Mitochondrial activation at the onset of contractions in isolated myofibres during successive contractile periods

Key points • During the transition in skeletal muscle from rest to steady state contractions, O2 consumption is limited and the exact mechanisms controlling respiration in intact cells are not completely understood. • Previous contractile activity can ‘prime’ skeletal muscle resulting in a faster enhancement of the O2 consumption during a subsequent bout of contractions. • Here, we showed in intact single muscle fibres that the mitochondrial electron transport chain was activated faster at the onset of contractions when the muscle cells were previously ‘primed’ by contractile activity. • Therefore, factors intrinsic to the muscle cells have a role in the delayed increase of mitochondrial respiration during the onset of contractions. Also, the control of mitochondrial respiration in intact cells is not simply dependent on substrate availability or a simple feedback mechanism but rather on a more complex system. Abstract  At the onset of skeletal muscle repetitive contractions, there is a significant delay in the time to achieve oxidative phosphorylation steady state. The purpose of the present study was to examine the factors that limit oxidative phosphorylation at the onset of contractions. NAD(P)H was measured in real time during two contractile periods (2 min each) separated by 5 min of rest in intact single muscle fibres (n= 7) isolated from Xenopus laevis. The fibres were then loaded with the dye tetramethylrhodamine methyl ester perchlorate (TMRM) to evaluate the kinetics of the mitochondrial membrane potential (Δψm) during two further successive contractile periods. At the onset of contractions in the first period, NAD(P)H exhibited a time delay (14.1 ± 1.3 s) before decreasing toward a steady state. In contrast, Δψm decreased immediately after the first contraction and started to be reestablished after 10.7 ± 0.9 s, with restoration to the pre-stimulation values after approximately 32 s. In the second contractile period (5 min after the first), NAD(P)H decreased immediately (i.e. no time delay) after the first contraction and had a significantly shorter time constant compared to the first contractile bout (3.3 ± 0.3 vs. 5.0 ± 0.2 s, P < 0.05). During the second bout, Δψm remained unchanged from pre-stimulation values. These results suggest: (1) that at the onset of contractions, oxidative phosphorylation is primarily limited by the activity of the electron transport chain complexes rather than by a limited level of substrates; and (2) when the muscle is ‘primed’ by previous contractile activity, the faster enhancement of the cellular respiratory rate is due to intrinsic factors within the myofibre.