Muscle as a consumer of lactate.

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.