surface or parallel cosine transmit/receive coils. Brain [Rb(+)] was safely brought up to 17-25 mEq/kg within 2-3 weeks of feeding. The characteristic patterns of [K(+)] decrease (with a sharp drop at 3-4 hr of ischemia) and [Na(+)] increase (at a rate of 31%/hr) observed previously in animals without Rb/K substitution were reproduced in ischemic cortex. The Rb/(Rb+K) ratio increased over time in ischemic areas (R = 0.91, P < 0.001), suggesting an additional index of ischemia progression. Preliminary (87)Rb MRI gave an estimate of 20-25 mEq Rb/kg brain weight (N = 2). In conclusion, brain Rb(+) is detectable by (87)Rb MRI and does not significantly interfere with ion dynamics in ischemic brain, which enables (87)Rb MRI studies of K(+) in ischemia.

Biomedical applications of 133Cs NMR

133Cs NMR is a valuable tool for non-invasively probing biological systems. As a congener of potassium, it accumulates in the intracellular space, primarily through the action of the Na+-K+ pump (ATPase). In addition, it is possible to resolve the MR signal of 133Cs in different tissue compartments on the basis of chemical shift or MR relaxation properties. This compartmental resolution applies not only to the intra- and extracellular spaces, but to subcellular compartments as well. In this review, we discuss the studies defining the ion transport, chemical shift and relaxation characteristics of 133Cs in living systems. We also review the application of 133Cs NMR to evaluation of ion transport across membranes and the kinetic/chemical environment of the intracellular space in systems ranging from red blood cells to rat brain.

Functional hepatocyte cation compartmentation demonstrated with 133Cs NMR

This study utilized the large intrinsic chemical shift range of (133)Cs, a potassium congener, in an NMR study of intracellular cation distribution. It demonstrates two distinct intracellular environments in isolated perfused hepatocytes from cesium-fed rats, evident as compartments with different (133)Cs chemical shifts and containing different proportions of total detected cesium. The chemical shifts of the two intracellular compartments were 2.44 +/- 0.07 and 1.21 +/- 0.18 ppm, relative to the cesium signal from the perfusate. The observation of two distinct intracellular cesium signals suggests slow exchange on an NMR chemical shift time-scale (k exchange > 0.02 s). The area of the high-frequency component represented 62 +/- 10% (N = 12) of the total intracellular cesium signal. Manipulation of the intracellular environment using anoxia with aglycemia or digitonin produced changes in the distribution between the two intracellular compartments, showing their dynamic nature. Changes measured in association with metabolic manipulation suggest cytoplasm and mitochondria as the origin of the high and low-frequency intracellular peaks, respectively.

Cesium-133: a potential reporter of the hepatic uptake of contrast agents

NMR spectroscopy of intracellularly located (133)Cs has been used to monitor the uptake of Gd-EOB-DTPA by the isolated rat liver. As shown by (31)P spectroscopy, accumulation of (133)Cs ions in hepatocytes does not produce detectable effects on the metabolism. The hepatic internalization of Gd-EOB-DTPA was followed by the paramagnetic relaxation enhancement of the intracellular (133)Cs ions, and confirmed by parallel quantitations of Gd and Cs run by inductively coupled plasma (ICP) analysis of liver samples and aliquots of perfusate. The relaxation data significantly underestimate the Gd content, suggesting a potential compartmentation of Cs(+) and/or the contrast agent. Magn Reson Med 45:711-715, 2001.

Nuclear magnetic resonance and oxygen affinity study of cesium binding in human erythrocytes

