LOW-MOLECULAR-WEIGHT IRON IN CEREBRAL ISCHEMIC ACIDOSIS IN-VIVO

Background and Purpose-Iron-catalyzed radical generation is a potentia lly significant mechanism by which extensive tissue acidosis exacerbat es brain injury during ischemia/reperfusion. We hypothesized that leve ls of low-molecular-weight (LMW) iron increase during in vivo global c erebral ischemia in a pH-dependent manner, potentially catalyzing oxid ant injury. The present study quantified regional differences in LMW i ron during global cerebral incomplete ischemia and determined whether augmenting the fall in ischemic tissue pH with hyperglycemia also ampl ifies free iron availability. Methods-Dogs anesthetized with pentobarb ital-fentanyl were treated with 30 minutes of global incomplete cerebr al ischemia produced by intracranial pressure elevation. Cerebral ener gy metabolites (ATP, phosphocreatine) and intracellular pH (pH(i)) wer e measured by P-31 magnetic resonance spectroscopy. Preischemic plasma glucose level was manipulated to titrate end-ischemic pH(i). After is chemia, brains were perfused with cold phosphate-buffered saline solut ion; then 16 different brain areas were sampled, filtered to separate the LMW fraction (<30 000 D), and assayed by rapid colorimetric assay for tissue iron. Total iron, LMW iron, and protein in each sample were measured in sham-operated (no ischemia, n=8), normoglycemic ischemia (ISCH [glucose 7+/-4 mmol/L], n=7), and hyperglycemic (GLU-ISCH [gluco se 31+/-3 mmol/L], n=9) groups. Results-High-energy phosphates fell to near zero values in both ISCH and GLU-ISCH groups by 30 minutes but r emained unchanged in the sham-operated group. As expected, pH(i) decre ased during ischemia but to a greater extent in GLU-ISCH (6.20+/-0.05 in ISCH, 6.08+/-0.04 in GLU-ISCH, P<.05). Iron could be detected in al l areas of the brain in sham-operated animals, with the highest amount s obtained from subcortical areas such as the hippocampus, pens, midbr ain, and medulla. Total iron was higher in ISCH relative to sham-opera ted animals and higher in cortex and pens relative to GLU-ISCH. Region al LMW (as a percentage of total iron; LMW/total iron) was elevated in numerous brain areas in ISCH, including cortical gray matter, cerebel lum, hippocampus, caudate, and midbrain. LMW/total iron was higher in GLU-ISCH versus ISCH in cortical gray matter only. In other brain area s, ischemic LMW/total iron was equivalent in glucose-treated or normog lycemic animals (white matter, thalamus, pens, medulla) or lower in th e glucose-treated group (cerebellum, hippocampus, caudate, midbrain). Conclusions-These data demonstrate that levels of total and LMW iron i ncrease with global cerebral ischemia in the majority of cortical and subcortical regions of normoglycemic brain. However, exacerbation of i schemic acidosis via glucose administration does not increase tissue i ron and produces a greater increase in the LMW fraction in cortical gr ay matter only. In other brain regions, total and LMW iron availabilit y is similar to that of nonischemic animals.