Temperature dependence of monovalent cation fluxes in isolated rat hearts: a magnetic resonance study

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.

Brain energetics and tolerance to anoxia in deep hypothermia

The remarkable time-resolution enhancement by deep lethargic hypothermia (15 degrees C rectal temperature, "cold narcosis," "anesthesia by internal cold") of metabolic events in the rat brain after oxygen deprivation has been exploited to monitor metabolic changes by in vivo (31)P-NMR. A correlation was established between the bioenergetic status of the brain and physiological descriptors of tolerance (survival and revival times) determined in parallel experiments with large series of animals. Spectral peak integrals were transformed into absolute concentrations by comparison to biochemically determined time series of data obtained in freeze-trapping experiments conducted under identical conditions. Serial spectra were used to reconstruct the time-course kinetics of intracellular brain pH and of concentration changes of inorganic phosphate, phosphocreatine, ATP, and ADP. Both the biochemical and NMR time series of data were simultaneously fitted by a set of exponential kinetic equations accounting for relationships imposed by the Lohmann and adenylate kinase reactions. Depletion profiles were then computed for a number of descriptors of brain energy status (energy charge, phosphorylation potential, total adenylate, and primary energy stores expressed as the sum of high-energy phosphate-bond equivalents). The results contribute to the understanding of the role of brain energetics in tolerance to oxygen deprivation.

Biomedical applications of 7Li NMR

The biomedical applications of 7Li MRS and MRI have been progressing slowly. The interest derives primarily from the clinical use of Li to treat bipolar disorder. One area of concern is the nature of ionic transport and binding, so as to elucidate the mechanism(s) of therapeutic action and toxicity. Another is the development of a non-invasive, in vivo analytical tool to measure brain Li concentration and environment in humans, both as an adjunct to treatment and as a mechanistic probe. Here we review the most recent progress toward these goals.

Rubidium-87 magnetic resonance spectroscopy and imaging for analysis of mammalian K+ transport

This review summarizes results 87Rb MRS/I studies of K+ transport in mammalian cells, organs and in vivo. It provides a brief description of K+ transport systems, their interactions with Rb+ and evidence that Rb+ is a best K+ congener. 87Rb MR studies have focused mostly on isolated perfused rat and pig hearts and to a lesser extent on kidney, skeletal muscle, salivary gland and red blood cells. The method has been used for three purposes: measurements of kinetics of unidirectional Rb+ uptake and efflux and steady-state Rb+ levels. In cardiovascular studies Rb+ has been used in the absence of shift reagent taking advantage of the predominantly intracellular Rb+/K+ distribution (approximately 20:1). Pharmacological analysis of Rb+ uptake and efflux allowed assessment of the contributions of various transporters to the total Rb+ fluxes in rat hearts. It was confirmed that Na+/K+ ATPase is responsible for the majority of K+ influx since Rb+ uptake is 80% ouabain-sensitive and dependent on the intracellular [Na+]. Energy deprivation caused by low-flow ischemia or metabolic inhibition reduced Rb+ uptake rate. Under normal conditions, Rb+ efflux is mediated mainly by voltage-gated K+ channels with a small contribution from the K+/Na+/2Cl- cotransporter. Intracellular alkalosis and osmotic swelling stimulated Rb+ efflux by activation of the putative K+/H+ antiporter. Activity of ATP-sensitive K+ (K(ATP)) channels was revealed by metabolic (2,4-dinitrophenol, ischemia) or pharmacological (K(ATP) opener, P-1075) stimulation of Rb+ efflux, which was reversed by the K(ATP) blocker, glibenclamide. Mitochondrial K+ transport was evaluated in hearts with saponin-permeabilized myocytes and under hypothermic conditions.Three-dimensional (3-D) spectroscopic MRI of isolated beating pig hearts has been used to obtain time series of Rb+ maps of normal and ischemic/infarcted hearts, which showed lower image intensity in the damaged area. Kinetics of Rb+ uptake in the ischemic areas depended on both regional flow and metabolism. The adrenergic agonist dobutamine stimulated Rb+ uptake in normal areas and did not affect uptake in ischemic areas. Drugs that may affect passive Rb+ transport (bumetanide, pinacidil, glibenclamide) did not change Rb+ uptake either in the normal or ischemic zones. 87Rb-MRI was also able to localize ischemia and infarction in blood-perfused hearts. 87Rb MRS/I is an excellent non-invasive research tool for studies of K+ transport in isolated organs and in vivo.

The Gárdos channel: a review of the Ca2+-activated K+ channel in human erythrocytes

Ca(2+)-dependent K(+) efflux from human erythrocytes was first described in the 1950s. Subsequent studies revealed that a K(+)-specific membrane protein (the Gárdos channel) was responsible for this phenomenon (the Gárdos effect). In recent years several types of Ca-activated K(+) channel have been identified and studied in a wide range of cells, with the erythrocyte Gárdos channel serving as both a model for a broader physiological perspective, and an intriguing component of erythrocyte function. The existence of this channel has raised a number of questions. For example, what is its role in the establishment and maintenance of ionic distribution across the red cell membrane? What role might it play in erythrocyte development? To what extent is it active in circulating erythrocytes? What are the cell-physiological implications of its dysfunction?This review summarises current knowledge of this membrane protein with respect to its function and structure, its physiological roles (some putative) and its contribution to various disease states, and it provides an introduction to adaptable NMR methods, which is our own area of technical expertise, for such ion transport analysis.

Observation of two inorganic phosphate NMR resonances in the perfused hypothermic rat heart

The effect of hypothermia on isolated perfused rat hearts was studied with 31P NMR. Hearts were continuously perfused with phosphate-free Krebs-Henseleit buffer while the perfusate temperature was adjusted. Perfusate pH was kept at 7.40 +/- 0.02 throughout the experiments. Using the chemical shift difference between PCr and Pi the intracellular pH was estimated. At 36, 20, and 10 degreesC a cytosolic alkalinization at a pH of 7.05 +/- 0.04, 7.21 +/- 0.05, and 7.40 +/- 0.03 was observed, respectively. At 10 degreesC two Pi resonances were observed with a separation of 0.25 ppm. This resonance corresponded to a Pi resonance of a cellular compartment with a local pH of 7.78 +/- 0.06, likely mitochondrial. This additional resonance disappeared upon warming of the hearts back to 36 degreesC.