We investigated the interaction of the cesium ion (Cs(+)) with the anionic intracellular components of human red blood cells (RBCs); the components studied included 2,3-bisphosphoglycerate (BPG), ADP, ATP, inorganic phosphate (P(i)), carbonmonoxy hemoglobin (COHb), and RBC membranes. We used spin-lattice (T(1)) and spin-spin (T(2)) (133)Cs NMR relaxation measurements to probe Cs(+) binding, and we found that Cs(+) bound more strongly to binding sites in BPG and in RBC membranes than in any other intracellular component in RBCs at physiologic concentrations. By using James-Noggle plots, we obtained Cs(+) binding constants per binding site in BPG (66 +/- 8 M(-1)), ADP (19 +/- 1 M(-1)), ATP (25 +/- 3 M(-1)), and RBC membranes (55 +/- 2 M(-1)) from the observed T(1) values. We also studied the effect of Cs(+) on the oxygen (O(2)) affinity of purified Hb and of Hb in intact RBCs in the absence and in the presence of BPG. In the absence of BPG, the O(2) affinity of Hb decreased upon addition of Cs(+). However, in the presence of BPG, the O(2) affinity of Hb increased upon addition of Cs(+). The O(2) affinity of Cs(+)-loaded human RBCs was larger than that of Cs(+)-free cells at the same BPG level. (31)P NMR studies on the pH dependence of the interaction between BPG and Hb indicated that the presence of Cs(+) resulted in a smaller fraction of BPG available to bind to the cleft of deoxyHb. Our NMR and O(2) affinity data indicate that a strong binding site for Cs(+) in human RBCs is BPG. A partial mechanism for Cs(+) toxicity might arise from competition between Cs(+) and deoxyHb for BPG, thereby increasing oxygenation of Hb in RBCs, and thus decreasing the ability of RBCs to give up oxygen in tissues. The presence of Cs(+) at 12.5 mM in intact human RBCs containing BPG at normal concentrations did not, however, alter significantly the O(2) affinity of Hb, thus ruling out the possibility of Cs(+)-BPG interactions accounting for Cs(+) toxicity in this cell type.

Short-term regulation of endothelial Na(+)-K(+)-pump activity by cGMP: a 133Cs magnetic resonance study

The effect of nitric oxide radicals (NO) on the activity of porcine aortic endothelial Na(+)-K(+)-ATPase is reported. Measurements were made using an in vitro cell system and 133Cs magnetic resonance (NMR). It is shown that NO, through stimulation of guanylate cyclase, results in a reduction of pump activity. Similar observations were made using 8-Br-cGMP. Measurement of the cytosolic volume indicated no changes in volume during incubation with 8-Br-cGMP. Our measurements indicate a continuous regulation of endothelial Na(+)-K(+)-ATPase activity by endogenous NO. This regulation could be removed by L-NAME, resulting in a small increase in pump activity.

Nuclear magnetic resonance studies of cesium-133 in the halophilic halotolerant bacterium Ba1. Chemical shift and transport studies

Ba1 bacteria (Halomonas israelensis) were grown on different salt concentrations 0.2-4 M. When the cells were transferred to a medium containing 25 mM CsCl without potassium there was an uptake of cesium by the cells. The intracellular cesium signal was shifted from the cesium signal in the medium without the use of a shift reagent. The shift was depended on the salt concentration in the growth medium. The intracellular cesium shift showed a much smaller dependence on the concentration of salts in the medium than the extracellular cesium; the same results were detected for cells grown on a medium containing 25 mM CsCl. The cesium transport through the cell membrane was mostly by active transport. The cesium concentration in the cell was higher than that of the medium, approximately 100 mM intracellular concentration compared to 25 mM in the medium. The first order constants for influx or efflux of cesium were from 2 x 10(-4) and up to 24 x 10(-4)/min for the different medium concentrations.

Regulation of endothelial Na(+)-K(+)-ATPase activity by cAMP

Using an in vitro cell system and Cs+ NMR techniques we were able to show that porcine aortic endothelial cells (PAEC) reduce their Na(+)-K(+)-ATPase activity upon an increase in intracellular cAMP. Reduction in the pump rate was due to phosphorylation of the alpha-subunit of the ATPase as shown by immunoprecipitation. Apart from a pump inhibiton using 8-Br-cAMP and IBMX, we were also able to show that changes in the Na(+)-K(+)-ATPase activity could be mediated by the adenosine-A2 and prostaglandin receptor agonists 5'-N-Ethylcarboxamidoadenosine and Iloprost, respectively. Parallel to a decrease in pump activity we also observed a decrease in intracellular Cs+, indicating opening of K+ channels.

A 133Cs nuclear magnetic resonance study of endothelial Na(+)-K(+)-ATPase activity: can actin regulate its activity?

