Glycogen synthesis from lactate in the three types of skeletal muscle.

Lactate utilization by skeletal muscle was studied in the perfused rat hindlimb. At lactate concentrations above 4 mu, the hindlimb consistently removed lactate from the perfusion medium. At the concentrations examined (up to 26 mu), the rate of lactate uptake was proportional to lactate concentration. When the hindlimb was perfused with medium containing 12 mu lactate as the only substrate, a rapid increase in glycogen occurred in the fast-twitch red and fast-twitch white types of muscle. In contrast, the rate of glycogen accumulation in the slow-twitch red type of muscle was so slow as to be of questionable physiological significance. [W]Lactate was rapidly incorporated into glycogen in the plantaris muscle which is composed predominantly (about 94%) of fast-twitch fibers, while there was little incorporation of [14C]lactate into glycogen in the soleus muscle which consists predominantly of slow-twitch red fibers. Measurement of fructose-1,6-bisphosphatase activity provided an explanation for the very slow rate of lactate conversion to glycogen in slow-twitch red muscle. It was found that in contrast to the fast-twitch red and white types of muscle, the slow-twitch red soleus muscle has an extremely low level of fructose1,6-bisphosphatase activity. There was a 24% inhibition of lactate uptake, and the amount of [14C]lactate incorporated into plantaris muscle glycogen was reduced by 30%, when 8.5 mu glucose and insulin were included in the perfusion medium. However, neither the rate of lactate uptake nor the rate of glycogen accumulation was affected by a high concentration of insulin alone.