EFFECT OF LACTATE CONCENTRATION AND METABOLIC-RATE ON NET LACTATE UPTAKE BY CANINE SKELETAL-MUSCLE

This study addressed two questions: 1) Does net lactate uptake (L) by muscle approach a saturation limit with increasing blood lactate concentration ([La])? 2) Is the muscle net L; response to increasing blood [La] affected by metabolic rate (VO2)? The gastrocnemius plantaris muscle group (GP) was isolated in situ in 20 anesthetized dogs. In three series of experiments, a lactate-lactic acid solution was infused into the arterial inflow of the GP to produce five different plasma [La] values: similar to 3, 9, 16, 22, and 30 mM, each of them maintained for 30 min. In one series, the GP remained at rest, whereas in the second series it contracted at 1 Hz and in the third series at 4 Hz. VO2 averaged similar to 3, 43, and 100 ml.kg(-1).min(-1) at rest and at 1 and 4 Hz, respectively. Within each of the three metabolic rates, increasing plasma [La] resulted in an increase in net L, which was well described (R > 0.98) by exponential equations. These equations predicted net L asymptotic values of 0.80, 0.72, and 1.09 mmol.kg(-1).min(-1) for rest and for 1 and 4 Hz, respectively. The corresponding plasma [La]s for half-maximal net L from the exponential equations were 16, 10, and 12 mM. Glucose uptake, pyruvate uptake/output, and alanine output by the muscles were not affected by the increasing [La] (and concomitant increases in net L) at any of the metabolic rates. Neither net glycogen synthesis nor depletion was changed by increasing [La]. These results answer the two questions posed: net L by skeletal muscle does approach a saturation limit as plasma [La] increases, and this relationship is altered by changing metabolic rate.