Biomedical subjects

R S Balaban

Publications and source records attributed to R S Balaban.

At least 91 records · Page 5Linked to original sources

PubMed1992

ATP-sensitive potassium channel is essential to maintain basal coronary vascular tone in vivo.

Glibenclamide, a known selective inhibitor of ATP-sensitive potassium channels, was infused into the coronary vasculature of anesthetized dogs and of isolated perfused rabbit hearts to assess the role of this channel in the maintenance of basal coronary resistance. Infusion of glibenclamide at a concentration of 55-80 microM in the dogs resulted in a twofold steady-state increase in coronary resistance with resultant tissue ischemia. Infusion of 1 microM glibenclamide in the isolated hearts resulted in a 67% increase in coronary resistance with resultant tissue ischemia. The ischemic changes were reversible upon removal of the drug. These findings indicate that the ATP-sensitive K+ channel plays a significant role in the maintenance of basal coronary resistance in vivo. Higher concentrations of glibenclamide (80-100 microM) in the in vivo dog heart consistently gave rise to an oscillating pattern of coronary flow. These oscillations were either eliminated or decreased in amplitude and frequency by the infusion of 8-phenyltheophylline, a specific competitive inhibitor of adenosine receptors. 31P-nuclear magnetic resonance spectroscopy performed at the peaks and troughs of these oscillations revealed oscillation of the phosphorylation potential at the same frequency. Thus adenosine release caused by tissue ischemia appears to play a major role in creating the oscillating pattern of coronary blood flow, that occurs during the inhibition of ATP-sensitive K+ channels by glibenclamide.

Adenosine Triphosphate↗

PubMed1992

Regulation of glycogen metabolism in canine myocardium: effects of insulin and epinephrine in vivo.

Myocardial glycogen synthesis and glucose, lactate, and oxygen extraction were measured in the hearts of anesthetized dogs during infusions of insulin and epinephrine. Glycogen was monitored in vivo using 13C-nuclear magnetic resonance during an infusion of [1-13C]glucose into the left anterior descending artery. Glycogen synthesis was observed during a venous infusion of insulin (1.8 microU.min-1.kg-1), and this newly synthesized glycogen was neither broken down nor was more glycogen synthesized during a subsequent epinephrine infusion (0.5 micrograms.min-1.kg-1). During recovery from epinephrine, glycogen synthesis occurred at 2.1 times the rate seen in the control period. Glycogen synthesis was not stimulated in the absence of epinephrine by control infusions of saline. Glucose uptake was increased by insulin during the control period (from 0.09 to 0.39 mumol.min-1.g-1), so that the combined extraction of glucose and lactate exceeded the requirement for oxidizable substrate calculated from oxygen consumption. The "excess" glucose (0.15 mumol.min-1.g wet wt-1) is presumably available for glycogen synthesis. During recovery from epinephrine, lactate uptake was increased over threefold. Because this additional lactate supplies most of the fuel required for oxidation, the excess glucose available for glycogen synthesis during this period was two times that seen before epinephrine (an average of 0.32 mumol.min-1.g wet wt-1 between 20 and 40 min postepinephrine). These data are consistent with the notion that glycogen synthesis can be activated in the heart without an accompanying increase in glucose uptake by providing an alternative substrate (i.e., lactate) for oxidation.

Animals↗

PubMed1992

Myocardial oxygenation in the isolated working rabbit heart as a function of work.

