Fiber type and metabolic dependence of T2 increases in stimulated rat muscles

This study examined the relationships between muscle fiber type, metabolism, and blood flow vs. the increase in skeletal muscle 1H-NMR transverse relaxation time (T2) after stimulation. Triceps surae muscles of anesthetized rats were stimulated in situ at 1–10 Hz for 6 min, and T2 was calculated from1H-NMR images acquired at 4.7 T immediately after stimulation. At low-to-intermediate frequencies (1–5 Hz), the stimulation-induced T2 increase was greater in the superficial, fast-twitch white portion of the gastrocnemius muscle compared with the deeper, more aerobic muscles of the triceps surae group. Although whole triceps muscle area changed in parallel with T2 after stimulation when blood flow was intact, clamping of the femoral artery during stimulation prevented an increase in muscle area but not an increase in T2. Partial inhibition of lactic acid production with iodoacetate diminished intracellular acidification (measured by 31P-NMR spectroscopy) during brief (1.5 min) stimulation but had no significa...