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Assessing the superposition of the intramuscular determinants of VO 2 kinetics during ramp‐incremental and ramp‐decremental exercise in humans
Author(s) -
Rossiter Harry B,
McIntyre Dominick J,
Howe Franklyn A,
Ward Susan A,
Griffiths John R,
Whipp Brian J
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.4.a411-a
Subject(s) - chemistry , kinetics , incremental exercise , zoology , medicine , endocrinology , heart rate , physics , biology , quantum mechanics , blood pressure
The pulmonary O 2 uptake (VO 2 ) response to ramp‐incremental (RI) exercise is consistent with a dynamically‐linear control system operating via mitochondrial feedback of intramuscular phosphates (e.g. PCr, ADP). The concept of superposition posits that the system input‐output relationship (i.e. VO 2 ‐[PCr]) is independent of the function used to ‘force’ it. We wished to assess superposition of VO 2 control, using RI and ramp‐decremental (RD) exercise. Seven males performed RI knee‐extension exercise at 1W/20s to the limit of tolerance in the bore of a 1.5T MR magnet. RD was performed on a separate day, where the work rate was increased abruptly to the maximum value attained in RI and then decremented at −1W/20s. Intramuscular PCr, Pi, ATP and pH were determined from the quadriceps by 31 P‐MRS, in concert with breath‐by‐breath VO 2 (mass‐spectrometer; turbine). At the limit of tolerance in RI, [PCr] and pH fell to 21.1±2.9 mM and 6.92±0.13, respectively, and VO 2 increased to 2.34±0.3 l/min, and were not different from the values at the extremes in RD (attained after ~3 min). As expected, the slope of the VO 2 –work rate relationship was greater in RD than RI. However, the gain of the VO 2 ‐[PCr] relationship was also greater in RD (–233±80 vs – 121±41 ml/min/mM in RD and RI respectively; p<0.05). Thus, RI and RD responses were not superposable, suggesting that control of intramuscular oxidative phosphorylation is not consistently first‐order. This may reflect fibre‐type‐dependent modulation of feedback control and is also consistent with pH‐ or PCr/Cr‐dependency of ADP‐stimulated mitochondrial oxidative phosphorylation.