Myocardial O2 consumption (MVO2) was stimulated up to two-fold by either increasing afterload or beta-receptor stimulation in working normothermic isolated rabbit hearts while noninvasively monitoring the O2 delivery or phosphate compounds (total n = 48). Intracellular O2 delivery was estimated with the use of myocardial optical absorbance changes centered at 603.5 and 582 nm that correlate with cytochrome aa3 redox and myoglobin oxygenation states. Phosphate-containing metabolites (ATP, phosphocreatine, free ADP) were assessed using 31P nuclear magnetic resonance spectroscopy. Measurements were made both with intact autoregulation and after maximal vasodilation by 1 microM nitroprusside (NP). When afterload was used to increase MVO2, absorbance decreased at 603.5 nm and increased at 582 nm, consistent with a 10-15% increase in myocardial oxygenation, without an associated change in cardiac phosphate compounds. NP caused a further increase in myocardial oxygenation and venous PO2 consistent with an increase in the O2 supply-to-demand ratio. Increases in MVO2 due to beta-stimulation alone were not associated with changes in 603.5-nm absorbance or phosphate compounds, but in combination with NP were accompanied by increased oxygenation, venous PO2, and cardiac phosphocreatine. KCl arrest caused maximal increases in oxygenation and phosphocreatine. These findings suggest that neither cytochrome aa3 nor myoglobin in the isolated working rabbit heart is fully oxidized or oxygenated, respectively. Furthermore, the oxygenation state of the tissue varied both with afterload-induced changes in cardiac work and with changes in O2 supply/demand.

Adenosine Triphosphate↗

PubMed1992

Magnetization transfer contrast in magnetic resonance imaging.

Magnetization transfer contrast (MTC) in magnetic resonance imaging (MRI) is the result of selectively observing the interaction of bulk water protons with the protons contained in macromolecules of a tissue. Since different tissues have different macromolecular compositions, the MTC can generate very high tissue contrast that is based on well-defined physiochemical properties. This is accomplished by combining a saturation transfer technique with standard MRI procedures. The specific practical and theoretical aspects of saturation transfer as it applies to the generation of MTC are reviewed and discussed. In the last 3 years, MTC has been applied to the study of the body, with useful applications demonstrated in evaluating the morphology of the knee joint, eye, brain, breast, and heart. The application of MTC to accentuate MR angiography and contrast agent studies has also been demonstrated. Thus, MTC is becoming another tool towards maximizing the quality and diagnostic potential of MRI. Recent studies on isolated macromolecules have suggested that the MTC effect is specific to the surface chemistry and correlation time of the macromolecules. These latter results indicate that the magnetization transfer process may provide a unique quantitative method of MR tissue characterization based on macromolecule dynamics and chemistry.

Animals↗

PubMed1991

19F NMR quantitation of lens aldose reductase activity using 3-deoxy-3-fluoro-D-glucose.

In the present study we have determined the kinetics of 3-deoxy-3-fluoro-D-glucose (3-FG) as a substrate for the aldose reductase reaction in vitro. In addition, we compared the 3-deoxy-3-fluoro-sorbitol (3-FS) production rates from 3-FG in the intact lens using 19F NMR with conventional aldose reductase determinations in extracts from the same lenses. The affinity of in vitro aldose reductase for 3-FG was approximately 20 times greater (9.3 mM) than that for glucose (188 mM). An excellent correlation between the rate of 3-FS production in the intact canine lens, determined with 19F NMR, and extracted aldose reductase activity was observed. The relatively high affinity of aldose reductase for 3-FG and the correlation of 3-FS production with enzyme activity in the intact lens suggests that 3-FS production from 3-FG detected by 19F NMR could provide an accurate noninvasive determination of aldose reductase activity in the eye lens.

Aldehyde Reductase↗

PubMed1991

Quantitative 1H magnetization transfer imaging in vivo.

A major factor contributing to proton (1H) spin-lattice relaxation in biological tissues is believed to be magnetization transfer between 1H in free bulk water and 1H restricted motion associated with macromolecules. We have shown recently that saturation transfer is an effective approach for studying this magnetization transfer process. Herein the determination of magnetization transfer rates in biological tissues is further analyzed by considering the time and power dependencies of saturation transfer. Following these analyses, quantitative magnetization transfer rate constant image maps were collected from the kidney in vivo. These rate constant images may prove useful in quantitative tissue characterization and in the determination of tissue-specific 1H relaxation mechanisms.

Agar↗

PubMed1991

Lipid bilayer and water proton magnetization transfer: effect of cholesterol.

