Ca²⁺-pumping impairment during repetitive fatiguing contractions in single myofibers: role of cross-bridge cycling

Abstract

Nogueira L, Shiah AA, Gandra PG, Hogan MC. Ca 2ϩ -pumping impairment during repetitive fatiguing contractions in single myofibers: role of cross-bridge cycling. Am J Physiol Regul Integr Comp Physiol 305: R118 -R125, 2013. First published May 15, 2013 doi:10.1152/ajpregu.00178.2013.-The energy cost of contractions in skeletal muscle involves activation of both actomyosin and sarcoplasmic reticulum (SR) Ca 2ϩ -pump (SERCA) ATPases, which together determine the overall ATP demand. During repetitive contractions leading to fatigue, the relaxation rate and Ca 2ϩ pumping become slowed, possibly because of intracellular metabolite accumulation. The role of the energy cost of cross-bridge cycling during contractile activity on Ca 2ϩ -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 Ca 2ϩ pumping is impaired during fatiguing contractions. Fibers were stimulated in the absence (control) and presence of BTS and cytosolic calcium ([Ca 2ϩ ]c) transients or intracellular pH (pHi) changes were measured. BTS treatment allowed normal [Ca 2ϩ ]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 [Ca 2ϩ ]c and the increase in basal [Ca 2ϩ ]c did not occur with BTS incubation. The progressively slower Ca 2ϩ 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 ϩ cyanide), there was no further slowing in SR Ca 2ϩ pumping during contractions compared with the BTS-alone condition. Furthermore, the fall in pHi was significantly less during the BTS ϩ 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 Ca 2ϩ pumping rate during fatiguing contractions. Ca 2ϩ uptake rate; skeletal muscle; cross-bridge cycling; fatigue; glycolysis; oxidative phophorylation DURING THE TRANSITION FROM rest to exercise in skeletal muscle, the demand for ATP increases several hundredfold, primarily because of the activation of both actomyosin (i.e., cross-bridge cycling) and sarcoplasmic reticulum (SR) Ca 2ϩ ATPase (SERCA) pumps (6). After membrane depolarization, cytosolic Ca 2ϩ concentration is transiently increased, thereby elevating actomyosin and SERCA ATPase activities. Although the Na ϩ -K ϩ -ATPase activity has a high contribution in the energy cost at rest [ϳ40% of total; (27)], it is quite small during contractions [ϳ1.5-7%; (6)]. Conversely, during contractions, actomyosin ATPase and SERCA account for ϳ60% and 40% of the total ATP demand, respectively (5,6,22,30). However, the energy cost of Ca 2ϩ handling by SERCA during contractions may be as high as 80% of the total energy cost in type IIB...

Cigarette smoke directly impairs skeletal muscle function through capillary regression and altered myofibre calcium kinetics in mice

“…Real‐time monitoring of intracellular Ca 2+ in isolated single fibres suggests that this slowing in relaxation in fibres from CSE‐treated mice is due to a decreased rate of SR Ca 2+ pumping (Fig. F and G ) (Nogueira & Hogan, ; Nogueira et al . 2013).…”

“…During contractions [Ca 2+ ] c was measured by illuminating the fibre with excitation wavelengths of 340 and 380 nm that were rapidly alternated (200 Hz). Fluorescence signals were collected at an emission of 510 nm and the ratio of signals excited at 340 nm to 380 nm were used to calculate the Ca 2+ ‐dependent signal (Nogueira et al . 2013).…”

“…β is the fluorescence ratio between high and no [Ca 2+ ] c at 380 nm, and R min and R max are the fluorescence ratios when Ca 2+ –FURA‐2 binding is minimal (at low cytosolic Ca 2+ ) and maximal (at high intracellular Ca 2+ ), respectively. R min and R max were determined using an internal in vivo calibration described in Nogueira et al . (2013), but modified for mouse fibres.…”

“…2018). Calculations of peak and basal [Ca 2+ ] c were previously described in Nogueira et al . (2013).…”

“…In order to measure the function of SR Ca 2+ pumping during relaxation (Nogueira et al . 2013), the elevated long tail of [Ca 2+ ] c decline (from 100 ms to 3.0 s after the end of the stimulation period during a single tetanic contraction) was used to determine the rate of [Ca 2+ ] c decline (−d[Ca 2+ ] c /d t ) and the rate of SR Ca 2+ pumping ( A , eqn ).…”