Using (133)Cs+ NMR, we developed a technique to repetitively measure, in vivo, Na(+)-K(+)-ATPase activity in endothelial cells. The measurements were made without the use of an exogenous shift reagent, because of the large chemical shift of 1.36 +/- 0.13 ppm between intra- and extracellular Cs+. Intracellularly we obtained a spin lattice relaxation time (T1) of 2.0 +/- 0.3 s, and extracellular T1 was 7.9 +/- 0.4 s. Na(+)-K+ pump activity in endothelial cells was determined at 12 +/- 3 nmol Cs+ x min(-1) x (mg Prot)[-1] under control conditions. When intracellular ATP was depleted by the addition of 5 mM 2-deoxy-D-glucose (DOG) and NaCN to about 5% of control, the pump rate decreased by 33%. After 80 min of perfusion with 5 mM DOG and NaCN, reperfusion with control medium rapidly reestablished the endothelial membrane Cs+ gradient. Using (133)Cs+ NMR as a convenient tool, we further addressed the proposed role of actin as a regulator of Na(+)-K+ pump activity in intact cells. Two models of actin rearrangement were tested. DOG caused a rearrangement of F-actin and an increase in G-actin, with a simultaneous decrease in ATP concentration. Cytochalasin D, however, caused an F-actin rearrangement different from that observed for DOG and an increase in G-actin, and cellular ATP levels remained unchanged. In both models, the Na(+)-K(+)-pump activity remained unchanged, as measured with (133)Cs NMR. Our results demonstrate that (133)Cs NMR can be used to repetitively measure Na(+)-K(+)-ATPase activity in endothelial cells. No evidence for a regulatory role of actin on Na(+)-K(+)-ATPase was found.

Inhibition of ion transport in septic rat heart: 133Cs+ as an NMR active K+ analog

Sepsis, the systemic response to severe infection, and the resulting multiorgan failure it induces are major contributors to intensive care unit morbidity and mortality. A number of abnormalities in ion transport processes and intracellular free Na+ ([Na+]i) and K+ ([K+]i) concentrations have been reported to occur during sepsis/endotoxemia. An effect of sepsis on the NA(+)-K(+)-ATPase may be an important contribution to changes in intracellular ion balance and the resultant pathophysiology of the disorder. The purpose of this study was to examine the effect of sepsis on the Na(+)-K(+)-ATPase in the isolated perfused rat heart using 133Cs+ nuclear magnetic resonance (NMR). Cs+ is a K+ analog, and 133Cs-NMR offers the opportunity to examine Na(+)-K(+)-ATPase activity in the intact organ via tracer kinetics. Sepsis was induced in halothane-anesthetized male Sprague-Dawley rats using the cecal ligation and perforation (CLP) model. Twenty-four to thirty-six hours after surgery, hearts from CLP or sham-operated rats were perfused with Krebs-Henseleit buffer containing 1.25 mM Cs+. The influx rate constant for Cs+ was decreased by 24% in septic rat hearts, i.e., 0.25 +/- 0.08 (SD) min 1 for controls and 0.19 +/- 0.04 (SD) min-1 for septic animals (P = 0.003). There was no difference for Cs+ efflux [0.005 +/- 0.001 (SD) min-1 for controls and 0.005 +/- 0.002 (SD) min-1 for septic animals; P = 0.8]. These results are consistent with an inhibition of the Na(+)-K(+)-ATPase pump during sepsis/endotoxemia. A decrease in the activity of the Na(+)-K(+)-ATPase pump may be responsible for or contribute to the changes in [Na+]i and [K+]i during the disorder.

Evaluation of intracellular diffusion in normal and globally-ischemic rat brain via 133Cs NMR

The question of whether the apparent diffusion coefficient (ADC) of intracellular water changes after brain injury was addressed by using 133Cs as an indicator to report on the state of the intracellular environment. Cesium is an NMR-detectable potassium analog that accumulates in the intracellular space and is detectable in rat brain after being added to the animal's diet. The ADC of cesium was measured before and after the death of the rat. The cesium ADC fell from 0.91 +/- 0.05 x 10(-3) mm2/s (mean +/- SEM, n=5) in the alive rat to 0.71 +/- 0.05 x 10(-3) mm2/s within 20 min (the best time resolution of the experiment) of the death of the animal and stayed at this value for at least 3 h (p < 0.001). Assuming that the ADC of cesium reflects motion in the intracellular environment, these results support the idea that there are changes associated with cell injury that would cause a reduction in the ADC of intracellular water. Hence, one factor contributing to the decrease in water ADC after brain injury is a change in the ADC of intracellular water.

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. CsCI 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 (mumol/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.