Magnetization transfer between macromolecules and water can be a significant factor contributing to tissue water 1H relaxation. Using saturation transfer techniques, the degree of magnetization transfer between the macromolecular matrix and bulk water 1H can be directly measured and magnetization transfer contrast (MTC) can be generated in MR images. A significant degree of MTC has been observed in tissues with high plasma membrane content such as kidney and brain. The purpose of this study was to establish whether lipid bilayers, as models for cell membranes, could exchange magnetization with the water solvent and whether this effect could contribute to MTC observed in intact tissues. Magnetization transfer was measured in aqueous dispersions of egg phosphatidylcholine (EPC) in the presence and absence of cholesterol. It was found that neither EPC bilayers nor cholesterol by themselves significantly exchanged magnetization with bulk water 1H. However, as the concentration of cholesterol was increased, the pseudo-first-order magnetization exchange rate increased to a maximum value of approximately 1 s-1. The cholesterol-induced 1H magnetization exchange may be related either to longer correlation times of the lipid or to an increase in the number of water molecules associated with the bilayer. These results indicate that EPC-cholesterol bilayers exchange 1H magnetization with bulk water. These results are consistent with lipid bilayer contributions to bulk water relaxation and MTC in intact biological tissues.

Animals↗

PubMed1991

Metabolism of the heart and brain during hypothermic cardiopulmonary bypass.

The alterations in tissue metabolism induced by hypothermic cardiopulmonary bypass are not completely known. Phosphorus-31 nuclear magnetic resonance spectroscopy was used to determine the effect of hypothermic cardiopulmonary bypass on energy states and intracellular pH of the heart and brain. Sheep were instrumented for cardiopulmonary bypass and had a radiofrequency coil placed over either the heart or skull. The animals were placed in a 4.7-T magnet at 37 degrees C and spectra obtained. The animals were cooled on cardiopulmonary bypass to either 26 degrees C (n = 17) or 18 degrees C (n = 14) for brain studies and to 26 degrees C (n = 12) for heart studies. Hypothermia increased the phosphocreatine/adenosine triphosphate ratio in the heart (2.38 +/- 0.23 versus 3.18 +/- 0.37, 37 degrees versus 26 degrees C, respectively, p = 0.03). The brain phosphocreatine/adenosine triphosphate ratio increased from 1.70 +/- 0.09 at 37 degrees C to 2.00 +/- 0.12 at 26 degrees C (p = 0.009) and 2.10 +/- 0.07 at 18 degrees C (p = 0.0001). Intracellular pH increased during hypothermia (heart: 7.05 +/- 0.02 to 7.18 +/- 0.02, 37 degrees versus 26 degrees C, p = 0.0001; and brain: 7.07 +/- 0.02 versus 7.32 +/- 0.02, 37 degrees versus 18 degrees C, p = 0.0001). The adenosine triphosphate resonance position is known to be sensitive to magnesium binding as well as temperature and was shifted upfield (p less than 0.01) in both the heart and brain. This effect could be totally explained by the temperature dependence of this process.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid-Base Equilibrium↗

PubMed1991

Magnetization transfer contrast: method for improving contrast in gradient-recalled-echo images.

A method that improves contrast in gradient-recalled-echo magnetic resonance images is described. The increase in contrast is produced by applying low-power radio-frequency irradiation 5-10 kHz from the main water proton resonance frequency to excite macromolecular hydrogen-1 nuclei as part of a conventional gradient-recalled-echo sequence. The contrast so obtained is theoretically different from T1 or T2 contrast and has been termed magnetization transfer contrast. Cat head images were acquired in vivo with this method at 4.7 and 1.5 T. Compared with conventional gradient-recalled-echo images, the magnetization transfer images demonstrate increased contrast between many tissue pairs, such as between white matter and blood and between gray matter and cerebrospinal fluid. The dependence of the magnetization transfer effect on repetition time and preirradiation power were also studied.

