Changes in crossbridge and non‐crossbridge energetics during moderate fatigue of frog muscle fibres.

Abstract

SUMMARY1. The effect of sarcomere length (SL) during a fatiguing series of isometric tetani of frog muscle fibres was investigated. Tetani at 2-3 ,um SL were more fatiguing than tetani at 3-2 ,sm SL, in that force declined twice as much and relaxation became much slower.2. In a second set of experiments the force and heat production were measured during a series of fatiguing tetani. Heat was separated into two components: (a) crossbridge heat which is dependent on filament overlap and interaction, and (b) non-crossbridge heat which is independent of filament overlap and due to Ca2" turnover.3. In a series of fifty tetani, force, crossbridge heat and non-crossbridge heat each declined by 25-30 % of its initial value.4. The 25 % reduction in non-crossbridge heat occurred completely during the first few tetani of the fatiguing series while force declined by less than 3 %. This may be due to a reduction in Ca2" binding to parvalbumin and to Ca2' remaining bound during the remainder of the fatigue series.5. After the first few tetani of the fatigue series the non-crossbridge heat hardly changed as force declined by a further 25 % of its initial value. Continuing reduction of force with constant Ca21 turnover indicates a reduction in the Ca21 sensitivity of the filaments, and/or a reduction in the average force per attached crossbridge.6. At the start of the fatiguing series, as force declines by about 7-5 % there is a much larger decline of crossbridge heat (17 %). The reason for this is unknown. Later in the series, force declined more rapidly than heat. This is probably due to a progressive accumulation of inorganic phosphate which acts by depressing force more than it depresses ATP breakdown.

Muscle cell function during prolonged activity: cellular mechanisms of fatigue

“…In all except the earliest (Allen et al 1989) of these studies a clear-cut reduction of the sensitivity was observed. The lack of any reduction in the earliest study is probably because the photoprotein aequorin was used to measure [Ca2+]i; its light emission is reduced by Mg2+, and we subsequently observed a marked increase in myoplasmic magnesium concentration ([Mg2+]i) in fatigue (Westerblad & Allen, 1992c Furthermore, fibres become more fatigue resistant when the number of energy-consuming cross-bridges is reduced by stimulating at long sarcomere lengths (Barclay, Curtin & Woledge, 1993). Finally, muscles fatigue more rapidly when the energy turnover is increased by active shortening during tetani (Cummins, Soomal & Curtin, 1989 Acidosis is one possible link between reduced Ca2' release and metabolism, since it has been shown that pH may affect the opening of isolated SR Ca2+ channels in lipid bilayers (Ma, Fill, Knudson, Campbell & Coronado, 1988).…”

Cross bridges account for only 20% of total ATP consumption during submaximal isometric contraction in mouse fast-twitch skeletal muscle

“…With respect to species, most earlier studies that estimated cross-bridge-dependent ATP consumption were performed on frog muscle, although studies on fish, chicken, mice, and rats have also been performed (2,7,25,31,41,42). Interestingly, recent studies (38) of fast-twitch (IIA/B and IIB) skinned human muscle fibers (at 20°C) demonstrated that cross bridges accounted for only 35-41% of total ATP consumption, whereas SR Ca 2ϩ pumping accounted for 52-55%.…”

Effects of series elastic compliance on muscle force summation and the rate of force rise

“…From an energetic perspective, these findings may have relevance to the neural control of motor unit behaviour during contractions in which total muscle work or force maintenance is of greater importance than the rate of force rise. Minimizing the activation level or frequency of activation required to achieve a target force is energetically desirable (Barclay et al, 1993(Barclay et al, , 2008, as the metabolic cost of the contraction will be reduced accordingly.…”