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.