Dissociation between lactate and proton exchange in muscle during intense exercise in man

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Abstract
Transport of lactate, H+ and fluid across muscle sarcolemma was studied in contracting muscles under varying blood acid–base conditions. Subjects performed two‐legged submaximal knee‐extensor exercise for 29–35 min consisting of warming up for 5 min followed by 10 min of leg exercise (L1), leg and arm exercise for 6–10 min (L2+A) and leg exercise for 10 min (L3). The experimental protocol was performed on two occasions; inspiring air (normoxia, N) or breathing 14% O2 in N2 (hypoxia, H). Leg blood flow was measured and femoral arterial and venous blood was sampled before and during each phase of exercise. Arterial blood lactate concentration increased progressively during exercise to 5.9 ± 0.8 (N) and 8.2 ± 0.8 mmol l−1 (H) (P≤ 0.05) after 5.5 min of L2+A. Arterial blood pH was higher (P≤ 0.05) in H than in N, whereas arterial blood HCO3‐ concentrations were the same. Leg lactate release was higher in H than in N (3.1 ± 0.7 vs. 2.0 mmol l−1 (P≤ 0.05) during L1. In L2+A a net uptake of lactate was observed in both N and H. The concentration of lactate in the red blood cells increased during exercise to 2.3 ± 0.4 (N) and 4.3 ± 0.7 mmol l−1 (H) (P≤ 0.05) after 5.5 min of L2+A, but no red blood cell femoral arterial–venous lactate difference was observed. Net proton release, estimated from actual base excess (ABE) adjusted for changes in reduced haemoglobin, was significant (P≤ 0.05) both at rest and during each phase of exercise. Furthermore, the difference between net proton and lactate release was positive throughout exercise and of similar magnitude in N and H. The present data suggest that (1) H+ exchange in muscle during submaximal exercise can to a large extent occur through mechanisms other than via coupling to lactate; (2) muscle transport of H+ is insensitive to changes in blood pH in the range of 0.02–0.08 pH units; (3) transport of lactate across the membrane of red blood cells appears to be of minor importance for lactate release from active muscles.

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