High Energy Phosphate Metabolism in Experimental Permanent Focal Cerebral Ischemia: An in vivo 31 P Magnetic Resonance Spectroscopy Study

Relative levels of phosphate metabolites in the brain were examined in vivo by ³¹P magnetic resonance spectroscopy (MRS) in 50 Sprague-Dawley rats before, during, and after induction of focal permanent cerebral ischemia. After acquisition of baseline spectra, rats were subjected to injury within the core of the MR spectrometer, and ³¹P spectra were collected for 60 min after injury: in 7 rats, permanent, acute focal cerebral ischemia was induced (ischemia group); in 6 rats, mild hypoxia (FiO₂ 14%) was induced at the time of the ischemic insult and was maintained for 20 min (ischemia-hypoxia group); in 6 rats, mild hypoxia (FiO₂ 14%) only was induced for 20 min (hypoxia group). Control studies were performed in 25 rats. Cerebral intracellular pH, calculated from the chemical shift of inorganic phosphate (Pi), decreased immediately after injury in the ischemia and ischemia-hypoxia groups. The first ³¹P spectrum obtained after injury was characterized by an increase in Pi and a decrease in phosphocreatine (PCr) in the ischemia and ischemia-hypoxia groups; these changes in spectra were significantly greater in the ischemia-hypoxia group. No significant changes in adenosine triphosphate (ATP) were found in either group. Within 60 min of occlusion, ³¹P spectra returned toward baseline spectra in both ischemia-hypoxia and ischemia groups. No significant changes were seen in spectra of rats subjected to hypoxia alone. These results confirm that ³¹P MRS is a sensitive measure of early changes of high energy metabolites in focal cerebral ischemia. The return of spectra toward baseline values within 1 h of injury despite the presence of permanent ischemic damage, however, suggests that caution should be used in attempts to interpret “recovery” of ³¹P MRS.