Sequential in Vivo Measurement of Cerebral Intracellular Metabolites with Phosphorus-31 Magnetic Resonance Spectroscopy during Global Cerebral Ischemia and Reperfusion in Rats

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Abstract
Phosphorus-31 magnetic resonance (31P MR) spectroscopy was used to obtain serial in vivo measurements of cerebral adenosine triphosphate (ATP), phosphocreatine (PCr), inorganic phosphate (Pi), and intracellular pH levels in rats during temporary global cerebral ischemia and reperfusion. Three groups of 4 rats each that recovered from permanent bilateral vertebral artery occlusion were placed in a MR spectrometer and subjected to remotely controlled bilateral carotid artery occlusion lasting 6, 15, or 30 minutes followed by 1 hour of reperfusion. Four additional rats that developed systemic hypotension (2 during a 6-minute occlusion and 2 during a 15-minute occlusion) were also studied. 31P MR spectra were obtained in each rat before, during, and after ischemia. Rats in which MR spectra showed metabolic recovery underwent a second occlusion followed by reperfusion and sacrifice. In the 12 normotensive rats, metabolic alterations began within 3 minutes after the onset of global ischemia. By the end of the occlusion period, cerebral ATP had decreased by 20 to 100% in 10 rats and PCr had decreased by 15 to 75% in all 12; Pi increased by 25 to 240%. The mean intracellular pH decreased from 7.33 to 6.9 ± 0.6. The degree of metabolic deterioration during ischemia was not related to the duration of occlusion. During reperfusion, ATP, PCr, Pi, and intracellular pH returned to normal in 4 rats; 5 rats had partial metabolic recovery, and 3 had minimal or transient metabolic recovery followed by progressive deterioration. All rats that developed systemic hypotension had a decrease in ATP, PCr, and intracellular pH and an increase in Pi during the initial occlusion. Each had transient partial recovery in ATP during reperfusion, and 2 had slight recovery of PCr. The onset of hypotension was followed by depletion of these metabolites, progressive increase in Pi, and progressive intracellular acidosis. All rats that deteriorated metabolically after reversal of carotid occlusion died by the end of the reperfusion period or soon after. The 8 rats that recovered from the first occlusion were subjected to a second period of ischemia, during which each rat showed severe depletion of metabolites. During the second reperfusion, only 1 rat showed significant metabolic recovery, which lasted only 30 minutes and was followed by progressive deterioration. Severe global cerebral ischemia was associated with a progressive decline in both ATP and PCr, whereas less complete ischemia seemed to be characterized by stabilization or recovery of ATP and continued depression of PCr. In the animal model studied, the variability in the degree of metabolic derangement during global ischemia and in the degree and rate of metabolic recovery during reperfusion was detected by MR spectroscopy. This study demonstrates that 31P MR spectroscopy can provide noninvasive, serial, in vivo measurements of critical high energy phosphate metabolites and intracellular pH during global cerebral ischemia and reperfusion in rats. (Neurosurgery 21:699-708, 1987)