On Intact Tissues

In recent years the applications of NMR spectroscopy to the analysis of living systems have grown at a rapid pace. Initially these measurements were limited to excised tissues, and later extended to studies of perfused organs. With the new improvements in magnet technology and FT NMR techniques, in vivo measurements of intact animals and humans became possible. Most of these studies concentrated on the measurement of endogenous phosphorylated compounds with P NMR spectroscopy which provided information on the bioenergetic and metabolic status of the tissues. Several excellent reviews on this topic are available (1,2). Other nuclei such as C, Na, H, and F have also been used as probes of metabolic changes taking place in tissues under varied physiological and pathological conditions. In vivo F NMR spectroscopy is a relatively new technique, with the first F surface coil study of an intact animal reported in 1983 (3). This nuclide is well suited to study the metabolic fate of exogenously administered substances such as flourinated drugs, many of which are in clinical use. Fluorine19 is the 100% abundant isotope of elemental fluorine, has a spin 1/2, NMR sensitivity about 83% that of proton and it occurs in living systems in trace amounts. The large chemical shift range of F (over 900 pprn) permits spectroscopic differentiation of similar compounds even at the lower field strengths used in the in vivo experiments. These characteristics make F an excellent NMR probe with which to monitor the distribution, retention or metabolism of drugs (or other compounds) in specific organ systems. Thus, it provides a potentially powerful method to evaluate the biochemical mechanisms involved in these processes. This article discusses applications of in vivo F NMR spectroscopy to the metabolic studies of three distinct classes of fluorinated compounds of biological importance. These are: fluorodeoxyglucose an-