Hypothermic circulatory arrest: 31-phosphorus nuclear magnetic resonsance of isolated perfused neonatal rat brain.

Deep hypothermic circulatory arrest facilitates repair of congenital cardiac anomalies in infants. However, despite its widespread use, little is known of the fundamental cellular and molecular changes induced. An isolated perfused brain model was developed in part to study high-energy phosphate metabolism with 31-phosphorus nuclear magnetic resonance (31-P NMR) at 109.3 mHz. Neonatal Sprague-Dawley rats cannulated through the ascending aorta were perfused with modified Krebs-Henseleit buffer. Soft tissues surrounding the calvarium and cervical and thoracic spine were excised and the preparation was lowered into an NMR tube (15 mm outer diameter). Then 200 to 400 free induction decays (FIDS) were averaged and transformed to produce each spectrum. The 31-P NMR spectra of well-perfused brain show six major resonances representing α, β, and γ phosphates of adenosine triphosphate (ATP), sugar phosphate, inorganic phosphate (Pi), and creatine phosphate (CrP). Preparations equilibrated to 37° C and 20° C were subjected to 20 minutes of ischemia with 15 to 20 minutes of reperfusion. Levels of CrP and ATP fell coordinately to 18% ± 1% and 34% ± 4% of control levels, respectively, by 15 minutes of normothermic ischemia. This is distinctly different from adult rat heart, wherein a fall in CrP significantly precedes the decrease in ATP: At 20° C CrP fell rapidly while ATP remained unchanged. This suggests isolation of creatine kinase from the ATP pool in brain. Following ischemia at 37° C, ATP and CrP recovered substantially but failed to return to control levels (51% ± 9% and 44% ± 12%, respectively). Following ischemia at 20° C, ATP and CrP returned to control levels by 20 minutes. Intracellular pH determinations by chemical shift of inorganic phosphate (Pi) revealed a decrease from 7.2 to 6.7 during ischemia at 37° C, whereas pH at 20° C remained unchanged above 7.2. NMR proved a valuable tool for studying high-energy phosphate metabolism in brain. This study suggests that permanent changes in ATP and CrP pools induced by 20 minutes of normothermic ischemia are attenuated whereas intracellular pH changes are abated by hypothermia.