Proton NMR spectroscopy and imaging of the rabbit kidney, in vivo, at 4.7 tesla

Abstract

Osmotically active organic solutes such as trimethylamines and polyols have been implicated in cell volume regulation in a variety of organisms, ranging from bacteria to mammals (I, 2). To avoid losing water in a hypertonic envionment (i.e., osmolality >300 mOsm/liter), animal cells must offset the osmotic pressure of their extracellular surroundings. Many cells have osmotically active organic solutes (“osmolytes”) to regulate their intracellular osmolality and hence their cell volume. The advantage of these organic solutes is that they do not increase intracell~ar ionic strength and thereby do not alter enzyme function; furthermore, studies have shown that these compounds protect enzyme function from the denaturing effects of urea which diffuses freely across cell membranes (I). Medullary cells of the mammalian kidney are exposed to a wide range of extracellular osmolalities, ranging from 50 to greater than 2000 mOsm/liter. To cope with this labile environment, renal medullary cells regulate the concentration of their osmolytes. The predominate osmolytes of the rabbit renal medulla include the trimethylamines betaine and glycerophosphorylcholine and the polyols sorbitol and inositol (2, 3). Averaged over the whole kidney, the concentration of these osmolytes is several millimolar; however, because these compounds are confined within the cells of the renal medulla, their local concentration may well exceed several hundred millimolar. Despite excellent sensitivity, in vivo ‘H NMR spectroscopy has lagged behind studies with other nuclei, such as 13C and 3’P, because of the dynamic range problem associated with a 110 A4 water signal and the limited spectral resolution afforded by a small chemical-shift range. These problems have been ameliorated by various techniques, such as binomial solvent suppression, soft pulse saturation, spin-echo relaxation, jump and-return-echo, and DANTE with homonuclear ‘H double-resonance editing sequences (4-10). Most of the in vivo ‘H NMR spectroscopy to date has been used to study brain metabolism (5, 7, 10). However, the sensitivity of the ‘H nucleus and the high concentration of osmolytes in the kidney provided good reason to try to use ‘H NMR spectroscopy to monitor osmoly-te concentrations. We have developed a kidney model in which ‘H NMR spectroscopy can monitor the regulation of these compounds, in vivo.