13C NMR spectroscopy as a tool to investigate renal metabolism

During the past 10 years NMR spectroscopy has been established as a non-invasive tool for physiological and biochemical investigations on living material [1,2]. Depending on the phenomenon to be studied, one of the following NMR-sensitive nuclei can be used: phosphorous (31P), sodium (23Na), the proton (1H), and carbon (13C).Renal cell metabolism [3] has thus far been characterized primarily by measuring substrate concentrations, concentrations of high energy phosphates, or enzyme activities in vitro. These parameters describe a certain metabolic state of a cell at a given time, but fail to reveal fluxes through metabolic pathways as they occur in the living cell. Flux studies are, however, possible when labelled substrates are used. Such studies using 14C-labelled compounds have revealed some overall features of metabolism; a quantitation of the relative velocities of the various pathways has not been possible because of the difficulties to isolate the labelled intermediates and to determine the site of labelling within the molecules. Such difficulties can be surmounted by using 13C-labelled substrates where each C atom, when labelled during its utilization in the cell, exhibits a unique resonance, thus making the mapping of intracellular pathways of a certain carbon possible. This review focuses on the application of 13C NMR spectroscopy for the elucidation of renal metabolic pathways. After providing a short description of the general properties of 13C NMR spectroscopy, studies on the proximal tubule [4–9] are used to examplify the advantage of the 13C spectroscopy compared to the more commonly used 14C method. These advantages are considered sufficient to compensate for the high cost of 13C-labelled compounds and the instrumentation required for the measurement.In more general terms, 13C NMR spectroscopy can be considered as a valuable addition to NMR experiments where other nuclei have been used. One of the most frequent nucleus used in non-invasive measurements is 31P, which allows the measurement of the concentration of energy-rich phosphates and the ATP turnover in renal tubule suspensions, intact kidneys or isolated cells. Another parameter for non-invasive investigations of renal transport linked to metabolism is 23Na, where changes in intra- and extracellular sodium can be recorded. The application of the latter two methods has been described recently [10–16]