Validity of in vivo nuclear magnetic resonance methods in measurement of intracellular water and sodium

31P and 23Na nuclear magnetic resonance study on forebrain ischemia in rats with shift reagent Dy(TTHA)

31P and 23Na nuclear magnetic resonance (NMR) spectroscopy was employed to study the dynamic changes in intracellular high-energy phosphates and sodium during 15 min of forebrain ischemia and recirculation in in vivo rat brain. In the presence of the shift reagent Dysprosium triethylenetetramine-N,N,N",N",N"',N"'-hexaacetic and [Dy(TTHA)], the sodium peak separated into two peaks, unshifted and shifted. During 15 min of ischemia, the unshifted sodium peak decreased and the shifted sodium peak increased. With recirculation, the unshifted and the shifted sodium peaks returned to the preischemia level within 10 min, but the shifted one increased during 30-60 min. Intracellular high-energy phosphates and intracellular pH (pHi) decreased during 15 min of ischemia and returned to the preischemia levels within 20 min of recirculation. We conclude that the decrease in unshifted sodium peak during ischemia is due to the decrease in subarachnoid sodium and the cellular influx of interstitial sodium would be minimum. The increase in shifted sodium peak during ischemia is considered to be due to the dilatation of cerebral blood vessels and the increase in interstitial sodium which was transported from subarachnoid space.

Shift reagent enhanced concurrent 23Na and 1H magnetic resonance spectroscopic studies of transcellular sodium distribution in the dog brain in vivo

The intracellular to extracellular sodium distribution is one of the primary determinants of action potentials necessary for the electrical function of organs such as brain, heart and skeletal muscle. The ability of shift reagent enhanced 23Na MRS to directly measure the intracellular and extracellular sodium distribution in brain is controversial and centers on the relative contributions of bulk magnetic susceptibility and hyperfine interactions to the observed chemical shifts. In this study, infusion of dysprosium (III) triethylenetetraminehexacetate (Dy(TTHA)-3), resulted in a 23Na MRS spectrum of dog brain with two well resolved peaks at 9 and 0.4 ppm. The 9 ppm peak corresponded to the resonance seen in aspirated blood. After disruption of the blood brain barrier, the single peak at 0.4 ppm split into two peaks at 3 and 0 ppm. The ability of Dy(TTHA)-3 enhanced 23Na MRS to follow global changes in brain sodium distribution was tested during cardiac arrest. The expected rapid Na influx into the intracellular space produced a marked decrease in the 3 ppm signal and a parallel increase in the 0 ppm peak. This is consistent with the assignment of the 3 ppm peak as interstitial sodium and the 0 ppm peak as intracellular sodium.

Two-site exchange revisited: a new method for extracting exchange parameters in biological systems

A new analysis is presented which links real volume fractions, relaxation rates, and intracompartmental exchange rates directly with apparent volume fractions and relaxation rates obtained from biexponential fits of transverse magnetization decay curves. The analysis differs from previous methods in that measurements from two paramagnetic doping levels are used to close the two-site exchange equations. Both the new method and one previously described by Herbst and Goldstein (HG) have been applied to paramagnetically doped whole-blood data sets. Significant differences in the calculated exchange parameters are found between the two methods. A small dependence of the intracellular relaxation rate on extracellular paramagnetic agent concentration, assumed nonexistent with the HG method, is inferred from the new analysis. The analysis was also applied to published data on perfused rat hearts, and we obtained a limited assessment of two-site exchange in this system.