Effects of feeding of either
creatine or its analog
beta-guanidinopropionic acid (beta-GPA) on endurance work capacity and oxygen consumption were studied in rats. Resting high-energy
phosphate contents in hindlimb muscles were lower in the beta-GPA group and higher in the
creatine group than in controls. The
glycogen contents in resting hindlimb muscles of rats fed beta-GPA were significantly higher than those in controls. The endurance run and swimming times to exhaustion were significantly greater (32-70%) in the beta-GPA group than in the control and
creatine groups. However, there were no beneficial effects on the maximum oxygen consumption (VO2max) and
oxygen transport capacity of blood by the feeding of beta-GPA. None of these parameters were significantly influenced by
creatine supply. Both maximum exercise time and VO2max in the beta-GPA group were not changed by normalization of
glycogen levels. The activities of mitochondrial
enzymes in skeletal muscles were higher in the beta-GPA group than in the controls. Thus endurance capacity is improved if the respiratory capacity of muscles is increased, even when the contents of high-energy
phosphates in muscles are lower. Increased endurance capacity was not directly associated with the elevated levels of muscle
glycogen,
oxygen transport capacity of blood, or VO2max.