Animals↗

PubMed1991

Magnetization transfer contrast: MR imaging of the knee.

The use of magnetization transfer contrast (MTC) in magnetic resonance imaging of the human knee was evaluated in this study. MTC is generated by irradiating the macromolecular protons in tissue with a low power off-resonance radio-frequency field. This results in a decrease in water proton signal intensity where a tight magnetic coupling between water and macromolecules exists. With this approach, the authors have demonstrated that MTC can improve contrast in standard single-section, gradient-recalled-echo images of the knee with regard to fat-muscle and cartilage-synovial fluid comparisons. The effect of changes in repetition time, echo time, and flip angle were also quantitatively evaluated. More important, MTC was shown to generate useful cartilage-synovial fluid contrast on high-resolution three-dimensional images, in which contrast is difficult to generate. This approach may not only provide better structural information about the knee, but may also provide noninvasive insight into the structure and biochemical composition of cartilage in vivo.

Humans↗

PubMed1991

Magnetization transfer contrast in MR imaging of the heart.

The use of magnetization transfer contrast (MTC) in short-echo-time (TE) cardiac magnetic resonance (MR) imaging was evaluated. For most cardiac MR imaging protocols, either long TE and short repetition time or exogenous intravascular agents are used for generating contrast between the ventricular wall and cavity as well as detecting pathologic conditions of the ventricular wall. The major problem with long-TE images is that the motion of the heart degrades the spatial resolution of the image during the TE period. However, MTC is generated by an off-resonance irradiation during the interpulse delay period that is relatively insensitive to motion artifacts. Short-TE (5-15 msec) gradient-recalled echo sequences were used for imaging the heart with and without MTC. These studies revealed that MTC can be used to greatly improve the contrast between the myocardial wall and blood chamber in short-TE images and may provide useful parameters for tissue characterization in pathologic cardiac muscle.

Adult↗

PubMed1991

Relationship of cerebral and myocardial intracellular pH to blood pH during hypothermia.

The regulation of tissue pH during hypothermia is important for cellular homeostasis. The present study was undertaken to determine the relationship between blood pH and intracellular pH in the brain and heart during hypothermia in sheep and to compare these data with those in humans. Intracellular pH (pHi) was determined by 31P nuclear magnetic resonance (NMR) spectroscopic data collected from the heart and brain of sheep during cardiopulmonary bypass (CPB). Alpha-stat and pH-stat blood pH management schemes were compared. When the blood pH was held constant (pH stat), the pHi of the heart increased from 7.01 +/- 0.01 at 37 degrees C to 7.18 +/- 0.02 at 26 degrees C, and the pHi of the brain increased from 7.04 +/- 0.02 at 37 degrees C to 7.23 +/- 0.02 at 26 degrees C and to 7.32 +/- 0.04 at 20 degrees C. Alpha-stat pH management resulted in similar increases in pH to that found with pH-stat [heart: 7.00 +/- 0.02 at 37 degrees C to 7.19 +/- 0.03 at 26 degrees C; brain: 7.07 +/- 0.02 at 37 degrees C to 7.29 +/- 0.02 at 26 degrees C, and 7.32 +/- 0.03 at 20 degrees C]. The tissue pH of the heart in humans showed similar findings during blood pH-stat regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Effects of beta-hydroxybutyrate on oxidative metabolism and phosphorylation potential in canine heart in vivo.

beta-Hydroxy-butyrate (HBA) is an effective substrate for mitochondrial respiration (MVO2) in the heart. Myocardial HBA oxidation is associated with high mitochondrial NADH and an inhibition of glycolytic flux. The purpose of this study was to investigate if the infusion of HBA in vivo could modify the coupling mechanisms between myocardial MVO2 and work in the presence and absence of epinephrine. The extraction of several oxidized metabolites, O2, and HBA was measured during the infusion of HBA as well as the high-energy phosphate metabolites using 31P-nuclear magnetic resonance spectroscopy. HBA infusion did not affect the MVO2 or function of the heart with or without epinephrine infusion. However, HBA increased the phosphorylation potential by decreasing inorganic phosphate and the calculated free ADP concentration under both conditions. This is consistent with HBA increasing the mitochondrial NADH, which results in an increase in the phosphorylation potential without modifying function. These data demonstrate that substrates, specifically HBA, can modulate the cardiac phosphorylation potential in vivo. The most likely mechanism for this effect is through the mitochondrial NADH concentration.

