Causes of differences in exercise-induced changes of base excess and blood lactate

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 (-Delta ABE) 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 (Delta[La](if+blood)) during exercise and only slightly larger during recovery. The discrepancy between -Delta ABE and Delta[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 HCO3-. 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 Delta[La](blood) instead to Delta[La](if+blood). When performing correct comparisons and considering Cl-/HCO3- - exchange between erythrocytes and extracellular fluid, neither the use of Delta ABE nor of Delta SBE provides evidence for differences in H+ and La- transport across the tissue cell membranes.