Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC330
Kinetic control of oxygen consumption in single Xenopus laevis skeletal muscle fibres is not first-order
R. C. Wüst1, H. B. Rossiter1, W. J. van der Laarse2
1. Institute of Membrane and Systems Biology, University of Leeds, Leeds, United Kingdom. 2. Institute for Cardiovascular Research, VU University Medical Centre, Amsterdam, Netherlands.
The means to increase muscular oxidative phosphorylation at the onset of contractions is the sine qua non of the ability to sustain exercise. This process is supported in vivo by increasing the delivery rate of substrates for mitochondrial electron transport and ATP synthesis. The mechanisms controlling the adaptation of oxygen consumption (VO2), however, are less well understood. In isolated mitochondria control is well described by a first-order reaction via [ADP], predicting exponential response kinetics . A first-order response, damped by [PCr], also provides a good approximation of VO2 control in recovery from contractions in frog muscle in vitro . We therefore aimed to determine whether a first-order control model could explain VO2 kinetics at the onset and cessation of contractions in single muscle fibres. For this, single muscle fibres (n=18) were isolated from the iliofibularis of Xenopus laevis, and suspended in oxygenated Ringer’s solution at 20 °C in an 170 μl glass chamber with a fast-response (τ=2s) polarographic PO2 electrode . [O2] and force production were measured during stimulated isometric contractions and recovery, and then deconvoluted and differentiated to provide VO2(every 1s). The twitch frequency was chosen to elicit VO2max whilst minimising fatigue. VO2 responses were fit to a first-order model using non-linear least squares regression to estimate the rate constants (kon and koff). R2 was used to estimate the goodness-of-fit. The VO2max of type 1 fibres (low oxidative; range, 0.012 to 0.027 nmol.s-1.mm-3) was lower than type 2 (0.046 to 0.130 nmol.s-1.mm-3) and type 3 (high oxidative; 0.118 to 0.133 nmol.s-1.mm-3, P<0.001) [3,4]. While the off-transient VO2 kinetics were well described by an exponential (R2=0.93±0.04) with a mean koff of 1.35 min-1 (range, 0.21 to 2.87 min-1), a first-order response did not provide a good characterisation of the on-transient kinetics (R2=0.83±0.06, P<0.001). Across fibre types the koff was proportional to VO2max (R2=0.80, P<0.001), but was unrelated to kon (R2=0.006, P=0.87). These data show a clear dissociation between the kinetic control of VO2 at the onset and cessation of contractions in single muscle fibres - behaviour inconsistent with first-order control. The significant correlation between koff and VO2max is consistent with the notion that VO2 kinetics are strongly related to [ADP] recovery and mitochondrial volume. However, this phosphate-control of VO2 kinetics may be modulated at exercise onset by the delivery of reducing equivalents or O2, allosteric activation of mitochondrial transporters or enzyme activities , and/or a fall in economy. Therefore, more complex models are required to fully understand the processes underlying the activation of mitochondrial oxidative phosphorylation at the start of skeletal muscle activity.
Where applicable, experiments conform with Society ethical requirements