Biomedical subjects
Publications and source records attributed to A L Busza.
23Na magnetic resonance imaging has been used to investigate sodium changes during and after cerebral ischaemia in a gerbil model. The sodium signal decreased within 4 minutes of the onset of ischaemia, and subsequently increased between 4 and 8 minutes after the onset of reperfusion. These observations may be reflecting the redistribution of tissue sodium resulting from energy failure and recovery.
23Na magnetic resonance imaging has been used in this preliminary study to investigate early changes in brain sodium signal intensity during and after cerebral ischaemia in a gerbil model. The total sodium signal in selected brain regions decreased between 15 and 30% within 4 min of the onset of ischaemia, and then remained constant throughout the ischaemic period. The same pattern was observed in the eyes. On reperfusion, there was no significant change in the sodium signal over the first 4 min, but by 8 min the signal intensity had returned to or passed through control levels in all regions measured, with the exception of the eyes. These observations are consistent with the loss and resynthesis of ATP as seen in this model, and may be reflecting the redistribution of tissue sodium resulting from energy failure and recovery.
The metabolic status of rat livers during hypothermia has been studied using 31P magnetic resonance spectroscopy. Perfusion with oxygenated buffer at 6-8 degrees C allowed maintenance of ATP, while pH increased to values in the range 7.7-7.9. In organs depleted of ATP by a short (2 h) period of cold ischemia, pH fell to 6.92 +/- 0.10. If these livers were reperfused with hypoxic buffer at hypothermia, two distinct responses were noted. In one group (responders), there was evidence of ATP resynthesis and in these organs pH returned to 7.90 +/- 0.28. In the second group (non-responders), there was no recovery of ATP synthesis and pH remained depressed at 6.97 +/- 0.07. In another group, adenine nucleotides were severely depleted by 24 h of cold ischemia, and in these livers there was again no significant recovery of ATP synthesis during hypoxic reperfusion and pH remained at 7.03 +/- 0.25. These results suggest that (a) there is an apparent relationship between energy metabolism and control of intracellular pH in the hypothermic mammalian liver, and (b) that intracellular pH may shift in liver at hypothermia to values predicted by the alpha-stat hypothesis.
The first practical demonstration that nuclear magnetic resonance (NMR) spectroscopy could be applied to the study of brain biochemistry in vivo came in 1980, with the studies of the rat brain using a surface coil. Since then the technique has been rapidly and extensively developed into a versatile, non-invasive tool for the investigation of various aspects of brain biochemistry, physiology and disease. NMR is non-destructive and can be used to examine a wide variety of samples, ranging from localized regions within the whole brain in humans or animals, through tissue preparations (perfused organ, tissue slices and homogenates), to isolated cells and aqueous solutions, such as tissue extracts. 31P and 1H NMR spectra deriving from endogenous compounds of the brain in situ allow assessment of tissue metabolites and provide information about high-energy phosphates, lactate, certain amino acids, intracellular pH and ionic concentrations. Exogenous substrates or probes labelled with stable isotopes can also be introduced into the brain and used to monitor metabolism. Animal models of brain diseases have given some impetus to rapid progress in clinical NMR spectroscopy and also magnetic imaging techniques. The purpose of this article is to highlight the type of information available from these NMR techniques, and to present this in a neuroscience context, emphasizing the biochemical, physiological and pathological information that can be obtained using these methods.
BACKGROUND AND PURPOSE: Magnetic resonance spectroscopy and imaging enable us to investigate biochemical and pathophysiological changes associated with cerebral ischemia. The specific aims of these studies were to establish the relationships between energy metabolites and regional cerebral blood flow and to determine whether diffusion-weighted imaging is sensitive to the known thresholds for cerebral tissue energy failure and disturbance of transmembrane ionic gradients in gerbils. METHODS: Magnetic resonance spectroscopy measurements of energy metabolites in the gerbil brain were obtained as a function of cerebral blood flow (measured with the hydrogen clearance technique) before, during, and after unilateral or bilateral occlusion of the common carotid arteries. Diffusion-weighted and T2-weighted images were obtained in a separate series of experiments. RESULTS: Major changes in brain energy metabolites were observed at flow values of 20 ml.100 g-1.min-1 and below. The cerebral blood flow threshold for maintenance of energy status was lowered in hypothermia, consistent with a protective effect. Diffusion-weighted imaging intensity increased at cerebral blood flow values of 15 to 20 ml.100 g-1.min-1 and below and increased gradually following the onset of severe global cerebral ischemia, but with a delay of about 2.5 minutes. CONCLUSIONS: The spectroscopic observations suggest that the flow thresholds for electrical function and edema are a direct consequence of energy failure. Comparison of the spectroscopy and imaging data suggests that diffusion-weighted imaging is sensitive to disruption of tissue energy metabolism or to a consequence of this disruption. The possibilities arise of visualizing energy failure with the spatial resolution characteristic of magnetic resonance imaging and detecting compromised but recoverable tissue.
