GLIAL ALKALINIZATION DETECTED IN-VIVO BY H-1-N-15 HETERONUCLEAR MULTIPLE-QUANTUM COHERENCE-TRANSFER NMR IN SEVERELY HYPERAMMONEMIC RAT

Brain [5-N-15] glutamine amide protons were selectively observed in vi vo by H-1-N-15 heteronuclear multiple-quantum coherence-transfer NMR i n spontaneously breathing, severely hyperammonemic rats during intrave nous [N-15]ammonium acetate infusion and the subsequent recovery perio d. The linewidth of brain [5-N-15]-glutamine amide proton H-z increase d from 36 +/- 2 Hz at 3.4 h to 58 +/- 6 Hz after 5.7 h of infusion, a net increase of 22 +/- 6 Hz. Concomitantly, brain ammonia concentratio n increased from 1.7 to 3.5 +/- 0.2 mu mol/g and the rat progressed fr om grade III to grade IV encephalopathy. On recovery to grade III and decrease of brain ammonia concentration to 1.3 mu mol/g, the linewidth returned to 37 +/- 2 Hz. In aqueous solution, [5-N-15]glutamine amide proton H-z underwent a 17-Hz linebroadening when pH was raised from 7 .1 to 7.5 at 37 degrees C, due to the increased rate of base-catalyzed exchange with water proton. Hence, linebroadening is a sensitive meas ure of changing intracellular pH. The 22-Hz linebroadening observed in vivo in severely hyperammonemic grade IV rats strongly suggests that the intracellular pH increases from 7.1 to about 7.4-7.5 in astrocytes where glutamine is synthesized and mainly stored. Probable mechanisms for the ammonia-induced alkalinization and decreased intraglial buffe ring capacity, as well as implications of the result for pathogenesis of hepatic encephalopathy, are discussed.