3-Hydroxybutyric Acid↗

PubMed1991

Effect of work on intracellular calcium of the intact heart.

Intracellular calcium has been proposed to play a key role in the orchestration of metabolic rate with contractile activity in the mammalian heart. Calcium is believed to accomplish this task by modulating the contractile apparatus as well as the metabolic process directly, and perhaps simultaneously, during alterations in cardiac work. The purpose of this study was to evaluate whether appropriate changes in intracellular calcium accompany alterations in cardiac work in the intact working rabbit heart. A range of myocardial oxygen consumption was obtained from 0.94 to 6.51 mumol.g LV wt-1.min-1 by changing afterload or beta-agonist addition. With the increase in work and associated increase in respiration, an increase in intracellular calcium was observed, on the basis of indo-1 fluorescence. These results indicate that intracellular calcium is a valid candidate as a cytosolic transducer contributing to the orchestration of myofibril adenosinetriphosphatase activity and oxidative phosphorylation in the intact heart.

Animals↗

PubMed1991

Magnetic resonance imaging of the rabbit eye. Improved anatomical detail using magnetization transfer contrast.

Proton nuclear magnetic resonance (NMR) imaging previously has been used to examine structure and pathologies of the eye. The present study investigates the use of a saturation-transfer technique, which exploits water-macromolecular proton magnetic interactions, to enhance image contrast in the rabbit eye in vivo. Upon steady-state saturation of the macromolecular-proton magnetization, the water-proton signal intensity will decrease in proportion to the degree of water-macromolecular proton magnetic interaction. NMR images of the eye collected using saturation transfer are shown to have superior contrast compared to conventional NMR imaging techniques, in regard to numerous ocular structures, including the iris, ciliary bodies, muscle, lens, and cornea.

Animals↗

PubMed1991

Low-flow hypothermic cardiopulmonary bypass protects the brain.

Cerebral protection during surgical procedures necessitating circulatory arrest or low flow remains the factor that most limits the critical time for repair of lesions. In vivo phosphorus-31 nuclear magnetic resonance spectroscopy was used to assess the metabolic state of the brain during circulatory arrest by measuring the concentration of high-energy phosphate compounds and the intracellular pH. The degree of cerebral protection during deep hypothermic cardiopulmonary bypass at low flow rates was compared with that obtained with a period of circulatory arrest interrupted by intermittent systemic perfusion. Sheep were instrumented with cannulas for cardiopulmonary bypass, and a radiofrequency coil was positioned on the skull. Animals were placed in the bore of a 4.7 Tesla magnet, cooled with the aid of cardiopulmonary bypass to 15 degrees C, and had either circulatory arrest (n = 5) or continuous low flow rates of 5 ml/kg/min (n = 6) or 10 ml/kg/min (n = 7) for 2 hours. A fourth group (n = 5) underwent 1 hour of circulatory arrest, systemic reperfusion for 30 minutes, then another hour of circulatory arrest. Both circulatory arrest and a flow rate of 5 ml/kg/min resulted in severe intracellular acidosis and depletion of high-energy phosphates. A flow of 10 ml/kg/min preserved high-energy phosphates and intracellular pH. Therefore deep hypothermia with cardiopulmonary bypass flows as low as 10 ml/kg/min can maintain brain high-energy phosphate concentrations and intracellular pH for 2 hours in sheep, whereas flows of 5 ml/kg/min or intermittent full-flow systemic perfusion between periods of circulatory arrest offers less protection. Previous studies from our laboratory have shown that improvement in nuclear magnetic resonance parameters positively correlates with improved survival and preservation of neurologic function.

