Brain oxidative metabolism of the newborn dog: correlation between 31P NMR spectroscopy and pyridine nucleotide redox state

A tale of two methods: combining near-infrared spectroscopy with MRI for studies of brain oxygenation and metabolism

Combining magnetic resonance imaging (MRI) with near-infrared spectroscopy (NIRS) leads to excellent synergies which can improve the interpretation of either method and can provide novel data with respect to measuring brain oxygenation and metabolism. MRI has good spatial resolution, can detect a range of physiological parameters and is sensitive to changes in deoxyhemoglobin content. NIRS has lower spatial resolution, but can detect, and with specific technologies, quantify, deoxyhemoglobin, oxyhemoglobin, total hemoglobin and cytochrome oxidase. This paper reviews the application of both methods, as a multimodal technology, for assessing changes in brain oxygenation that may occur with changes in functional activation state or metabolic rate. Examples of hypoxia and ischemia are shown. Data support the concept of reduced metabolic rate resulting from hypoxia/ischemia and that metabolic rate in brain is not close to oxygen limitation during normoxia. We show that multimodal MRI and NIRS can provide novel information for studies of brain metabolism.

Real-time hemodynamic response and mitochondrial function changes with intracarotid mannitol injection

Disruption of blood brain barrier (BBB) is used to enhance chemotherapeutic drug delivery. The purpose of this study was to understand the time course of hemodynamic and metabolic response to intraarterial (IA) mannitol infusions in order to optimize the delivery of drugs for treating brain tumors.

PRINCIPAL RESULTS

We compared hemodynamic response, EEG changes, and mitochondrial function as judged by relative changes in tissue NADH concentrations, after intracarotid (IC) infusion of equal volumes of normal saline and mannitol in our rabbit IC drug delivery model. We observed significantly greater, though transient, hyperemic response to IC infusion of mannitol compared to normal saline. Infusion of mannitol also resulted in a greater increase in tissue NADH concentrations relative to the baseline. These hemodynamic, and metabolic changes returned to baseline within 5min of mannitol injection.

CONCLUSION

Significant, though transient, changes in blood flow and brain metabolism occur with IA mannitol infusion. The observed transient hyperemia would suggest that intravenous (IV) chemotherapy should be administered either just before, or concurrent with IA mannitol injections. On the other hand, IA chemotherapy should be delayed until the peak hyperemic response has subsided.

Measuring oxygenation in vivo with MRS/MRI--from gas exchange to the cell

Oxygen plays a major role as a substrate in metabolic processes in numerous signaling pathways, in redox metabolism, and in free radical metabolism. To study the role of oxygen in normal and pathophysiological states, methods that can be used noninvasively are required. This review examines the potential of nuclear magnetic resonance techniques to study tissue oxygenation. It is written from a systems perspective, looking at detection methods with respect to the path that oxygen takes in the mammalian system-from the lungs, through the vascular system, into the interstitial space, and finally into the cell. Methods discussed range from those that are quantifiable, such as the assessment of spin lattice relaxation time in fluorocarbon solutions, to those that are more correlative, such as assessment of lactate and high energy phosphates. Since the methods vary in their site of application, sensitivity, and specificity to the quantification of oxygen, this review provides examples of how each method has been applied. This may facilitate the reader's understanding of how to optimally apply different methods to study specific biomedical problems.

Oxygen transport and intracellular bioenergetics on stimulated cat skeletal muscle

A unique multiparameter recording of skeletal muscle bioenergetics, biochemistry and biomechanics has permitted determination of novel relationships among hemodynamics, cellular high-energy metabolites and mitochondrial bioenergetics in feline skeletal muscle. The study utilizes 31P NMR, NIR, and NADH fluorescence spectrophotometry, biochemical assays and muscle performance. Seven cats were anesthetized and mechanically ventilated. Calf muscles were stimulated through sciatic nerve electrical stimulation and tension was monitored by a strain gauge connected to the Achilles tendon. We stimulated the muscle to produce several workloads up to Vmax. We also changed FiO2 from normoxia to hypoxia for each %Vmax. From these results, the most sensitive indicators of cellular hypoxia leading to a reduction in muscle performance can be determined. Hemoglobin deoxygenation generally does not correlate with cellular hypoxia, although when the HbO2 drops below 30% saturation there is an increased incidence of cellular hypoxia. The [ADP], which is known to regulate mitochondrial function, has a close relation to the work, not to the hypoxia. On the other hand, the mitochondrial NADH does respond to cellular PO2. The degree of oxidation (NADH decrease) due to the ATP flux shifts with oxygen availability in mild to moderate hypoxia (at FiO2 down to 9%). As cellular hypoxia causes decreases in muscle performance (moderate to severe hypoxia), NADH is being reduced rather than oxidized with increasing workloads.

Three-dimensional redox image of the normal gerbil brain

Regional differences in the redox ratio were studied in the gerbil brain. Brains were frozen using an in situ funnel-freezing method, and sliced coronally for scanning of mitochondrial redox imaging. The relative local redox ratio of nicotinamide-adenosine dinucleotide to its reduced form was calculated from fluorescence signals of intrinsic fluorochromes, i.e. reduced nicotinamide-adenosine dinucleotide and flavoproteins, using a high resolution fluorometer developed in our laboratory. Twelve consecutive coronal images were obtained from each of 10 gerbils. The mean value of the regional redox ratio in both the cerebral and cerebellar gray matter were found to be significantly lower than that in the cerebral and cerebellar white matter (P < 0.01, Mann-Whitney test). Local differences in the redox ratio were also found among subregions of gray matter. The redox ratio in the globus pallidus was significantly higher than values in other subregions of gray matter (P < 0.01, Mann-Whitney test) We postulate that a high concentration of the reduced form of pyridine nucleotide is maintained to provide redox energy for rapid turnover of ATP in the areas of high energy consumption.

