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A P Koretsky

Publications and source records attributed to A P Koretsky.

At least 55 records · Page 3Linked to original sources
PubMed1995

NMR measurement of perfusion using arterial spin labeling without saturation of macromolecular spins.

When measuring perfusion by arterial spin labeling, saturation of tissue macromolecular spins during arterial spin labeling greatly decreases tissue water magnetization, reducing the sensitivity of the technique. In this work, a theory has been developed for perfusion measurement by arterial spin labeling without saturation of macromolecular spins. A two-coil system was used to achieve arterial spin labeling without saturation of brain tissue macromolecular spins for NMR measurement of rat cerebral perfusion. The effects of cross-relaxation on the measurement of perfusion have been studied in the absence of macromolecular spin saturation, and it is demonstrated that at 4.7 Tesla, perfusion is underestimated by approximately 17% when the effect of cross-relaxation is neglected in the calculation of perfusion. However, assuming water to be a freely diffusable tracer, the effect of cross-relaxation is predicted to be flow independent, and it can, thus, be accounted for in the calculation of perfusion. The theory and experiments are presented to estimate tissue perfusion, magnetization transfer rate constants, and spin-lattice relaxation times of water and macromolecular spins in rat brain.

Algorithms↗
PubMed1995

Radiofrequency magnetic field gradient echoes have reduced sensitivity to susceptibility gradients.

The amplitudes of gradient-echoes produced using static field gradients are sensitive to diffusion of tissue water during the echo evolution time. Gradient-echoes have been used to produce MR images in which image intensity is proportional to the self-diffusion coefficient of water. However, such measurements are subject to error due to the presence of background magnetic field gradients caused by variations in local magnetic susceptibility. These local gradients add to the applied gradients. The use of radiofrequency (RF) gradients to produce gradient-echoes may avoid this problem. The RF magnetic field is orthogonal to the offset field produced by local magnetic susceptibility gradients. Thus, the effect of the local gradients on RF gradient-echo amplitude is small if the RF field is strong enough to minimize resonance offset effects. The effects of susceptibility gradients can be further reduced by storing magnetization longitudinally during the echo evolution period. A water phantom was used to evaluate the effects of background gradients on the amplitudes of RF gradient-echoes. A surface coil was used to produce an RF gradient of between 1.3 and 1.6 gauss/cm. Gradient-echoes were detected with and without a 0.16 gauss/cm static magnetic field gradient applied along the same direction as the RF gradient. The background static field gradient had no significant effect on the decay of RF gradient-echo amplitude as a function of echo evolution time. In contrast, the effect of the background gradient on echoes produced using a 1.6 gauss/cm static field gradient is calculated to be significant. This analysis suggests that RF gradient-echoes can produce MR images in which signal intensity is a function of the self-diffusion coefficient of water, but is not significantly affected by background gradients.

Electromagnetic Fields↗
PubMed1995

In situ 31P nuclear magnetic resonance for observation of polyphosphate and catabolite responses of chemostat-cultivated Saccharomyces cerevisiae after alkalinization.

The proposed pH buffering and phosphagenic functions of polyphosphate were investigated by subjecting chemostat-cultivated Saccharomyces cerevisiae to alkalinization (NaOH addition) and anaerobiosis. The subsequent changes in intracellular phosphate-containing species were observed in situ by nuclear magnetic resonance (NMR) spectroscopy by using the NMR cultivator we developed. For the alkalinization experiments, changes in catabolite secretion were also measured in parallel experiments. Additionally, a range of potential neutralization capacity was investigated: a dilute culture and concentrated cultures with low or high polyphosphate content. The concentrated cultures displayed increased cytosolic pH and rapid polyphosphate degradation to small chains. The pH changes and extent of polyphosphate degradation depended inversely on initial polyphosphate content. The dilute culture restored extracellular pH rapidly and secreted acetate. The concentrated culture with low polyphosphate reserves also secreted acetate. In contrast to the alkalinization-induced polyphosphate dynamics, anaerobiosis resulted in the complete hydrolysis of polyphosphate to P(i), as opposed to small chains, and reduced cytosolic pH. The results and calculations suggest that the bulk of NMR-observable polyphosphate (vacuolar) degradation to short polymers conceivably contributes to neutralizing added alkalinity. In other circumstances, such as anaerobiosis, degradation serves other functions, such as phosphorylation potential regulation.

Anaerobiosis↗
PubMed1995

Functional and energetic effects of the inotropic agents EMD-57033 and BAPTA on the isolated rat heart.

