Hepatic lactate uptake versus leg lactate output during exercise in humans.

The exponential rise in blood lactate with exercise intensity may be influenced by hepatic lactate uptake. We compared muscle-derived lactate to the hepatic elimination during 2 h prolonged cycling (62 +/- 4% of maximal O(2) uptake, (.)Vo(2max)) followed by incremental exercise in seven healthy men. Hepatic blood flow was assessed by indocyanine green dye elimination and leg blood flow by thermodilution. During prolonged exercise, the hepatic glucose output was lower than the leg glucose uptake (3.8 +/- 0.5 vs. 6.5 +/- 0.6 mmol/min; mean +/- SE) and at an arterial lactate of 2.0 +/- 0.2 mM, the leg lactate output of 3.0 +/- 1.8 mmol/min was about fourfold higher than the hepatic lactate uptake (0.7 +/- 0.3 mmol/min). During incremental exercise, the hepatic glucose output was about one-third of the leg glucose uptake (2.0 +/- 0.4 vs. 6.2 +/- 1.3 mmol/min) and the arterial lactate reached 6.0 +/- 1.1 mM because the leg lactate output of 8.9 +/- 2.7 mmol/min was markedly higher than the lactate taken up by the liver (1.1 +/- 0.6 mmol/min). Compared with prolonged exercise, the hepatic lactate uptake increased during incremental exercise, but the relative hepatic lactate uptake decreased to about one-tenth of the lactate released by the legs. This drop in relative hepatic lactate extraction may contribute to the increase in arterial lactate during intense exercise.