Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC68
A simple phosphate feedback control mechanism does not explain the kinetics of canine skeletal muscle oxygen consumption
R. C. Wüst1, B. Grassi2, M. C. Hogan3, R. A. Howlett3, L. Gladden4, H. B. Rossiter1
1. Institute of Membrane and Systems Biology, University of Leeds, Leeds, United Kingdom. 2. Dipartimento di Scienze e Tecnologie Biomediche, Universit? degli Studi di Udine, Udine, Italy. 3. Division of Physiology, Department of Medicine, University of California, San Diego, California, United States. 4. Department of Kinesiology, Auburn University, Auburn, Texas, United States.
Fast kinetics of muscle oxygen consumption (VO2) is characteristic of effective physiological systems integration. The mechanism of VO2 kinetic control is equivocal, but current theories include: A feedback mechanism based on linear [PCr]-VO2 dynamics ; ADP feedback via classical hyperbolic (nH=1)  or higher-order (nH>1) functions ; and parallel activation of ATP consumption and roduction . Distinguishing between these control models in vivo is complicated by the twin difficulties of making high-frequency direct measurements of VO2 and intramuscular metabolites, and in attaining high [ADP] (due to the attenuating influence of a metabolic acidosis): Complexities that can be overcome utilising highly-aerobic canine muscle for the investigation of the transition from rest to contractions at VO2max. The gastrocnemius complex of anaesthetised, ventilated, dogs (n=6) were surgically isolated and the tendon attached to a force transducer. Isometric tetanic contractions (50Hz; 200ms duration; 1 s-1) were elicited via sciatic nerve stimulation. Muscle VO2 and lactate efflux were determined from direct Fick measurements using an ultrasonic flowmeter (T108, Transonic Systems) and blood analysis (IL1304 and IL282, Instrumentation Laboratories). Muscle biopsies were taken at rest and during contractions (approx. 10,20,30,45,60,75,100,140 and 180 s), immediately frozen, and analysed for [phosphates], [lactate] and pH. From rest to VO2max, VO2 increased from 3±2 to 123±46 (mean,SD) mL.min-1.kg-1; [PCr] decreased from 24.2±1.6 to 7.6±2.9mM; and [ADP] increased from 17±7 to 131±68μM. The [PCr]-VO2 and [ADP]-VO2 temporal relationships were not well fit by linear or classical hyperbolic models (respectively), due to the high sensitivity of VO2 to metabolic perturbations early in the transient. A higher-order function (nH=3.7±1.4) improved the early [ADP]-VO2 model fit, but not for the region approaching VO2max. A parallel activation model (nH=1; Km=50μM) revealed an increase in the maximum rate of ADP-stimulated VO2 from 9±8 (rest), to 58±32 (10s), to 87±39% of VO2max by 20s of contractions: the time constant of which (11±9s) was significantly lower than both VO2 (31±13s; P=0.03) and blood flow (25±11s; P=0.03, ANOVA). These findings suggest that higher-order  or parallel activation  models are required to explain the control of VO2 kinetics. The apparent ~10-fold increase in the maximum rate of ADP-stimulated VO2 early in contractions may be achieved via activation of regulated enzymes, mitochondrial complexes and/or transporters with a time-course that is quicker than both O2 delivery and utilisation. Simple phosphate feedback models alone [1,2], however, are not sufficient to explain the dynamic control of VO2 across the entire range of aerobic function in mammalian skeletal muscle.
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