NMR-BASED IDENTIFICATION OF INTRA-CELLULAR AND EXTRACELLULAR COMPARTMENTS OF THE BRAIN P-I PEAK

The P-i peak in a P-31 NMR spectrum of the brain can be deconvoluted i nto six separate Lorentzian peaks with the same linewidth as that of t he phosphocreatine peak in the spectrum. In an earlier communication w e showed that the six P-i peaks in normal brain represent two extracel lular and four intracellular compartments. In that report we have iden tified the first of the extracellular peaks by marking plasma with inf used P-i, thereby substantially increasing the amplitude of the single peak at pH 7.35, 2-Deoxyglucose-6-phosphate (2-DG-6-P) was placed in the brain interstitial space by microdialysis, The resulting 2-DG-6-P peak was deconvoluted into three separate peaks. The chemical shift of the principle 2-DG-6-P peak gave a calculated pH of 7.24 +/- 0.02 for interstitial fluid pH, a value that agreed well with the pH of the se cond extracellular P-i peak at pH 7.25 +/- 0.01, We identified the int racellular compartments by selectively stressing cellular energy metab olism in three of the four intracellular spaces. A seizure-producing c hemical, flurothyl, was used to activate the neuron, thereby causing a demand for energy that could not be completely met by oxidative phosp horylation alone. The resulting loss of high-energy phosphate reserves caused a significant increase in intracellular P-i only in those cell s associated with the P-i peak at pH 6.95 +/- 0.01. This suggests that this compartment represents the neuron. Ammonia is detoxified in the astrocyte (glutamine synthetase) by incorporating it into glutamine, a process that requires large amounts of glucose and ATP, The intraarte rial infusion of ammonium acetate into the brain stressed astrocyte en ergy metabolism resulting in an increase in the P-i of the cells at pH of 7.05 +/- 0.01 and 7.15 +/- 0.02. This finding, coupled with our ob servation that these same cells take up infused P-i probably via the a strocyte end-foot processes, lead us to conclude that these two compar tments represent two different types of astrocytes, probably protoplas mic and fibrous, respectively, As a result of this study, we now belie ve the brain contains four extracellular and four intracellular compar tments.