P-31 NMR quantitation of phosphorus metabolites in rat heart and skeletal muscle in vivo

The aim of this study was to examine two methods of P-31 NMR quantitation o f phosphocreatine (PCr), ATP, and P-i in rat heart and skeletal muscle in v ivo. The first method employed an external standard of phenylphosphonic aci d (PPA; 10 mM), and the second method used an enzymatic measurement of tiss ue ATP equated to the area under the beta ATP peak. With the use of the ext ernal standard, the concentrations of ATP, PCr, and Pi in the rat heart wer e 4.48 +/-0.33, 9.21 +/-0.65, and 2.25 +/-0.16 mu mol/g wet wt, respectivel y. With the use of the internal ATP standard, measured on the same tissue, the contents (means +/- SE) were 4.78 +/-0.19, 9.83 +/-0.18, and 2.51 +/-0. 33 mu mol/g wet wt, respectively (n = 7). In skeletal muscle, ATP, PCr, and P-i were 6.09 +/-0.19, 23.44 +/-0.88, and 1.81 +/-0.18 mu mol/g wet wt usi ng the PPA standard and 6.03 +/-0.19, 23.30 +/-1.30, and 1.82 +/-0.19 mu mo l/g wet wt using the internal ATP standard (n = 6). There was no significan t difference for each metabolite as measured by the two methods of quantifi cation in heart or skeletal muscle. The results validate the use of an exte rnal reference positioned symmetrically above the coil and imply that each has similar NMR sensitivities (similar signal amplitude per mole of P-31 be tween PPA and tissue phosphorus compounds). We conclude that PCr, ATP, and P-i are nearly 100% visible in the normoxic heart and nonworking skeletal m uscle given the errors of measurement.