G. D. Marsh

Two-dimensional time correlation relaxometry of skeletal muscle in vivo at 3 Tesla

A hybrid two‐dimensional relaxometry (2DR) sequence was used to simultaneously measure both the spin‐spin (R2) and spin‐lattice relaxation rates (R1) of skeletal muscle in vivo. The 2DR sequence involved a 180° inversion pulse followed by a variable delay time (30 values from 40 to 7000 ms); a projection presaturation (PP) scheme to localize a 16‐ml cylindrical voxel; and a CPMG sequence (950 even echoes, effective echo spacing = 1.2 ms, equilibrium time = 12 s). The 2DR data were collected at 3.0 Tesla from the flexor digitorum profundus of eight healthy males, 26 ± 2 years old. Analysis was performed with a 2D version of the non‐negative least‐squares algorithm and a one‐way ANOVA. All subjects exhibited at least three spin‐groups (R2 < 200 s−1), designated B, C, and D, with R2 values of 42.7, 26.5, and 8.1 s−1, and fractional volumes of 52, 35, and 7%, respectively. The R1 values of B and C were similar, ≅0.7 s−1, but different from that of D (P < 0.001), which had an R1 of 1.0 s−1. The results suggest that exchange between B and C ranges from 0.7–16.2 s−1, while exchange between either of these spin‐groups with D is slower. If the data are interpreted with a compartment model, in which spin‐groups with short and long R2 values are attributed to extra‐ and intracellular fluid, respectively, the exchange of water across the cell membrane in living skeletal muscle is slow or intermediate relative to both R1 and R2. Magn Reson Med 46:1093–1098, 2001.

Muscle metabolic status and acid-base balance during 10-s work:5-s recovery intermittent and continuous exercise

Gastrocnemius muscle phosphocreatine ([PCr]) and hydrogen ion ([H(+)]) were measured using (31)P-magnetic resonance spectroscopy during repeated bouts of 10-s heavy-intensity (HI) exercise and 5-s rest compared with continuous (CONT) HI exercise. Recreationally active male subjects (n = 7; 28 yr ± 9 yr) performed on separate occasions 12 min of isotonic plantar flexion (0.75 Hz) CONT and intermittent (INT; 10-s exercise, 5-s rest) exercise. The HI power output in both CONT and INT was set at 50% of the difference between the power output associated with the onset of intracellular acidosis and peak exercise determined from a prior incremental plantar flexion protocol. Intracellular concentrations of [PCr] and [H(+)] were calculated at 4 s and 9 s of the work period and at 4 s of the rest period in INT and during CONT exercise. [PCr] and [H(+)] (mean ± SE) were greater at 4 s of the rest periods vs. 9 s of exercise over the course of the INT exercise bout: [PCr] (20.7 mM ± 0.6 vs. 18.7 mM ± 0.5; P < 0.01); [H(+)] (370 nM ± 13.50 vs. 284 nM ± 13.6; P < 0.05). Average [H(+)] was similar for CONT vs. INT. We therefore suggest that there is a glycolytic contribution to ATP recovery during the very short rest period (<5 s) of INT and that the greater average power output of CONT did not manifest in greater [H(+)] and greater glycolytic contribution compared with INT exercise.

Post-polio fatigue : a 31P magnetic resonance spectroscopy investigation

Changes in high energy phosphates (HEP) and intramuscular pH during exercise were measured in 17 patients with post-polio fatigue and in 28 healthy controls using 31P magnetic resonance spectroscopy (MRS). Subjects performed a dynamic hand grip exercise at low and high intensity. Mean changes in the HEP and pH showed no significant differences between the groups, although the post-polio groups response was highly variable. Six patients showed evidence of a lower lactate accumulation during the high intensity exercise when compared with controls. These data suggest that the whole body fatigue experienced by polio survivors is not related to any systemic metabolic abnormality.

Early Detection of Cancer Cachexia in the Rat Using 31P Magnetic Resonance Spectroscopy of the Liver and a Fructose Stress Test

The dynamic metabolic effects of a fructose infusion challenge on hepatic intracellular levels of adenosine 5′‐triphosphate (ATP), inorganic phosphate (Pi) and phosphomonoesters (PME) were monitored noninvasively by 31P MRS in a remote tumour‐bearing rat model. Fisher male rats were inoculated with a methylcholanthrene‐induced sarcoma. Seventeen rats were randomized into three groups: control (n=6), low tumour burden (LTB, n=6), or moderate tumour burden (MTB, n=5). The LTB group had tumour burdens of 0.2–2.0% while the MTB group had tumour burdens of 2.6–5.7%. All rats were in the pre‐clinical phase of cancer cachexia as determined by food intake and body weight. Rats were infused with 1.2 g/kg of fructose i.v. and the metabolic response of the liver was monitored with time over 1 h via 31P MRS. In all groups an immediate increase in hepatic levels of PME was noted, which returned to baseline values over the course of the experiment, reflecting the phosphorylation of fructose to fructose 1‐phosphate. For the MTB rats, the return to baseline levels was more rapid than in the control or LTB group. All groups experienced a 20% decrease in hepatic ATP levels which did not return to baseline over the 1 h observation period. As well, all groups experienced an initial fall in Pi, which recovered to prefructose levels or greater. MTB rats demonstrated a 30–40% increase in Pi concentration and a 60–70% increase in Pi/ATP ratio after infusion with fructose as compared to LTB and control rats (ANOVA; p<0.05). This is consistent with cachexia‐induced enhancement of hepatic gluconeogenic activity, and hence more rapid release of Pi from the phosphorylated metabolites in the MTB rats. Thus fructose infusion and hepatic 31P MRS permit pre‐clinical detection of cancer cachexia as reflected by increased Pi generation and more rapid removal of PME.