Phosphorus nuclear magnetic resonance of fast- and slow-twitch muscle

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Abstract
Phosphorus nuclear magnetic resonance (NMR) spectra were obtained at 109.3 MHz from isolated, arterially perfused cat biceps brachi (greater than 75% fast-twitch, glycolytic fibers) and soleus (greater than 92% slow-twitch, oxidative) muscles at 30 degrees C. The perfused muscles were stable with respect to O2 consumption, twitch characteristics, and ATP and phosphocreatine (PC) levels for up to 10 h. NMR spectra showed a higher PC/Pi ratio in the biceps (11) than in the soleus (1.7). Relatively higher Pi levels were observed in extracts of clamp-frozen muscles than in the intact muscles. This difference could be accounted for by artifactual hydrolysis of PC during muscle freezing. Based on the NMR and chemical data, the free cytosolic ADP level, calculated from the creatine kinase equilibrium, was 14 microM in the soleus and less than 1 microM in the biceps. Intracellular Pi concentration was 10 mM in the soleus and 3 mM in the biceps. Intracellular pH, estimated from the chemical shift of phosphate or 2-deoxyglucose 6-phosphate, was 7.0 in both muscles (perfusate pH 7.2). Both extracellular space and pH measurements were obtained from NMR spectra of muscles perfused with 10 mM sodium phenylphosphonate added to the perfusate. These results document larger differences in the phosphate metabolites in the two types of mammalian muscles than previously reported.