Cerebral glutamine metabolism under hyperammonemia determined in vivo by localized 1H and 15N NMR spectroscopy

摘要

Brain glutamine synthetase (GS) is an integral part of the glutamate–glutamine cycle and occurs in the glial compartment. In vivo Magnetic Resonance Spectroscopy (MRS) allows noninvasive measurements of the concentrations and synthesis rates of metabolites. ¹⁵N MRS is an alternative approach to ¹³C MRS. Incorporation of labeled ¹⁵N from ammonia in cerebral glutamine allows to measure several metabolic reactions related to nitrogen metabolism, including the glutamate–glutamine cycle. To measure ¹⁵N incorporation into the position 5N of glutamine and position 2N of glutamate and glutamine, we developed a novel ¹⁵N pulse sequence to simultaneously detect, for the first time, [5-¹⁵N]Gln and [2-¹⁵N]Gln+Glu in vivo in the rat brain. In addition, we also measured for the first time in the same experiment localized ¹H spectra for a direct measurement of the net glutamine accumulation. Mathematical modeling of ¹H and ¹⁵N MRS data allowed to reduce the number of assumptions and provided reliable determination of GS (0.30±0.050 μmol/g per minute), apparent neurotransmission (0.26±0.030 μmol/g per minute), glutamate dehydrogenase (0.029±0.002 μmol/g per minute), and net glutamine accumulation (0.033±0.001 μmol/g per minute). These results showed an increase of GS and net glutamine accumulation under hyperammonemia, supporting the concept of their implication in cerebral ammonia detoxification.