Abstract

894 The basal phosphate content, and in particular the inorganic phosphate (Pi) to phosphocreatine (PCr) ratio (Pi/PCr) has been shown to increase during denervation atrophy. In addition, skinned muscle fiber studies have shown that Pi is an important inhibitor of force generation in skeletal muscle. The purpose of this study was to measure the basal phosphate content in patients with immobilization-induced atrophy and to determine its relationship to force development. 11 healthy control subjects (C) and 10 patients (A) whose lower legs were casted for a period of 6-8 weeks following a malleolus fracture were studied. 31P·Magnetic Resonance Spectroscopy (MRS) and peak torque measurements were performed at time points of 1, 5 and 10 weeks post-immobilization (PIM). 31P-MRS measurements were made of the medial gastrocnemius using a 2.0 Tesla magnet and a 6 x 8cm oblong surface coil. Rest spectra were acquired with a repetition time of 30s for a total of 9 min. Isometric peak torque was determined at 0 ° plantar flexion in both the involved and uninvolved leg. Comparison between the patients and controls showed a 50% increase in the basal Pi concentration(PiC=5.8±0.0mM, PiA=8.6±1.2mM) at 1w-PIM. The isometric peak torque at 1w-PIM was 75.7±19.7 Nm and 42.8±14.0 Nm in the uninvolved and the involved legs of the patients, respectively. During recovery, a progressive decrease in the Pi content and Pi/PCr ratio was observed along with an increase in the isometric peak torque. Comparison between the two measurements showed a strong linear inverse relationship between basal Pi content and isometric peak torque in the patient population during recovery (r2=0.4662, p=0.0013). Other studies have attributed decreased force production during sustained exercise to end-product inhibition of the powerstroke in the actin-myosin cross-bridge cycle by increased Pi and decreased pH. This study suggests that increases in basal Pi content may also contribute to the loss of force observed in non-fatigued, atrophic skeletal muscle.

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