| dc.description | Lactate metabolism and transport were studied in skeletal muscle. Experiments examining the metabolism of inactive muscle during exercise revealed that lactate accumulates in inactive muscle due to increased glycogenolytic flux. This glycogenolysis is dissociated from the muscle energetic demand. Further studies in the perfused rat hindlimb showed that lactate is taken up by muscle and can be used to synthesize glycogen (glyconeogenesis). The rate of glyconeogenesis is related to the fiber type distribution of the muscle (FG $>$ FOG $>$ SO). Glyconeogenesis is sensitive to pH but is not altered by hormones (Corticosterone and Insulin). To determine the nature of the membrane permeability to L-lactate, the transmembrane flux of lactate was studied in two preparations: (a) purified plasma membrane vesicles, and (b) intact muscle strips. Transport (zero-trans) and uptake experiments indicated saturability with increasing L-lactate concentrations, stereospecificity, sensitivity to inhibitor compounds such as pyruvate and N-ethylmaleimide, and stimulation by an inwardly directed pH gradient. These data implicate a carrier mediated transport system as the predominant mode for lactate transport in skeletal muscle. Further studies on the lactate transport system suggest that, in contrast to glucose uptake in muscle, lactate uptake is not increased after an acute bout (30min.) of contractile activity. Lactate transport studies using skeletal muscle sarcolemmal vesicles (SLV) isolated from rats that were subjected to chronic exercise (CE) for five weeks, revealed that lactate transport was enhanced in the CE membranes compared to membranes from a sedentary control group (S), when the experiments were performed at a low concentration of L-lactate (1mM), however, at an external concentration of 50mM L-lactate there was no difference in membrane transport between the CE and S groups. In summary, lactate may be produced in a muscle without increased contractile activity, and it can be used as a precursor for glyconeogenesis. Lactate transport in muscle is mediated by a carrier system which displays similar characteristics to the lactate carrier described for several other tissue types. However, lactate transport is not altered by muscle contractility, although some adaptation in transport capacity is evident after a period of chronic exercise. | en_US |