This investigation studied the functional and energetic effects of the novel positive inotropic agent EMD-57033 and the negative inotropic agent 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) in the paced Langendorff-perfused rat heart. EMD-57033 is a calcium-sensitizing agent that has previously been shown to increase contractility without affecting calcium transients. Its effects were compared with that of dobutamine, which increases contractility by increasing calcium transient amplitude. EMD-57033 (2 microM) and dobutamine (0.2 microM) induced a 40% increase in developed pressure. Myocardial oxygen consumption (MVO2) increased significantly with dobutamine. However, there was no significant change in MVO2 with EMD-57033. There was no change in phosphate metabolite concentrations as detected by 31P-nuclear magnetic resonance with either agent. Lactate production and basal metabolism were unaffected by either agent. Thus EMD-57033 increased contractility in a more energetically economical manner than did dobutamine. Contractility was decreased with BAPTA, an intracellular calcium chelator that decreases contractility by binding free calcium. The metabolic effects of BAPTA (2.2 microM) were compared with those of verapamil (10 nM), an agent which decreases calcium fluxes. Both agents decreased developed pressure 60%. MVO2 decreased 14% with BAPTA and 50% with verapamil. Neither agent altered the concentrations of phosphate metabolites, lactate production, or basal metabolism. Thus BAPTA lowered contractility in a less energetically economical manner than verapamil. These data suggest that in rat hearts, inotropic agents with different effects on calcium handling have different effects on energetics.

Animals↗
PubMed1995

Insights into cellular energy metabolism from transgenic mice.

Rapid progress in manipulating the mouse genome now makes it possible to express any cloned gene in virtually any cell type of the mouse. Homologous recombination in embryonic stem cells allows the specific mutation of any gene. These tools are finding widespread application to problems in cell biology and physiology. After a brief description of some of the molecular genetic techniques available for the mouse, the application of transgenic mice to problems in cellular energy metabolism is discussed. The effects of increasing glucose transport on carbohydrate metabolism in muscle and fat are presented. Experiments using a transgenic mouse expressing creatine kinase in liver to understand ATP catabolism and regulation of oxidative phosphorylation are discussed. Finally, mice either lacking or misexpressing different isoforms of creatine kinase have been useful for understanding the detailed role of this important enzyme in cellular energy metabolism.

Adipose Tissue↗
PubMed1994

Tissue specific perfusion imaging using arterial spin labeling.

Quantitative magnetic resonance measurements of regional tissue perfusion can be obtained using magnetically labeled arterial water as a diffusable tracer. Continuous labeling is achieved in flowing spins using adiabatic inversion. The effects of continuous labeling of proximal arterial spins and T1 relaxation in distal tissue magnetization result in a steady-state change in tissue magnetization which is tissue specific, i.e., it can be quantified in units of blood flow per gram of tissue per unit time. This magnetization is sampled using standard imaging sequences. The theoretical basis for this method, including the effects of macromolecular spin saturation, is reviewed. Recent results demonstrating the successful implementation of this technique in vitro and in vivo in rat brain, heart, and kidney, and in human brain and kidney are presented, as well as the use of a separate RF coil for arterial labeling to produce selective perfusion images in rat brain. This approach allows quantitative perfusion images to be obtained completely non-invasively at the resolution of 1H MRI, and is useful in the clinical and investigational evaluation of organ physiology.

Animals↗
PubMed1994

Perfusion imaging of the rat kidney with MR.

PURPOSE: To develop a technique for measurement of regional renal perfusion with magnetic resonance (MR) imaging. MATERIALS AND METHODS: Quantitative renal perfusion images in rats were obtained by measurement of the reduction in kidney MR image signal intensity after steady state magnetic labeling of arterial blood in the suprarenal aorta. Labeling was achieved with adiabatic fast passage inversion of arterial water. RESULTS: Cortical renal blood flow was 4.9 mL/g/min +/- 0.15 (12 rats), which correlated well with previous measurements obtained with other techniques. Serial perfusion images obtained every 5 minutes during intravenous infusion of either acetylcholine or angiotensin II showed that one increased and the other decreased renal blood flow, respectively, also correlating with previous measurements. CONCLUSION: Quantitative measurement of cortical renal blood flow can be obtained with proton MR imaging techniques, with use of endogenous arterial water as a tracer. This technique should be readily applicable to measurement of renal perfusion in humans.

Acetylcholine↗
PubMed1993

Role of oxygen vs. glucose in energy metabolism in a mammary carcinoma perfused ex vivo: direct measurement by 31P NMR.

The role of glycolysis vs. respiration in tumor energy metabolism has been studied, to date, primarily in vitro by using single cells, multicellular spheroids, or tissue slices. With the advent of in vivo NMR spectroscopy, several investigators have shown that tumor energy status depends on its blood flow. Since manipulation of blood flow alters both oxygen and glucose delivery to a solid tumor, these studies have not been able to separate the relative contribution of oxygen vs. glucose in energy metabolism in vivo. In the present study, we have overcome this problem by combining two methods: the tissue-isolated R3230AC mammary adenocarcinoma perfused ex vivo and 31P NMR spectroscopy. The isolated tumor permits one to control the perfusion pressure as well as the metabolite concentrations in the perfusate. NMR spectroscopy permits one to measure the ratio of nucleoside triphosphate to inorganic phosphate (NTP/Pi) and pH. Our results show that (i) the NTP/Pi ratio ex vivo is similar to that observed in vivo prior to surgery, (ii) the NTP/Pi ratio is insensitive to flow changes at high flow rates but is proportional to flow rate at flows comparable to those found in vivo, (iii) the NTP/Pi ratio of these tumors is resistant to hypoxia and is not maintained when glucose is removed or replaced with glutamine, and (iv) although both O2 and glucose are consumed by these tumors, the effect of perfusate flow rate appears to be mediated largely through glucose delivery. The current approach not only provides information about the role of glycolysis vs. respiration in a rodent tumor but also is general and versatile enough to provide similar data in human tumors perfused ex vivo.

