Phosphorus J coupling constants of ATP in human myocardium and calf muscle.

Proton-decoupled 31P NMR spectroscopy of the heart and calf muscle of healthy volunteers was performed with a 1.5 T whole-body imager. By use of two-dimensional chemical-shift imaging in combination with slice-selective excitation, well-resolved localized spectra (elements of 38 ml) were obtained within 20 to 35 min from which the homonuclear J coupling constants of ATP could be determined. In myocardium, J gamma beta = 16.03 +/- 0.17 Hz and J alpha beta = 15.82 +/- 0.23 Hz were obtained, while the values in calf muscle were J gamma beta = 17.16 +/- 0.12 Hz and J alpha beta = 16.04 +/- 0.09 Hz. The difference in J gamma beta was significant. According to the literature, a possible reason for greater ATP J coupling constants is a smaller fraction of ATP complexed to magnesium. However, the chemical-shift difference between alpha- and beta-ATP, which is also a measure for the fraction of ATP complexed to magnesium, showed only a small difference in ATP complexation: 88% in myocardium and 90% in calf muscle. This small difference cannot account for the observed difference in J gamma beta.