A bolus injection of [14C]lactate was used to measure lactate turnover rates at rest and during recovery from exhaustive exercise in coho salmon (Oncorhynchus kisutch) and starry flounder (Platichthys stellatus). At rest, lactate turnover rate in salmon was almost double that in flounder (1.33 versus 0.76 mumol min-1 kg-1), which reflected the higher blood lactate level in salmon (1.00 versus 0.12 mmol l-1). From 2 to 4 h after exercise, when blood lactate levels were at their peak and constant, turnover rates were elevated in both species, though to a greater extent in salmon than in flounder (11.88 versus 2.27 mumol min-1 kg-1). Lactate concentration and turnover rate were linearly correlated in both species. The higher turnover rate in salmon was solely a consequence of the higher blood lactate levels since, at similar blood lactate concentrations, turnover rates in flounder and salmon were the same. Therefore, the lower blood lactate levels in flounder after exercise were not a consequence of higher turnover. In neither species was the turnover rate adequate to account for the rate of lactate clearance from the muscle, suggesting a large portion was retained within the muscle and metabolized in situ. Furthermore, following injection of [14C]lactate, greater than 80% of the total blood activity was recovered as lactate, indicating that little label was incorporated into other products (e.g. glucose). These data suggest that the Cori cycle plays a minimal role in the metabolism of lactate in salmon and flounder. Furthermore, at least in flounder, there was no correlation between the kinetics of lactate clearance and O2 consumption, suggesting that the classical concept of ‘O2 debt’ is not applicable in this species.

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