The energetic cost of activation in mouse fast-twitch muscle is the same whether measured using reduced filament overlap or N-benzyl-p-toluenesulphonamide

Aim: Force generation and transmembrane ion pumping account for the majority of energy expended by contracting skeletal muscles. Energy turnover for ion pumping, activation energy turnover (E-A), can be determined by measuring the energy turnover when force generation has been inhibited. Most measurements show that activation accounts for 25-40% of isometric energy turnover. It was recently reported that when force generation in mouse fast-twitch muscle was inhibited using N-benzyl-p-toluenesulphonamide (BTS), activation accounted for as much as 80% of total energy turnover during submaximal contractions. The purpose of this study was to compare E-A measured by inhibiting force generation by: (1) the conventional method of reducing contractile filament overlap; and (2) pharmacological inhibition using BTS. Methods: Experiments were performed in vitro using bundles of fibres from mouse fast-twitch extensor digitorum longus (EDL) muscle. Energy turnover was quantified by measuring the heat produced during 1-s maximal and submaximal tetanic contractions at 20 and 30 degrees C. Results: E-A measured using reduced filament overlap was 0.36 +/- 0.04 (n = 8) at 20 degrees C and 0.31 +/- 0.05 (n = 6) at 30 degrees C. The corresponding values measured using BTS in maximal contractions were 0.46 +/- 0.06 and 0.38 +/- 0.06 (n = 6 in both cases). There were no significant differences among these values. E-A was also no different when measured using BTS in submaximal contractions. Conclusion: Activation energy turnover is the same whether measured using BTS or reduced filament overlap and accounts for slightly more than one-third of isometric energy turnover in mouse EDL muscle.