Response to mitre et al.: 'Analysis of continuous glucose monitoring data to assess outpatient closed-loop studies: considerations for different sensors'

摘要

Key points: Muscle glycogen (the storage form of glucose) is consumed during muscle work and the depletion of glycogen is thought to be a main contributor to muscle fatigue. In this study, we used a novel approach to first measure fatigue-induced reductions in force and tetanic Ca2+ in isolated single mouse muscle fibres following repeated contractions and subsequently quantify the subcellular distribution of glycogen in the same fibre. Using this approach, we investigated whether the decreased tetanic Ca2+ induced by repeated contractions was associated with glycogen depletion in certain subcellular regions. The results show a positive correlation between depletion of glycogen located within the myofibrils and low tetanic Ca2+ after repetitive stimulation. We conclude that subcellular glycogen depletion has a central role in the decrease in tetanic Ca2+ that occurs during repetitive contractions. In skeletal muscle fibres, glycogen has been shown to be stored at different subcellular locations: (i) between the myofibrils (intermyofibrillar); (ii) within the myofibrils (intramyofibrillar); and (iii) subsarcolemmal. Of these, intramyofibrillar glycogen has been implied as a critical regulator of sarcoplasmic reticulum Ca2+ release. The aim of the present study was to test directly how the decrease in cytoplasmic free Ca2+ ([Ca2+]i) during repeated tetanic contractions relates to the subcellular glycogen distribution. Single fibres of mouse flexor digitorum brevis muscles were fatigued with 70 Hz, 350 ms tetani given at 2 s (high-intensity fatigue, HIF) or 10 s (low-intensity fatigue, LIF) intervals, while force and [Ca2+]i were measured. Stimulation continued until force decreased to 30% of its initial value. Fibres were then prepared for analyses of subcellular glycogen distribution by transmission electron microscopy. At fatigue, tetanic [Ca2+]i was reduced to 70 ± 4% and 54 ± 4% of the initial in HIF (P 0.01, n = 9) and LIF (P 0.01, n = 5) fibres, respectively. At fatigue, the mean inter- and intramyofibrillar glycogen content was 60-75% lower than in rested control fibres (P 0.05), whereas subsarcolemmal glycogen was similar to control. Individual fibres showed a good correlation between the fatigue-induced decrease in tetanic [Ca2+]i and the reduction in intermyofibrillar (P = 0.051) and intramyofibrillar (P = 0.0008) glycogen. In conclusion, the fatigue-induced decrease in tetanic [Ca2+]i, and hence force, is accompanied by major reductions in inter- and intramyofibrillar glycogen. The stronger correlation between decreased tetanic [Ca2+]i and reduced intramyofibrillar glycogen implies that sarcoplasmic reticulum Ca2+ release critically depends on energy supply from the intramyofibrillar glycogen pool.