Purpose: Sodium bicarbonate was used to investigate the effect of blood pH on [latin capital V with dot above]O2 kinetics during heavy exercise. Methods: On separate days, 10 active subjects performed two 6-min cycling bouts (208 +/- 12 W) at 25 W above their ventilatory threshold. Each subject ingested 0.3 g[middle dot]kg-1 of sodium bicarbonate with ~1 L of water or water alone 1 h before exercise. [latin capital V with dot above]O2 kinetics were examined by means of a three-component mono-exponential model. Results: Bicarbonate ingestion caused a significant increase in the preexercise blood pH (7.512 +/- 0.009 vs 7.425 +/- 0.007; P < 0.001). In the bicarbonate trial, the time constant for the rapid component (27.9 +/- 3.5 s) was slower than the control trial (20.8 +/- 2.4 s; P = 0.017). The higher blood pH after bicarbonate ingestion would have diminished local blood flow and caused a leftward shift of the oxygen-hemoglobin dissociation curve both of which would slow oxygen delivery to working muscle. In addition, bicarbonate ingestion diminished the amplitude of the slow component 29% (463 +/- 43 vs 649 +/- 53 mL[middle dot]min-1; P = 0.040). The primary cause of the slow component during heavy exercise is fatigue of working fibers and an accompanying increase of motor unit recruitment. Elevated plasma bicarbonate concentration is reported to stimulate the efflux of H+ from muscle fibers and to increase intramuscular pH. Conclusions: The slower time constant during the rapid component suggested that oxygen delivery is a limiting factor of [latin capital V with dot above]O2 kinetics during the onset of heavy exercise. Also, these results imply that bicarbonate ingestion diminished fatigue in working fibers during the slow component.
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