1. 31P n.m.r. spectroscopy in vivo was used to study the effect of l-alanine infusion on the concentrations of gluconeogenic intermediates in normal human liver. Studies were performed in six healthy male subjects (34–44 years, fasted overnight) using a chemical shift imaging pulse sequence on a whole-body n.m.r. system operating at 1.6T. Hepatic 31P n.m.r. spectra were obtained from 10 min before to 70 min after intravenous administration of 0.70 (n = 2), 1.40 (n = 3) or 2.80 (n = 5) nmol of l-alanine/kg body weight over 4.5 min. Concentrations of phosphomonoesters, Pi and phosphodiesters relative to ATP were calculated from peak areas in the n.m.r. spectra, using the β-ATP peak as a reference. 2. Dose-dependent spectral changes were observed for [phosphomonoesters]/[ATP] and [Pi]/[ATP]. At the highest dose given, maximal changes in [phosphomonoesters]/[ATP] (mean ± sem: 98 ± 12%, P<0.005) and [Pi]/[ATP] (−33 ± 3%, P<0.001) were observed approximately 45 min after the l-alanine infusion. [Phosphodiesters]/[ATP] showed a maximal increase of 24 ± 6% (P<0.05), which was independent of the l-alanine dose. Hepatic ATP levels and pH did not change. 3. To identify the metabolites responsible for the changes observed in vivo, male Wistar rats were infused with 11.2 mmol of l-alanine/kg body weight. After 15 min, livers were freeze-clamped and were extracted according to standard procedures. In vitro, 31P n.m.r. spectra obtained at 8.4 or 11.7 T revealed sharp increases in the concentrations of 3-phosphoglycerate and phosphoenolpyruvate after l-alanine infusion. No significant increases in other metabolites contributing to the phosphomonoester or phosphodiester resonances in vivo were observed, suggesting that the rise in [phosphomonoesters] observed in vivo was caused by increasing concentrations of 3-phosphoglycerate, and that phosphoenolpyruvate contributed to the rise in [phosphodiesters]. 5. These results show that l-alanine infusion leads to consistent changes in the 31P n.m.r. spectra of the human liver owing to increased concentrations of gluconeogenic intermediates. The ‘n.m.r.-alanine test’ may constitute a useful tool for studies of gluconeogenesis and hepatic biochemical pathology in vivo. |