Creatine uptake in isolated soleus muscle: kinetics and dependence on sodium, but not on insulin

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 full characterization. Consequently, we have investigated 14C-labelled creatine uptake in isolated, incubated rat soleus (type I) muscle preparations at 37 degrees C. We found that the apparent Km for creatine uptake was 73 microM and the Vmax was 77 nmol h-1 gww-1. Creatine uptake was 82% inhibited by 2 mM beta-guanidinopropionic acid, the structural analogue of creatine. In addition, a decrease in buffer Na+ concentration, from 145 to 25 mM, reduced the rate of 14C-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 microM 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.