Intracellular acidification and volume increases explain R₂ decreases in exercising muscle

Abstract

Exercise-induced decreases in the 1 H transverse relaxation rate (R 2 ) of muscle have been well documented, but the mechanism remains unclear. In this study, the hypothesis was tested that R 2 decreases could be explained by pH decreases and apparent intracellular volume (V i ) increases. 31 P and 1 H spectroscopy, biexponential R 2 analysis, and imaging were performed prior to and following fatiguing exercise in iodoacetate-treated (IAA, to inhibit glycolysis), NaCN-treated (to inhibit oxidative phosphorylation), and untreated frog gastrocnemii. In all exercised muscles, the apparent intracellular R 2 (R 2i ) and pH decreased, while intracellular osmolytes and V i increased. These effects were larger in NaCN-treated and untreated muscles than in IAAtreated muscles. Multiple regression analysis showed that pH and V i changes explain 70% of the R 2i variance. Separate experiments in unexercised muscles demonstrated causal relationships between pH and R 2i and between V i and R 2i . These data indicate that the R 2 change of exercise is primarily an intracellular phenomenon caused by the accumulation of the end-products of anaerobic metabolism. In the NaCN-treated and untreated muscles, the R 2i change increased as field strength increased, suggesting a role for pH-modulated chemical exchange. Key words: skeletal muscle; exercise; hydrogen-ion concentration; osmolarity; nuclear magnetic resonance Exercise-induced increases in the apparent 1 H transverse relaxation time (T 2 ) of muscle water have been well documented. The amount of T 2 change increases as exercise intensity increases; T 2 plateaus 3-4 min after the onset of exercise (1). Following isometric leg extension to fatigue, T 2 recovery follows an approximately exponential time course and is complete after ϳ35 min. (2). The increase in signal intensity from active muscles in T 2 -weighted images allows muscle activity to be detected reliably and noninvasively, which may aid in the placement of regions of interest or surface coils in spectroscopic studies (2,3). With a full understanding of the mechanism(s) of the T 2 change, this phenomenon may also be useful in functional studies of muscle activation during exercise (e.g., Refs. 4 -6), noninvasive studies of pathologic conditions in which the T 2 response to exercise is altered (e.g., Refs. 7 and 8), and as a means of studying noninvasively those physiological changes that increase T 2 .The most universally offered and best-studied explanation for the T 2 increase during exercise is the concomitant increase in muscle volume. However, the mechanism of the T 2 increase is more complex than total water accumulation. For example, recovery of the muscle's anatomical cross-sectional area is faster than T 2 recovery (2), and enhancement of the extracellular fluid volume increase during exercise is not proportional to the T 2 increase (9). A further complication is that both ex vivo amphibian (10) and in vivo mammalian (11) studies suggest that exchange between the intra-and extracellular spaces in muscle is sl...

Cluster analysis of muscle functional MRI data

“…It has also been used to detect changes in muscle function consequent to clinical conditions such as peripheral vascular disease (31). The interpretation of these data, however, is limited by the absence of a full theoretical understanding of which physiological variables produce the mfMRI response and how their contributions may differ during and after exercise.Multiexponential relaxation analysis has shown that the major contribution to signal intensity (SI) changes in mfMRI is made by the increase in the transverse relaxation time constant (T2) of intracellular water protons (4,22,23). Proposed explanations for intracellular T2 increases include the accumulation of end products of cellular energy metabolism, which cause water to move into the cell (4, 21), decreased intracellular pH (4, 8), and water shifts from an intracellular compartment possessing a short T2 (ϳ20 ms) to a second intracellular compartment having a longer T2 (ϳ40 ms) (22, 23).…”

“…Multiexponential relaxation analysis has shown that the major contribution to signal intensity (SI) changes in mfMRI is made by the increase in the transverse relaxation time constant (T2) of intracellular water protons (4,22,23). Proposed explanations for intracellular T2 increases include the accumulation of end products of cellular energy metabolism, which cause water to move into the cell (4,21), decreased intracellular pH (4,8), and water shifts from an intracellular compartment possessing a short T2 (ϳ20 ms) to a second intracellular compartment having a longer T2 (ϳ40 ms) (22,23).…”

“…Proposed explanations for intracellular T2 increases include the accumulation of end products of cellular energy metabolism, which cause water to move into the cell (4,21), decreased intracellular pH (4,8), and water shifts from an intracellular compartment possessing a short T2 (ϳ20 ms) to a second intracellular compartment having a longer T2 (ϳ40 ms) (22,23). Any of these possible explanations, acting alone or in concert with the others, suggests that the intracellular T2 increase results from changes in the chemical behavior of water resulting from increased flux through energy metabolism pathways.…”

“…Increases in interstitial volume [such as those caused by exercise (24) or leg negative pressure (18)] increase SI in T2-weighted images (6,18), although the low interstitial volume fraction diminishes the importance of these changes (4,18). Recently, Hu et al (12) showed through the injection of an extracellular contrast agent that postexercise hyperemia affects SI in T2-weighted images as well.…”

Substrate Channelling in a Creatine Kinase System of Rat Skeletal Muscle Under Various pH Conditions

“…Non-invasive NMR studies revealed different values of acidification; however, all results are within a pH range of 6.0-7.0 (Meynial- Denis et al 1993;Mizuno et al 1994;Damon et al 2002). Iwanaga et al (1991) described an even more significant pH decrease after the contraction of muscle using 31 P-MRS spectra measurement.…”