Energetic driving forces are maintained in resting rat skeletal muscle after dietary creatine supplementation

The total creatine (TCr) pool of skeletal muscle is composed of creatine (Cr) and phosphocreatine (PCr). In resting skeletal muscle, the ratio of PCr to TCr (PCr/TCr; PCr energy charge) is ∼0.6-0.8, depending on the fiber type. PCr/TCr is linked to the cellular free energy of ATP hydrolysis by the Cr kinase equilibrium. Dietary Cr supplementation increases TCr in skeletal muscle. However, many previous studies have reported data indicating that PCr/TCr falls after supplementation, which would suggest that Cr supplementation alters the resting energetic state of myocytes. This study investigated the effect of Cr supplementation on the energy phosphates of resting skeletal muscle. Male rats were fed either rodent chow (control) or chow supplemented with 2% (wt/wt) Cr. After 2 wk on the diet, the gastrocnemius and soleus muscles were freeze clamped and removed from anesthetized animals. Cr supplementation increased TCr, PCr, and Cr levels in the gastrocnemius by 20, 22, and 17%, respectively (P < 0.05). A numerical 6% higher mean soleus TCr in Cr-supplemented rats was not statistically significant. All other energy phosphate concentrations, free energy of ATP hydrolysis, and PCr/TCr were not different between the two groups in either muscle. We conclude that Cr supplementation simply increased TCr in fast-twitch rat skeletal muscle but did not otherwise alter resting cellular energetic state.