title: Ca²⁺-pumping impairment during repetitive fatiguing contractions in single myofibers: role of cross-bridge cycling.
abstract: The energy cost of contractions in skeletal muscle involves activation of both actomyosin and sarcoplasmic reticulum (SR) Ca²⁺-pump (SERCA) ATPases, which together determine the overall ATP demand. During repetitive contractions leading to fatigue, the relaxation rate and Ca²⁺ pumping become slowed, possibly because of intracellular metabolite accumulation. The role of the energy cost of cross-bridge cycling during contractile activity on Ca²⁺-pumping properties has not been investigated. Therefore, we inhibited cross-bridge cycling by incubating isolated Xenopus single fibers with N-benzyl-p-toluene sulfonamide (BTS) to study the mechanisms by which SR Ca²⁺ pumping is impaired during fatiguing contractions. Fibers were stimulated in the absence (control) and presence of BTS and cytosolic calcium ([Ca²⁺]c) transients or intracellular pH (pHi) changes were measured. BTS treatment allowed normal [Ca²⁺]c transients during stimulation without cross-bridge activation. At the time point that tension was reduced to 50% in the control condition, the fall in the peak [Ca²⁺]c and the increase in basal [Ca²⁺]c did not occur with BTS incubation. The progressively slower Ca²⁺ pumping rate and the fall in pHi during repetitive contractions were reduced during BTS conditions. However, when mitochondrial ATP supply was blocked during contractions with BTS present (BTS + cyanide), there was no further slowing in SR Ca²⁺ pumping during contractions compared with the BTS-alone condition. Furthermore, the fall in pHi was significantly less during the BTS + cyanide condition than in the control conditions. These results demonstrate that factors related to the energetic cost of cross-bridge cycling, possibly the accumulation of metabolites, inhibit the Ca²⁺ pumping rate during fatiguing contractions.
authors.affiliation: Department of Medicine, University of California, San Diego, La Jolla, CA 92093, USA. lnogueira@ucsd.edu
authors.forename: Leonardo
authors.initials: L
authors.lastname: Nogueira
authors.forename: Amy A
authors.initials: AA
authors.lastname: Shiah
authors.forename: Paulo G
authors.initials: PG
authors.lastname: Gandra
authors.forename: Michael C
authors.initials: MC
authors.lastname: Hogan
mesh_terms: D000205:Actomyosin / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004734:Energy Metabolism / Q000502:physiology; D005260:Female; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D000205:Actomyosin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium
keywords: Ca2+ uptake rate; cross-bridge cycling; fatigue; glycolysis; oxidative phophorylation; skeletal muscle