In Vivo Cross-Relaxation in ATP in Skeletal Muscle Measured by 31P Saturation Transfer MRS

Christine Nabuurs¹, Bertolt Huijbregts¹, Andor Veltien¹, Be Wieringa², Cees Hilbers³, Arend Heerschap¹

ATP involvement in multiple enzymatic exchange reactions has been extensively studied by 31P saturation transfer (ST) MR spectroscopy. So far, however, little attention has been paid to in vivo 31P-31P cross-relaxation processes possibly interfering in the analysis of the exchange rates. Here we performed 31P ST experiments on hind-limb muscle of mice with deficiencies in adenylate and creatine kinase. In these conditions the enzyme contributions to exchange effects can be excluded and cross relaxation contributions to ST effects observed. However, in experiments on ATP, free in solution, cross relaxation effects did not become manifest. This was corroborated by theoretical calculations. These results indicate that in vivo ATP is involved in the formation of short lived complexes where cross relaxation is effective and saturation effects can be can be transferred to the free ATP.