Changes in muscle proton transverse relaxation times and acidosis during exercise and recovery

Abstract

We studied changes in muscle proton (1H) transverse relaxation times (T2) by magnetic resonance imaging during exercise and compared these changes with alterations in muscle metabolism measured by phosphorus-31 magnetic resonance spectroscopy (31P-MRS). Eleven subjects completed two trials of intermittent incremental forearm wrist flexion exercise requiring 30 contractions/min for 5 min, 7 min of recovery between stages, and 5-N load increments/stage. Between stages of the first trial, T2 images of muscle 1H were obtained. Muscle T2 increased from 27.3 +/- 1.1 (SD) ms at rest to 35.8 +/- 3.6 ms after volitional fatigue (P < 0.05), whereas less active wrist extensor muscle T2 remained unchanged (26.8 +/- 0.9 to 28.8 +/- 1.6 ms; P > 0.05). After localizing the predominant muscle recruited from the T2 images, subjects completed an identical trial at least 1 wk later but involving surface coil 31P-MRS of the T2-enhanced muscle to measure the H+ concentration ([H+]). Intramuscular [H+] of T2-enhancing muscle increased from 1.1 +/- 0.1 x 10(-7) M at rest to 4.1 +/- 2.0 x 10(-7) M after volitional fatigue. Both muscle T2 and intramuscular [H+] increased in a bimodal manner, with T2 increasing before muscle [H+] (P < 0.05). The correlation coefficient between the percent change in T2 and muscle [H+] during exercise was +0.74 (range 0.48-0.98; P < 0.05) and +0.47 during recovery. After 12 min of recovery, muscle [H+] decreased to 1.4 +/- 0.3 x 10(-7) M (P < 0.05), and T2 remained close to postexercise values (32.2 +/- 3.1 ms, P > 0.05). The data indicate that 1) the T2 increases during increases in exercise intensity are nonlinear, 2) the T2 increases during exercise are significantly correlated with increases in [H+], and 3) the slow recovery of T2 compared with [H+] indicates that [H+] has a minor contribution to the recovery in T2.

Muscle Functional MRI as an Imaging Tool to Evaluate Muscle Activity

“…They demonstrated a linear association for the lumbar paraspinals (R 2 = 0.92, P.001) and revealed that, for both muscles, an increase of 10% exercise intensity corresponds with an increase of the T2 value of 1.18 (95% CI: 0.89, 1.47) milliseconds. Studies by Fleckenstein et al, 33 Mayer et al, 42 and Cheng et al, 14 however, did not support a linear relationship between T2 times and all levels of intensity of muscular activity but, rather, supported a sigmoid-shaped relationship. Differences in outcomes between these studies may be attributed to differences in their statistical approach (linear regression analysis versus mixed-model analysis) and methodology.…”

Nonischemic Myocardial Changes Detected by Cardiac Magnetic Resonance in Critical Care Patients with Sepsis

“…Although there are studies that have identified such mediators as biomarkers for the diagnosis and/or grading of asthma (7,8), prospective data on their predictive value for asthma are scarce (9). We therefore investigated whether plasma cytokine levels in EW could be used in early prediction of allergic asthma (AA).…”

Intracellular acidification and volume increases explain R2 decreases in exercising muscle

“…T 2 is inversely related to intracellular pH (pH i ) in chemically-skinned rabbit psoas muscle (12) and in isolated rat liver (13). End-exercise pH i and T 2 are also correlated (2,14); however, during incremental arm ergometer exercise, pH i changes lag T 2 changes, and during recovery from this exercise, pH i recovers more quickly than T 2 (15). Together, these findings imply that pH i changes may contribute to, but cannot be the sole cause of, the T 2 increase during exercise.…”