Causes of differences in exercise-induced changes of base excess and blood lactate
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It has been concluded from comparisons of base excess (BE) and lactic acid (La) concentration changes in blood during exercise-induced acidosis that more H+ than La− leave the muscle and enter interstitial fluid and blood. To examine this, we performed incremental cycle tests in 13 untrained males and measured acid–base status and [La] in arterialized blood, plasma, and red cells until 21 min after exhaustion. The decrease of actual BE (−ΔABE) was 2.2 ± 0.5 (SEM) mmol l−1 larger than the increase of [La]blood at exhaustion, and the difference rose to 4.8 ± 0.5 mmol l−1 during the first minutes of recovery. The decrease of standard BE (SBE), a measure of mean BE of interstitial fluid (if) and blood, however, was smaller than the increase of [La] in the corresponding volume (Δ[La]if+blood) during exercise and only slightly larger during recovery. The discrepancy between −ΔABE and Δ[La]blood mainly results from the Donnan effect hindering the rise of [La]erythrocyte to equal values like [La]plasma. The changing Donnan effect during acidosis causes that Cl− from the interstitial fluid enter plasma and erythrocytes in exchange for HCO 3 − . A corresponding amount of La− remains outside the blood. SBE is not influenced by ion shifts among these compartments and therefore is a rather exact measure of acid movements across tissue cell membranes, but changes have been compared previously to Δ[La]blood instead to Δ[La]if+blood. When performing correct comparisons and considering Cl−/HCO 3 − exchange between erythrocytes and extracellular fluid, neither the use of ΔABE nor of ΔSBE provides evidence for differences in H+ and La− transport across the tissue cell membranes.
KeywordsAcid–base status Buffering Donnan equilibrium Erythrocytes Lactate in erythrocytes
The authors thank all test subjects for their willing cooperation and Mrs. J. Nadol for technical assistance.
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