Diffusion-weighted images of the rat brain were obtained using the pulsed-gradient spin-echo method. An attempt was made to extract perfusion-related parameters from signal intensity data taken from the caudate-putamen region of the images, by using a nonlinear least-squares calculation to fit the Le Bihan biexponential expression (Le Bihan et al., Radiology, 168, 497 (1988)) to the data. The perfusion-related parameters could not be obtained with sufficient accuracy to be useful, although the perfusion-weighted images appear to contain meaningful qualitative information. An analysis of the perfusion model is presented and shows why the Le Bihan pseudo-diffusion coefficient is particularly difficult to measure with reasonable accuracy.
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BACKGROUND AND PURPOSE: Diffusion-weighted magnetic resonance imaging has been shown to be particularly suited to the study of the acute phase of cerebral ischemia in animal models. The studies reported in this paper were undertaken to determine whether this technique is sensitive to the known ischemic thresholds for cerebral tissue energy failure and disturbance of membrane ion gradients. METHODS: Diffusion-weighted images of the gerbil brain were acquired under two sets of experimental conditions: as a function of cerebral blood flow after controlled graded occlusion of the common carotid arteries (partial ischemia), as a function of time following complete bilateral carotid artery occlusion (severe global ischemia), and on deocclusion after 60 minutes of ischemia. RESULTS: During partial cerebral ischemia, the diffusion-weighted images remained unchanged until the cerebral blood flow was reduced to 15-20 ml.100 g-1.min-1 and below, when image intensity increased as the cerebral blood flow was lowered further. This is similar to the critical flow threshold for maintenance of tissue high-energy metabolites and ion homeostasis. After the onset of severe global cerebral ischemia, diffusion-weighted image intensity increased gradually after a delay of approximately 2.5 minutes, consistent with complete loss of tissue adenosine triphosphate and with the time course of increase in extracellular potassium. This hyperintensity decreased on deocclusion following 60 minutes of ischemia. CONCLUSIONS: The data suggest that diffusion-weighted imaging is sensitive to the disruption of tissue energy metabolism or a consequence of this disruption. This raises the possibility of imaging energy failure noninvasively. In humans, this could have potential in visualizing brain regions where energy metabolism is impaired, particularly during the acute phase following stroke.
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We report here how magnetic resonance imaging can be used to gain definitive information about tissue pathology by the combined use of spin-echo and gradient-echo sequences. We also show how artifacts arising from respiratory motion can be eliminated by using a simple respiratory gating technique.
Cerebral ischaemia was produced in 2 groups of gerbils by occlusion of the common carotid arteries for 30 minutes, resulting in cerebral oedema. In group 1 cerebral oedema was measured by specific gravity microgravimetry, and in group 2 brain metabolism and blood flow were measured by 31P and 1H NMR spectroscopy and hydrogen clearance respectively. In group 1 the brain water content did not return to control levels by 180 minutes of reperfusion. Energy metabolism, determined by 31P NMR spectroscopy returned to control by 12 minutes, intracellular pH (pHi) by 20 minutes, and lactate, determined by 1H NMR spectroscopy, by 50 minutes. There was a lag of about 10 minutes before lactate began to be cleared from the brain. We suggest that while pHi is low, Na+/H+ exchange will negate the Na+ extrusion driven by the Na+/K+ ATPase. When pHi approaches normal there will be a net extrusion of Na+, taking osmotic water with it, and presumably with passive washout of lactate. This may be the cause of the initial delay in lactate clearance.
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The metabolic response of the rat liver to flushing and reflushing with Marshall's solution at pH 7.2 or pH 7.8 has been studied by 31P nuclear magnetic resonance spectroscopy. The changes in intracellular pH, inorganic phosphate, ATP and phosphomonoesters have been determined from the 31P spectra. We show that the intracellular pH at any stage of the flushing protocol is largely independent of the pH of the medium when using these solutions. However, we demonstrate that there are differences between the efficiency of the two solutions in respect of the rates of hydrolysis of ATP and accumulation of phosphomonoesters. There were also differences in the response of the livers upon reflushing--those livers reflushed at pH 7.2 resynthesized ATP from a lower initial concentration to achieve ATP concentrations similar to those restored in livers reflushed at pH 7.8. These trends were mirrored in the responses of the phosphomonoester peaks (which contain a contribution from AMP). We conclude that short-term control of liver metabolism during hypothermia is possible by use of solutions of different pH, but that for longer-term storage, other approaches may be necessary to maintain metabolic integrity.
