Lactate transport in skeletal muscle cells: uptake in L6 myoblasts.

During exercise, lactate is produced by degradation of glucose-6-phosphate during glycolysis in the contracting muscles. This lactate is metabolized during and after exercise in the muscle itself and also in the liver and other muscles, which can use it as an energy metabolite or can resynthetize glycogen. Lactate is transported in the blood, and the rate of muscular utilization may be limited by two factors: the rate of metabolic utilization by the muscle cell; and the rate of transport across the membrane regulating lactate transfer from the blood to the cell. We have studied lactate uptake in L6 muscle cells by incorporation of 14C-lactate. The uptake rates were linear for 20 seconds with 5 mM lactate and 10 seconds with 20 mM. The uptake during 10 seconds for physiological lactate concentrations (1-20 mM) gave a straight line passing through the origin. Lactate uptake was not altered by specific inhibitors of lactate transport (2.5 mM alpha cyano-4-hydroxycinnamic acid. 5 microM 4,4'-diisothiocyanostilbene-2,2'disulphonic acid) or by the stereospecific D-lactate inhibitor. The results suggest that L-lactate uptake in L6 cells occurs by passive diffusion.