Cerebral mitochondrial redox states during metabolic stress in the immature rat

The brain mitochondrial NAD+/NADH ratio, as a reflection of the oxidation-reduction (redox) state of cellular compartment, was determined under conditions of hypoxia, anoxia, hypoxia-ischemia, complete ischemia and hypoglycemia in immature rats. NAD+/NADH ratios were calculated from changes in the concentrations of specific oxidative substrates and calculated intracellular pH during cerebral metabolic stress. The results suggest that the use of the acetoacetate/beta-hydroxybutyrate substrate couple provides a more accurate prediction of the mitochondrial redox state under adverse conditions than use of the alpha-ketoglutarate/glutamate couple. It is possible that the mitochondrial oxidation seen with the latter substrate couple during cerebral metabolic stress might reflect a population of cells (neurons or glia) which are substrate-deprived relative to the rest of the brain in the setting of metabolic stress produced by oxygen deficiency.

Effect of acetyl-L-carnitine on recovery of brain phosphorus metabolites and lactic acid level during reperfusion after cerebral ischemia in the rat--study by 13P- and 1H-NMR spectroscopy

The effects of acetyl-L-carnitine (ALCAR) treatment on brain energy state recovery and lactic acid levels following 20 min ischemia and 2, 24 and 48 h reperfusion were investigated by 31P and 1H-NMR spectroscopy. Transient forebrain ischemia was induced by four-vessel occlusion method in fed 6-month-old Fischer rats. ALCAR or saline was administered by intraperitoneal route immediately after 20 min ischemia and again at 1, 4, 24 and 30 h during reperfusion. Twenty-min severe forebrain ischemia was associated with a marked decrease in phosphocreatine (PCr) and ATP levels and a corresponding increase in lactic acid, inorganic phosphate (Pi), AMP, creatine, glycerol 3-phosphate and alanine levels. Following reperfusion, a general tendency to restore pre-ischemic metabolite levels was observed. However, after 2 h reperfusion in saline-treated rats, lactic acid and Pi levels remained significantly higher, while ATP levels were still significantly lower than in non-ischemic controls. On the contrary, in ALCAR-treated animals a complete recovery of all metabolites including Pi and ATP was observed, while PCr levels were even more elevated compared with those in saline-treated rats. Furthermore lactic acid content was significantly lower than that in both saline-treated and non-ischemic control rats. It is concluded that a potential therapeutic role may be claimed for ALCAR in the treatment of cerebral ischemia through mechanisms that include faster recovery and improvement of brain energy production as well as a decreased lactic acid content during early post-ischemic reperfusion.

Effects of age on the metabolic, ionic and electrical responses to anoxia in the newborn dog brain in vivo

The interrelation between brain energy metabolism, electrical activity and ion homeostasis developing under experimental anoxia in animals of different ages is of significant value in the understanding of brain damage occurring under similar conditions of clinical neuropathology. The purpose of the present study was to compare brain energy states and extracellular ion homeostasis during anoxia in newborn puppies of various ages. We have developed and used a multiparametric monitoring device by which various functions of the brain can be recorded in a real-time mode from a 5 mm diameter area on the surface of the cortex. Intracellular oxygen balance was evaluated in newborn puppies of various ages by monitoring the intramitochondrial NADH redox state using a fluorescence technique. The electrical activity was measured by recording the spontaneous ECoG (electrocorticogram) and DC (direct current) steady potential. Ion homeostasis was evaluated using surface potassium and calcium mini-electrodes. Newborn puppies were anesthetized, the dura mater was removed and the multiprobe assembly was placed on the brain and cemented to the skull. Five groups of puppies (0-1, 2-7, 8-14, 15-21 days and 3-24 weeks) were exposed to 5 minutes of complete O2 deprivation (100% nitrogen exposure) and were monitored during the recovery period until all parameters returned to baseline values. The results may be summarized as follows: 1. Resting baseline levels of extracellular K+ were in the same range as described for other young and adult mammals (2.9 +/- 0.05 mM). 2. Extracellular Ca2+ levels were higher than those published for other mammals (1.6 +/- 0.07 mM). 3. During 5 minutes of anoxia, a significant increase in K+ levels was recorded. This increase was not accompanied by measurable changes in extracellular Ca2+. 4. The effect of age on the length of time to the elevation of the extracellular K+ concentration and on the rate of K+ accumulation from the onset of the anoxic condition was significant, i.e., the younger the animal the longer the time and the lower the rate. 5. The rate of energy depletion was age dependent as indicated by the rate of NADH accumulation during anoxia. However, no significant effect of age on the basal aerobic metabolism was found as measured by the maximum percent increase of NADH during anoxia.(ABSTRACT TRUNCATED AT 400 WORDS)

Complicated migraine studied by phosphorus magnetic resonance spectroscopy

The brain and skeletal muscle of eight adult patients with migraine with prolonged auras or migraine strokes leaving a permanent hemianopic defect were studied by phosphorus magnetic resonance spectroscopy. Biochemical assays performed on muscle biopsy and platelets had revealed abnormal mitochondrial enzyme activities. Brain magnetic resonance spectroscopy showed an abnormally low phosphocreatine to inorganic phosphate ratio in all patients, apparently due to decreased phosphocreatine and increased inorganic phosphate contents. Muscle phosphorus magnetic resonance spectroscopy showed low recovery from exercise in seven patients. Three patients had an increased phosphocreatine/inorganic phosphate ratio at rest, and the exercise transfer characteristics were abnormal in four patients for relatively low levels of exercise. The mitochondrial metabolic defects present in platelets and muscle of complicated migraine patients are therefore also expressed in the brain.