Department of Biochemistry, University of Oxford, South Parks Road, Oxford, UK ABSTRACT The increased use of creatine by athletes as a dietary supplement to improve their physical performance assumes that increased serum creatine levels will increase intracellular skeletal muscle creatine. Despite this common assumption, skeletal muscle creatine uptake awaits fullcharacterization. Consequently, we have investigated. C-labelled creatine uptake in isolated, incubated rat soleus (type 1) muscle preparations at 37° C. We found that the apparent Km for creatine uptake was 73 by 2mM buffer Na concentration, from 145 to 25mM, reduced the rate of C-labelled creatine uptake by 77%, indicating that uptake is largely Na dependent in soleus muscle. Insulin had no effect on the rate of creatine uptake in vitro. The total creatine content was 34% lower, but the rate of creatine uptake in the presence of 100 extensor digitorum longus (type II) muscle. However, at 1 mM extracellular creatine, the maximal rate of uptake was not significantly different for the two muscle types, implying that soleus muscle has a lower Km for creatine uptake. We suggest that intracellular creatine levels may play a role in the regulation of skeletal muscle creatine uptake.
μM and the Vmax was 77nmol h gww. Creatine uptake was 82% inhibitedβ-guanidinopropionic acid, the structural analogue of creatine. In addition, a decrease inμM extracellular creatine was 45% higher, in soleus than in extensor digitorum longus (type II) muscle. However, at 1 mM extracellular creatine, the maximal rate of uptake was not significantly different for the two muscle types, implying that soleus muscle has a lower Km for creatine uptake. We suggest that intracellular creatine levels may play a role in the regulation of skeletal muscle creatine uptake.
