GLADDEN, L. B., Muscle as a consumer of lactate. Med. Sci. Sports Exerc., Vol. 32, No. 4, pp. 764–771, 2000. Historically, muscle has been viewed primarily as a producer of lactate but is now considered also to be a primary consumer of lactate. Among the most important factors that regulate net lactate uptake and consumption are metabolic rate, blood flow, lactate concentration ([La]), hydrogen ion concentration ([H+]), fiber type, and exercise training. Muscles probably consume more lactate during steady state exercise or contractions because of increased lactate oxidation since enhancements in lactate transport due to acute activity are small. For optimal lactate consumption, blood flow should be adequate to maintain ideal [La] and [H+] gradients from outside to inside muscles. However, it is not clear that greater than normal blood flow will enhance lactate exchange. A widening of the [La] gradient from outside to inside muscle cells along with an increase in muscle [La] enhances both lactate utilization and sarcolemmal lactate transport. Similarly, a significant outside to inside [H+] gradient will stimulate sarcolemmal lactate influx, whereas an increased intramuscular [H+] may stimulate exogenous lactate utilization by inhibiting endogenous lactate production. Oxidative muscle fibers are metabolically suited for lactate oxidation, and they have a greater capacity for sarcolemmal lactate transport than do glycolytic muscle fibers. Endurance training improves muscle capacity for lactate utilization and increases membrane transport of lactate probably via an increase in Type I monocarboxylate transport protein (MCT1) and perhaps other MCT isoforms as well. The future challenge is to understand the regulatory roles of both lactate metabolism and membrane transport of lactate.
At rest, muscles slowly release lactate into the blood on a net basis, although at times they may show a small net uptake. During exercise, particularly short-term high-intensity exercise, muscles produce lactate rapidly, whereas lactate clearance is slowed. This results in an increased intramuscular lactate concentration ([La]) and an increased net output of lactate from the muscles into the blood. Later, during recovery from short-term exercise, or even during continued, prolonged exercise, there is net lactate uptake from the blood by resting muscles or by other muscles that are doing mild to moderate exercise. During prolonged exercise of low to moderate intensity, the muscles that originally showed net lactate output at the onset of the exercise may actually reverse to net lactate uptake (33). This shuttling of lactate (14–18) between adjacent muscles and between muscles and blood raises intriguing questions concerning how and why lactate is produced or consumed by skeletal muscles. The present paper will discuss factors that might regulate lactate consumption by muscle. A partial list of the most important factors studied to date includes: metabolic rate, blood flow, [La], hydrogen ion concentration ([H+]), fiber type, and exercise training.