In vivo monitoring of extracellular 13N-glutamine derived from blood-borne 13N-ammonia in rat striatum using microdialysis with radio-LC method.

Glutamine synthetase (GS) is selectively localized in astrocytes and has important roles in the central nervous system (CNS). Cerebral extracellular excess ammonia and glutamate are taken up by astrocytes and converted to glutamine via GS to protect the CNS against neurotoxicity. In this study, we monitored cerebral extracellular 13N-glutamine derived from 13N-ammonia as a potential marker for astroglial metabolism using in vivo microdialysis combined with ultra performance liquid chromatography-radiometric detection. This method allowed rapid and highly sensitive radiometric analysis of 13N-ammonia and its metabolite, 13N-glutamine, in striatal extracellular fluid with good time resolution. Inhibition of GS with methionine sulfoximine resulted in a decrease of extracellular 13N-glutamine accompanied by an increase of 13N-ammonia as compared with control. Fluorocitrate, a selective inhibitor of glial metabolism, also decreased 13N-glutamine production and increased unmetabolized 13N-ammonia. In contrast, 13N-glutamine was increased with 5 mmol/kg of ammonium acetate without significant changes in 13N-ammonia as compared with control. These results suggest that the concentration of extracellular 13N-glutamine strongly reflects the biological changes in the metabolic activity of astroglial cells.

Radionuclide production and partial radiochemical characterization following fast neutron irradiation of mouse spleens.

When radiotherapy patients are irradiated with fast neutron beams (energies greater than 20 MeV), positron emitting radionuclides (15O, 13N, 11C) are created in their tissues. Capillary blood flow can be determined in the irradiated tissue by measuring the washout of the 15O. The chemical form of these positron emitting nuclides is important when assumptions about their transport in the development of a blood flow model are being made. In the present work, normal mouse spleen tissue was used as a model system for these studies. The mouse spleen was activated by whole mouse irradiation using the M.D. Anderson Hospital neutron beam produced by 42 MeV protons impinging on a beryllium target. Results of cellular studies and large molecule precipitation measurements (1) show that at least 65% of 15O created in situ in mouse spleen is capable of being transported out of the spleen by the blood supply, (2) suggest that 13N may be 100% biologically mobile, and (3) indicate that 11C appears to be evenly divided between a population associated with small biologically mobile molecules and a population associated with larger, biologically trapped or immobile molecules.

Measurement of regional myocardial blood flow with N-13 ammonia and positron-emission tomography in intact dogs.

N-13 ammonia mimics certain properties of microspheres. It rapidly clears from blood into myocardium where it becomes fixed in proportion to myocardial blood flow. Used with positron emission tomography as a means for quantifying in vivo myocardial indicator concentrations, N-13 ammonia may be useful for noninvasive determination of myocardial blood flow with the arterial reference sampling technique. This possibility was examined in 27 experiments in 10 chronically instrumented dogs at control, high and low blood flows. Myocardial blood flow was calculated in vivo from the myocardial N-13 tissue activity concentrations derived from serial cross-sectional images of the heart, the 2 minute arterial input function and the withdrawal rate of arterial blood. These calculations were compared with blood flow determined by the standard microsphere technique. Blood flow determined in vivo with N-13 ammonia and positron emission tomography correlated with microsphere blood flow by y = -36.2 + 1.53x -0.0027x2 (r = 0.94 with a standard error of the estimate of 16 ml/min per 100 g). For flows from 44 to 200 ml/min per 100 g, the relation between in vivo and in vitro measured myocardial blood flow was nearly linear but reached a plateau at flows higher than 200 ml/min per 100 g. These results indicate that in dogs, blood flow in the physiologic range can be quantified in vivo with N-13 ammonia and positron emission tomography.