An analysis of intracellular ²³Na relaxation using the double-quantum filtered NMR signal from the perfused rat salivary gland

The intracellular sodium of the perfused rat mandibular salivary gland was measured by double-quantum filtered ²³Na-NMR spectroscopy at 2.34, 4.7 and 8.45 T. Biexponential relaxation of the intracellular ²³Na signal was observed, and its intensity was increased by administration of acetylcholine with ouabain at 25°C. The transverse and longitudinal relaxation rate constants were determined by the 'transverse experiment' (D-90°-τ/2-180°-τ/2-90°-δ-90°-acquire) and the 'longitudinal experiment' (D-180°-τ-54.7°-δ-90°-acquire), respectively. From observed dependencies on B₀ and temperature (5-37°C), a possibility of exchange between two populations of intracellular Na⁺ was suggested. A small fraction of Na⁺ is in the slow-motion condition (with a quadrupole coupling constant of approx. 1.75 MHz and a correlation time of 6·10^-8 s). The major portion of intracellular Na⁺ is in the extreme narrowing condition with a transverse relaxation rate constant of approx. 100 s^-1, which corresponds to a viscosity of approx. 5 cP.