Flow Thresholds for Cerebral Energy Disturbance and Na+ Pump Failure as Studied by in vivo 31P and 23Na Nuclear Magnetic Resonance Spectroscopy

Abstract

The relationships among CBF, cerebral energy metabolism, Na+ pump activity, and electrocorticograms (ECoG) following graded hypotension were studied in 48 gerbils. Energy metabolism and Na⁺ pump activity were estimated by in vivo ³¹p and ²³Na nuclear magnetic resonance (NMR) spectroscopy, and CBF was determined by [¹⁴C]iodoantipyrine methods at the end of the experiments. The CBF measured in normotensive animals was 0.51 ± 0.07 ml/g brain/min. Following graded hypotension, no ³¹P spectral change was observed until CBF fell to 0.21–0.27 ml/g brain/min, at which level the intracellular pH began to decrease in association with ECoG voltage reduction. At a CBF level of 0.18–0.23 ml/g brain/min, phosphocreatine (PCr) began to decrease in association with inorganic phosphate (Pi) elevation. At this level, ECoG became isoelectric, although no adenosine triphosphate (ATP) change yet resulted. At a flow level of 0.12–0.14 ml/g brain/min, ATP began to decrease gradually. At 0.04–0.05 ml/g brain/min, PCr and ATP virtually disappeared, and the ²³Na signal intensity suddenly changed. The present study demonstrated flow thresholds for the development of tissue acidosis, PCr–Pi changes, and ATP reduction. It appears that functional suppression occurs prior to ATP changes, whereas Na⁺ pump failure results after ATP depletion.