In Vivo Microdialysis of 2-Deoxyglucose 6-Phosphate into Brain

1471-4159

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Abstract : A unique method for simultaneously measuring interstitial(pHe) as well as intracellular (pHi) pH in the brains oflightly anesthetized rats is described. A 4-mm microdialysis probe wasinserted acutely into the right frontal lobe in the center of the area sampledby a surface coil tuned for the collection of 31P-NMR spectra.2-Deoxyglucose 6-phosphate (2-DG-6-P) was microdialyzed into the rat until asingle NMR peak was detected in the phosphomonoester region of the31P spectrum. pHe and pHi values werecalculated from the chemical shift of 2-DG-6-P and inorganic phosphate,respectively, relative to the phosphocreatine peak. The average in vivopHe was 7.24 ± 0.01, whereas the average pHi was7.05 ± 0.01 (n = 7). The average pHe value and the averageCSF bicarbonate value (23.5 ± 0.1 mEq/L) were used to calculate aninterstitial Pco2 of 55 mm Hg. Rats were then subjected to a 15-minperiod of either hypercapnia, by addition of CO2 (2.5, 5, or 10%)to the ventilator gases, or hypocapnia (Pco2 〈 30 mm Hg), byincreasing the ventilation rate and volume. pHe responded inverselyto arterial Pco2 and was well described (r2 =0.91) by the Henderson-Hassel-balch equation, assuming apKa for the bicarbonate buffer system of 6.1 and asolubility coefficient for CO2 of 0.031. This confirms the viewthat the bicarbonate buffer system is dominant in the interstitial space.pHi responded inversely and linearly to arterial Pco2.The intracellular effect was muted as compared with pHe (slope =-0.0025, r2 = 0.60). pHe and pHivalues were also monitored during the first 12 min of ischemia produced bycardiac arrest. pHe decreases more rapidly than pHiduring the first 5 min of ischemia. After 12 min of ischemia, pHeand pHi values were not significantly different (6.44 ± 0.02and 6.44 ± 0.03, respectively). The limitations, advantages, and futureuses of the combined microdialysis/31P-NMR method for measurementof pHe and pHi are discussed.

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