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NMR‐Based Identification of Intra‐ and Extracellular Compartments of the Brain Pi Peak

Abstract: The Pi peak in a 31P NMR spectrum of the brain can be deconvoluted into six separate Lorentzian peaks with the same linewidth as that of the phosphocreatine peak in the spectrum. In an earlier communication we showed that the six Pi peaks in normal brain represent two extracellular and four intracellular compartments. In that report we have identified the first of the extracellular peaks by marking plasma with infused Pi, 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 second extracellular Pi peak at pH 7.25 ± 0.01. We identified the intracellular compartments by selectively stressing cellular energy metabolism in three of the four intracellular spaces. A seizure‐producing chemical, flurothyl, was used to activate the neuron, thereby causing a demand for energy that could not be completely met by oxidative phosphorylation alone. The resulting loss of high‐energy phosphate reserves caused a significant increase in intracellular Pi only in those cells associated with the Pi 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 intraarterial infusion of ammonium acetate into the brain stressed astrocyte energy metabolism resulting in an increase in the Pi of the cells at pH of 7.05 ± 0.01 and 7.15 ± 0.02. This finding, coupled with our observation that these same cells take up infused Pi probably via the astrocyte end‐foot processes, lead us to conclude that these two compartments represent two different types of astrocytes, probably protoplasmic and fibrous, respectively. As a result of this study, we now believe the brain contains four extracellular and four intracellular compartments.