Disuse Versus Reloading Induced Change In Energy Rich Phosphate Content In Human Skeletal Muscle

Abstract

Cast immobilization is associated with decreases in muscle contractile area, specific force and functional ability. Additionally, recent studies have shown that following immobilization, there is a change in the resting energy rich phosphate content. Specifically, there is an elevation in the inorganic phosphate (Pi) concentration, which may contribute to the loss in force production. The mechanism underlying the change in energy rich phosphate content following immobilization is not clear. PURPOSE To determine if the change in resting energy rich phosphate content following immobilization is related to disuse or reloading. METHODS Six patients with a surgically stabilized, unilateral ankle malleolar fracture participated in the study. Patients were immobilized for about six weeks, followed by reloading. Image guided localized 31-P magnetic resonance spectroscopy data were acquired using chemical shift imaging from the medial gastrocnemius muscle in a 1.5 Tesla magnet. Measurements were performed during immobilization (at 3 weeks), at the end of immobilization, and at 2–4 days of reloading. The subjects' uninvolved leg was used for comparison purposes. Resting values of Pi/ATP, phosphocreatine (PCr) /ATP and Pi/PCr ratios were determined. Intracellular pH was calculated based on the chemical shift of the Pi peak. T-tests were used for comparisons with significance set at p < 0.05. RESULTS A progressive increase in the Pi /ATP and the Pi /PCr ratios was noted in the involved limb during immobilization and subsequent reloading. The greatest increase occurred during the immobilization period (Pi /ATP: 70 %; Pi/PCr: 65%). Significant differences were noted between the immobilized and uninvolved limbs at 6 weeks of immobilization for both the Pi/ATP (0.91±0.08 vs. 0.47±0.05; p < 0.05) and the Pi /PCr ratios (0.19±0.03 vs. 0.11±0.01, p < 0.05). However, from 6 weeks of immobilization to 2–4 days of reloading, no significant changes were observed in the Pi /ATP (p = 0.26) and Pi /PCr ratios (p = 0.29), although the average values increased by an additional 30% and 35% for Pi /ATP and Pi /PCr ratios, respectively. No significant changes were noted in the resting PCr /ATP ratio or basal pH at any time point. CONCLUSION Our findings suggest that the shift in energy rich phosphate content following immobilization is mainly a consequence of pathophysiological processes during disuse. Supported by NIH- RO1HD37645, NIH-RO1HD40850.