Nuclear magnetic resonance studies of sodium/calcium exchange in frog perfused, beating hearts

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.

Characteristics of extracellular sodium relaxation in perfused hearts with pathologic interventions

Sodium spectroscopy and imaging sequences designed to emphasize fast T2 decay or the multiple quantum signal have previously demonstrated a high contrast between normal and pathologic tissue which may be due to changes in intracellular versus extracellular sodium distribution. Since alterations in the amount of signal with fast T2 decay have previously been shown to occur with changes in intracellular sodium content, this study investigated the fast T2 relaxation characteristics of extracellular sodium during pathologic interventions on nonsubmerged perfused rat hearts. T2 data on total sodium content were obtained while global ischemia (stopping all perfusate flow) and extracellular edema (due to long perfusion times) were induced in the heart. The data were fit to a biexponential, with Mf(T2f) the magnitude (time constant) of the fast component of decay. Mf increased significantly in both pathologies (to 319 +/- 26%, n = 3, of baseline for ischemia and to 527 +/- 284%, n = 3, of baseline for edema); the increase with edema was demonstrated to be due to extracellular sodium by intermittently perfusing the heart for a short period with shift reagent. When shift reagent was not used until the conclusion of the edema experiment, Mf increased to 169 +/- 35% of baseline, also due mainly to extracellular sodium. T2f did not exhibit any trends with these experiments, with values ranging from 1.7 to 5.5 ms. We believe that these results indicate that compartmental sodium content will most likely not be quantifiable in pathologic states in the heart with relaxation-based techniques. However, correlations between the pathologic state of the tissue and the sodium NMR signal obtained with pulse sequences or images that emphasize a particular aspect of relaxation may prove to be useful.