The use of nuclear magnetic resonance (NMR) spectroscopy to assess metabolic viability in organ preservation is discussed. A brief coverage of the physical principles involved and the biochemical information available from NMR spectroscopy is given. We also present the advantages and disadvantages of the method and outline the future possibilities of the technique in relation to organ preservation.
31P nuclear magnetic resonance spectroscopy has been used to study rat livers following flushing with the University of Wisconsin (UW) lactobionate/raffinose solution (N. Jamieson, R. Sundberg, S. Lindell, J. Southard, and F.O. Belzer, Cryobiology 24, 573-574, 1987; M. Kalayoglu, H. Sollinger, R. Stratta, A. D'Alessandro, R. Hoffman, J. Pirsch, and F. O. Belzer, Lancet 1, 617-619, 1988). These studies have revealed that despite the improved storage properties that have been reported for this solution, hepatic ATP and ADP declined at a rate similar to that seen in the more commonly used Marshall's or Collins' solutions. However, there was a significant inhibition of the developing acidosis, such that by 5 hr postflush, the intracellular pH was 7.17 +/- 0.06 (mean +/- SD, n = 5) compared to 6.90 +/- 0.06 for Marshall's solution (4 hr postflush) and 6.94 +/- 0.04 for Collins' solution (4 hr postflush). This did not appear to be due to a buffering effect of the solution, as this was found to be relatively low, but was probably due to a modification of hepatic metabolism caused by the solution itself.
Nuclear magnetic resonance (NMR) spectroscopy was used in the study of rat livers following flushing with a clinically used preservation solution containing either 12 or 30% (v/v) Me2SO. The extent of equilibration of Me2SO in the tissue after 10-15 min of perfusion with Me2SO and again after subsequent washout with Me2SO-free medium was assessed by 1H NMR spectroscopy. 31P NMR spectroscopy was used to follow the changes in ATP, ADP, inorganic phosphate, and tissue pH. The data show that 1H NMR spectroscopy can be used as a sensitive and rapid method of assessing the equilibration and concentration of compounds such as Me2SO, since these compounds are likely to be present at concentrations greatly in excess of other constituents of the medium and will therefore give rise to strong, easily detected signals. At the same time, 31P NMR spectroscopy can be used to monitor the metabolic status of the tissue reflected in the levels of ATP, ADP, and inorganic phosphate, as well as being a noninvasive monitor of intracellular pH. The possibility of determining the tissue pH in the presence of solutes such as Me2SO is discussed.
31P NMR spectroscopy was used in conjunction with conventional biochemical techniques to study metabolic abnormalities in normal rats, in rats with CCl4-induced acute liver failure and in rats with paracetamol-induced acute liver failure. Studies were carried out before and after a metabolic challenge in the form of a fructose infusion. Prior to fructose infusion the ATP levels in the groups with acute liver failure were significantly lower than in control rats, despite their having a similar ATP/Pi ratio. Following a fructose infusion in control animals, the changes in phosphomonoesters, ATP and inorganic phosphate were consistent with previously reported findings. However, in both test groups the percentage changes in phosphomonoesters and ATP were significantly smaller. Intracellular pH declined in control animals, but did not change significantly in the animals with acute liver failure. Fructose clearance from the blood in control animals was not significantly different from that in animals with CCl4-induced acute liver failure. Metabolic changes measured using a horizontal magnet were very similar to those obtained with a vertical magnet.
CBF has been measured with the hydrogen clearance technique in the two cerebral hemispheres of the gerbil under halothane anaesthesia. At the same time, intracellular pH and the concentrations of lactate and high-energy phosphates were measured in the brain using 1H and 31P nuclear magnetic resonance spectroscopy. Flow and metabolism have been followed during either a 15- or a 30-min ischaemic period (induced by bilateral carotid occlusion) and for up to 1 h of recovery. There was no significant difference between the flow characteristics of the two experimental groups. High-energy phosphate levels and pH returned to control within approximately 20 min of the end of the ischaemic period. Lactate clearance, following a 30-min occlusion, was slower than the recovery of pH. The concentration of free ADP, calculated from the creatine kinase equilibrium, was lower during the recovery phase than under control conditions.