Effects of temperature and field strength on the NMR relaxation times of 23Na in frog striated muscle

Pulsed NMR techniques have been applied to the study of the relaxation parameters characterizing ²³Na within frog striated muscle. Experiments were performed at 3°C, 22–24°C and 39°C at a Larmor frequency of 15.7 MHz; at 22–24°C, measurements were obtained both at 15.7 MHz and at 7.85 MHz.As previously reported, only a single spine-lattice relaxation time (T₁) was observed, but both slow (T₂)_I and fast (T₂)_II components of the spin-spin relaxation time were measured. The effect of temperature (θ) upon (1/T₁) was qualitatively similar to that reported for ²³Na in free solution; (θ) did not significantly affect (1/T₂) over the range of temperatures studied. (1/T₂)_I, and to a lesser degreee, (1/T₁) exhibited a modest inverse dependence of doubtful significance on the Larmor frequency.The data are examined within the framework of a simple specific model; a conservative values in assumed for the quadrupolar coupling constant characterizing immobilized intracellular Na⁺. Within this framework, the results suggest that the fraction of bound ions whose molecular tumbling is severely restricted does not exceed some few percent of the total sodium population.