Effect of muscle creatine content manipulation on contractile properties in mouse muscles.

The effects of muscle creatine manipulation on contractile properties in oxidative and glycolytic muscles were evaluated. Whereas control mice (NMRi; n = 12) received normal chow (5 g daily), three experimental groups were created by adding creatine monohydrate (CR group; 5%, 1 week; n = 13); beta-guanidinoproprionic acid, an inhibitor of cellular creatine uptake (beta-GPA group; 1%, 2 weeks; n = 12); or CR following beta-GPA (beta-GPA+CR group; n = 11). Total creatine (TCr) and the contractile properties of incubated soleus and extensor digitorum longus (EDL) muscles were determined. For the soleus, compared with control, TCr increased in the CR group (+25%), decreased in beta-GPA group (-50%), and remained stable in the beta-GPA+CR group, whereas, for the EDL, TCr was similar in the CR, and lower in the beta-GPA (-40%) and beta-GPA+CR (-15%) groups. None of the experimental groups (CR, beta-GPA, or beta-GPA+CR) showed changes in peak tension (P(peak)), time to peak tension, or relaxation in soleus or EDL during twitch or tetanic stimulation. For the soleus, fatigue reduced P(peak) to approximately 60% of initial P(peak); 5 min of recovery restored P(peak) to values approximately 15% higher in CR than in controls. P(peak) recovery was not affected by beta-GPA or beta-GPA+CR in the soleus or any treatment in the EDL. Thus, peak tension recovery is enhanced by creatine intake in oxidative but not glycolytic muscles. This may be implicated in the beneficial action of creatine loading.