title: Subcellular distribution of glycogen and decreased tetanic Ca2+ in fatigued single intact mouse muscle fibres.
abstract: 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 Ca(2+) release. The aim of the present study was to test directly how the decrease in cytoplasmic free Ca(2+) ([Ca(2+)]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 [Ca(2+)]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 [Ca(2+)]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 [Ca(2+)]i and the reduction in intermyofibrillar (P = 0.051) and intramyofibrillar (P = 0.0008) glycogen. In conclusion, the fatigue-induced decrease in tetanic [Ca(2+)]i, and hence force, is accompanied by major reductions in inter- and intramyofibrillar glycogen. The stronger correlation between decreased tetanic [Ca(2+)]i and reduced intramyofibrillar glycogen implies that sarcoplasmic reticulum Ca(2+) release critically depends on energy supply from the intramyofibrillar glycogen pool.
authors.affiliation: jnielsen@health.sdu.dk.
authors.forename: Joachim
authors.initials: J
authors.lastname: Nielsen
authors.forename: Arthur J
authors.initials: AJ
authors.lastname: Cheng
authors.forename: Niels
authors.initials: N
authors.lastname: Ørtenblad
authors.forename: Hakan
authors.initials: H
authors.lastname: Westerblad
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005260:Female; D006003:Glycogen / Q000032:analysis / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D018763:Muscle Fatigue / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000737:chemistry / Q000378:metabolism*; D009924:Organ Culture Techniques; D012032:Refractory Period, Electrophysiological / Q000502:physiology*; D013347:Subcellular Fractions / Q000737:chemistry / Q000378:metabolism
chemical_list: D006003:Glycogen; D002118:Calcium