1. The ability to metabolize lactate at rest and during exercise was studied in six male subjects by using a constant infusion of sodium l(+)-lactate at a rate of 0·05 mmol min−1 kg−1. Twenty minute periods of rest and exercise at two work rates were used, amounting to 25% and 50% of maximal O2 uptake (V̇o2 max.) in four subjects and 50% and 66% of V̇o2 max. in two subjects. Control measurements were made with saline infusion. In all studies a steady state in blood lactate was achieved.

2. At rest lactate infusion was associated with an increase of 3·51 mmol/l (± sd 0·70) in plasma lactate. The increase was smaller in exercise and in a given subject was the same at both work rates; plasma lactate was on average 1·21 mmol/l (±sd 1·11) higher during lactate infusion than the control measurement at the same power output. In one subject lactate values in exercise were unchanged by lactate infusion.

3. At rest lactate infusion was associated with an increase in O2 intake and CO2 output, the respiratory exchange ratio was unchanged, and plasma HCO3 rose by 1·85 mmol/l.

4. During exercise lactate infusion was associated with a smaller and variable increase in O2 intake. CO2 output was less, the respiratory exchange ratio fell, and plasma HCO3 rose by 6·1 mmol/l.

5. Exercise is accompanied by an increased capacity to metabolize lactate aerobically. Decreasing lactate metabolism appears to play no part in the increase in plasma lactate concentration with increasing exercise, at least to 66% of the maximal O2 intake.

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