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1 School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
2 Department of Medical Biophysics, The University of Western Ontario, London, Ontario, Canada
3 School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; Department of Medical Biophysics, The University of Western Ontario, London, Ontario, Canada; Lawson Health Research Institute, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: gdmarsh{at}uwo.ca.
During heavy-intensity exercise, the mechanisms responsible for the continued slow decline in [PCr] (PCr slow component) have not been established. In this study we tested the hypothesis that a reduced intracellular acidosis would result in a greater oxidative flux and consequently a reduced magnitude of the PCr slow component. Subjects (n = 10) performed isotonic wrist-flexion in a control (CON) trial and in an induced alkalosis (ALK) trial (0.3g/kg oral dose of NaHCO3, 90 min prior to testing). Wrist-flexion, at a contraction rate of 0.5 Hz, was performed for 9 min at moderate- (75% of onset of acidosis; TpHi) and heavy- (125% TpHi) intensity exercise. 31P-MRS was used to measure intracellular [H+], [PCr], [Pi], and [ATP]. The initial recovery data was used to estimate the rate of ATP synthesis and oxidative flux at the end of heavy-intensity exercise. In repeated trials venous blood sampling was used to measure plasma [H+], [HCO3-], and [Lac-]. Throughout rest and exercise plasma [H+] was lower (p<0.05) and [HCO3-] was elevated (p<0.05) in ALK compared to CON. During the final 3 min of heavy-intensity exercise, ALK caused a lower (p<0.05) intracellular [H+] (246 (117) vs. 291 (129) nmol/l; mean (SD)), a greater (p<0.05) [PCr] (12.7 (7.0) vs. 9.9 (6.0) mmol/l), and a reduced accumulation of [ADP] (0.065 (0.031) vs. 0.098 (0.059) mmol/l). Oxidative flux was similar (p>0.05) in the conditions at the end of heavy-intensity exercise. In conclusion, our results are consistent with a reduced intracellular acidosis causing a decrease in the magnitude of the PCr slow component. The decreased PCr slow component in ALK did not appear to be due to an elevated oxidative flux.
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