Use of glycolytic energy sources by human skeletal muscle under anoxic conditions in vitro and during moderate exercise in vivo.

To test a new in vitro model for investigations of muscle metabolism, the most important metabolites of muscle anaerobic metabolism (ATP, creatine phosphate, glucose, glycogen, and lactate) were measured in muscle biopsies from healthy male subjects, endurance-trained cyclists, strength-trained weight lifters, intensively trained long-distance runners, and speed-trained sprinters. The samples were taken at rest and after moderate muscle exercise for 30 s. These samples were analysed as such or after in vitro incubation for 30 s, 60 s, and 5 min under anoxic conditions. Anoxic conditions were created by incubating the muscle specimens in mineral oil through which N2 was bubbled. Under anoxic conditions in vitro, the concentrations of glycogen fell significantly in muscle specimens from sprinters and long-distance runners. The same tendency was observed in all other groups. The concentrations of high-energy phosphates. ATP and creatine phosphate, did not deplete under anoxic conditions. Under anoxic conditions, the rate of energy use, i.e., the metabolic rate calculated from the changes in the energy sources in terms of high-energy phosphate (P) use, was highest during the first 30 s, and declined to zero after 60 s. In the endurance-trained cyclists the metabolic rate was significantly lower than in any other group. In conclusion, muscle tissue behaves in quite a different way under anoxic conditions in vitro as compared with its behavior during exercise in vivo, and so the in vitro model is not suitable to compensate for studies of muscle metabolism in vivo. The in vitro model is of value, however, in, for example, determining the maximal metabolic rate of muscle.