NMR measurements of intra- and extravesicular sodium in renal microvilli

Noninvasive quantification of total sodium concentrations in acute reperfused myocardial infarction using 23Na MRI

The transport of sodium and potassium between the intra- and extracellular pools and the maintenance of the transmembrane concentration gradients are important to cell function and integrity. The early disruption of the sodium pump in myocardial infarction in response to the exhaustion of energy reserves following ischemia and reperfusion results in increased intracellular (and thus total) sodium levels. In this study a method for noninvasively quantifying myocardial sodium levels directly from sodium (23Na) MRI is presented. It was used to measure total myocardial sodium on a clinical 1.5T system in six normal dogs and five dogs with experimentally-induced myocardial infarction (MI). The technique was validated by comparing total sodium content measured by 23Na MRI with that measured by atomic absorption spectrophotometry (AAS) in biopsied tissue. Total sodium measured by 23Na MRI was significantly elevated in regions of infarction (81.3 +/- 14.3 mmol/kg wet wt, mean +/- SD) compared to noninfarcted myocardial tissue from both infarcted dogs (36.2 +/- 1.1, P < 0.001) and from normal controls (34.4 +/- 2.8, P < 0.0001). Myocardial tissue sodium content as measured by 23Na MRI did not vary regionally in the lateral, anterior, or inferior regions in normal hearts (ANOVA, P = NS). Sodium content measured by 23Na MRI agreed with the mean AAS estimates of 31.3 +/- 5.6 mmol/kg wet wt (P = NS) in normal hearts, and did not differ significantly from AAS measurements in MI (P = NS). Thus, local tissue sodium levels can be accurately quantified noninvasively using 23Na MRI in normal and acutely reperfused MI. The detection of regional myocardial sodium elevations may help differentiate viable from nonviable, infarcted tissue.

The longitudinal relaxation time (T1) of the intracellular 23Na NMR signal in the isolated perfused rat heart during hypoxia and reoxygenation

Each of six perfused rat hearts was subjected to 30 min of hypoxia followed by 60 min of reoxygenation. Inversion-recovery data on the intracellular Na NMR signal, differentiated by a shift reagent, 6 mM Dy(PPP)2, were obtained every 5 min, and T1 values were calculated. The T1 of the intracellular Na signal did not show any significant change either during hypoxia or upon reoxygenation, although the level of Nai increased about 50%. Such an increase of total Nai is expected to reduce the observed relaxation rate by diluting the fraction of Nai ions that interact with intracellular polyelectrolytes. The observed constancy of T1 in our study is explained on the basis of the typical values of the dissociation constants of sodium ions, in aqueous solutions, in interaction with polyelectrolytes. Although the constancy of intracellular sodium T1 during hypoxia may preclude the utilization of T1 weighting for the monitoring of pathology, its determination could be important for setting optimal acquisition times in high time-resolution experiments.

Phosphate-dependent sodium transport in S. faecalis investigated by 23Na and 31P NMR

Na+ movements in S. faecalis were studied by 23Na NMR. They proved to be dependent on phosphate concentration in the buffer during the de-energization step. K+ and H+ were also studied respectively by potentiometry and 31P NMR and were shown not to be implicated. For de-energized cells the internal phosphate concentration, on the contrary, was directly linked to the external phosphate contained in the buffer. The experiments showed a Na+/Pi dependence in this prokaryote so far known only in eukaryotes.