Adenosine Triphosphate↗

PubMed1990

Combination of 31P-NMR magnetization transfer and radioisotope exchange methods for assessment of an enzyme reaction mechanism: rate-determining steps of the creatine kinase reaction.

The theoretical analysis of a reversible enzyme reaction performed in this work shows that the 31P-NMR magnetization (saturation) transfer technique combined with a radioisotope exchange method may potentially provide information on the position of rate-determining step(s). It depends on chemical shifts of NMR signals of nuclei of interest in free and enzyme-bound forms of substrate(s) and product(s) of the reaction. The creatine kinase reaction (MgATP + creatine----MgADP + P-creatine) has been used as a model. Chemical shifts of 31P in binary, ternary and transitional state substrate-enzyme complexes have been estimated by the variable frequency saturation transfer (VFST) method. This method is based on selective irradiation of numerous points in the spectrum and observation of changes in the intensity of visible line(s) which occur due to chemical exchange between it and lines which are not visible in the routine spectrum. Also, dissociation rate constants of MgADP-containing complexes were determined. Magnetization exchange rates, P-creatine----[gamma-P]MgATP and [beta-P]MgADP----[beta-P]MgATP, were compared with radioisotope exchange rates, [gamma-32P-MgATP----P-creatine and [3H]MgADP----MgATP at different [P-creatine]/[creatine] ratios and at different temperatures. All these exchange rates were close to each other at 30-37 degrees C and [PCr]/[Cr] ratios lower than 2. It is concluded that phosphoryl group transfer is the rate-determining step of the overall creatine kinase reaction under these conditions. However, at lower temperatures (below 25 degrees C) or at high [PCr]/[Cr] ratios ([ADP] less than 20 microM) the rate-determining step seems to be shifted toward dissociation of nucleotide substrates from enzyme-substrate complexes, since exchange rates became significantly different. This approach is useful for analysis of mechanism of enzymatic reactions and also can be applied to non-enzymatic reactions and evaluation of small rapidly exchangeable metabolite pools.

Adenosine Diphosphate↗

PubMed1990

In vivo metabolism of 3-deoxy-3-fluoro-D-glucose.

Recent studies have demonstrated that 3-deoxy-3-fluoro-D-glucose (3-FG) is metabolized to 3-deoxy-3-fluoro-D-sorbitol (3-FS), via aldose reductase, and 3-deoxy-3-fluoro-D-fructose (3-FF), via the sorbitol dehydrogenase reaction with 3-FS, in rat cerebral tissue (Kwee, I. L., Nakada, T., and Card, P. J. (1987) J. Neurochem. 49, 428-433). However, the biochemistry of 3-FG in other mammalian organs has not been investigated making the application of 3-FG as a metabolic tracer uncertain. To address this issue we investigated 3-FG metabolism and distribution in isolated cell lines and in rabbit tissues in vivo with 19F NMR and gas chromatography-mass spectrometry. In general, the production of 3-FS is well correlated with the known distribution of aldose reductase in all the systems studied. Further metabolism of 3-FS to 3-FF was verified to occur in cerebral tissue. Surprisingly, two new fluorinated compounds were found in the liver and kidney cortex. These compounds are identified as 3-deoxy-3-fluoro-D-gluconic acid, which is produced via glucose dehydrogenase activity on 3-FG, and 3-deoxy-3-fluoro-D-gluconate-6-phosphate. Based on enzyme studies, it is argued that the 3-deoxy-3-fluoro-D-gluconate-6-phosphate is derived directly from 3-deoxy-3-fluoro-D-gluconic acid and not as a product of pentose phosphate activity. Direct oxidation and reduction are the major metabolic routes of 3-FG, not metabolism through glycolysis or the pentose phosphate shunt. Thus, 3-FG metabolism coupled with 19F NMR appears to be very useful for monitoring aldose reductase and glucose dehydrogenase activity in vivo.

Aldehyde Reductase↗