Adenocarcinoma↗
PubMed1993

Response of normal and reperfused livers to glucagon stimulation: NMR detection of blood flow and high-energy phosphates.

The effects of glucagon on blood flow and high-energy phosphates in control and in rat livers damaged by ischemia were studied using in vivo nuclear magnetic resonance (NMR) spectroscopy. Normal livers and livers which had been made ischemic for 20, 40, and 60 min followed by 60 min of reperfusion were studied. Ischemia led to a loss in adenosine triphosphate (ATP) within 30 min. Reperfusion after 20 min of ischemia led to complete recovery of ATP. 60 min of reperfusion after 40 or 60 min of ischemia led to only a 76% and 48% recovery of ATP, respectively. Glucagon, at doses up to 2.5 mg/kg body weight, caused no changes in the inorganic phosphate (P(i)) to ATP ratio in normal livers as measured by 31P-NMR spectroscopy. In livers which had been made ischemic for 20, 40, or 60 min, glucagon caused an increase in the P(i)/ATP ratio of 18%, 40%, and 40%, respectively. 19F-NMR detection of the washout of trifluoromethane from liver was used to measure blood flow. Glucagon-stimulated flow in the normal liver in a dose-dependent manner, with 2.5 mg glucagon/kg body weight leading to a 95% increase in flow. Ischemia for 20, 40, and 60 min followed by 60 min of reperfusion led to hepatic blood flows which were 63%, 68%, and 58% lower than control liver. In reperfused livers, blood flow after glucagon-stimulation was reduced to 56%, 43%, and 48% of control glucagon-stimulated flow after 20, 40, and 60 min of ischemia. These results indicate that ischemia followed by reperfusion leads to decreases in hepatic blood flow prior to alterations in ATP and the response of the liver to glucagon is altered in the reperfused liver.

Adenosine Triphosphate↗
PubMed1993

Measurement of rat brain perfusion by NMR using spin labeling of arterial water: in vivo determination of the degree of spin labeling.

In vivo NMR experiments are performed to determine the degree of spin labeling for measurement of tissue perfusion by NMR using spin labeling of arterial water by adiabatic fast passage. Arterial water spins are labeled using flow in the presence of a field gradient and B1 irradiation to fulfill the conditions for adiabatic fast passage spin inversion. It is demonstrated that the NMR-measured tissue perfusion is not affected by changing the degree of spin labeling as long as the degree of spin labeling is determined and accounted for according to the model used for calculating perfusion. By measuring the degree of spin labeling with different arterial blood flow velocities induced by different arterial pCO2, it is also demonstrated that, when spin labeling is carried out by adiabatic fast passage, the degree of spin labeling is not affected by changes in arterial blood flow velocity over a broad range.

Animals↗
PubMed1993

Magnetic resonance imaging of perfusion in the isolated rat heart using spin inversion of arterial water.

Measurement of regional myocardial perfusion is important for the diagnosis and treatment of coronary artery disease. Currently used methods for the measurement of myocardial tissue perfusion are either invasive or not quantitative. Here, we demonstrate a technique for the measurement of myocardial perfusion using magnetic resonance imaging (MRI) with spin tagging of arterial water. In addition, it is shown that changes in perfusion can be quantitated by measuring changes in tissue T1. Perfusion images are obtained in Langendorff-perfused, isolated rat hearts for perfusion rates ranging from 5 to 22 ml/g/min. The MRI-determined perfusion rates are in excellent agreement with perfusion rates determined from measurement of bulk perfusate flow (r = 0.98). The predicted linear dependence of the measured T1 (T1app) on perfusion is also demonstrated. The ability of perfusion imaging to measure regional variations in flow is demonstrated with hearts in which perfusion defects were created by ligation of a coronary artery. These results indicate that MRI of perfusion using spin inversion of arterial water gives quantitative maps of cardiac perfusion.

Animals↗
PubMed1993

Altering creatine kinase isoenzymes in transgenic mouse muscle by overexpression of the B subunit.

To change the levels of expression and isoenzyme distribution of creatine kinase (CK) in muscle, transgenic technology was used to express the B subunit of CK in mouse muscle. Normally, mammalian skeletal muscle contains the MM dimer of CK. The BB dimer and MB heterodimer of CK can be found in brain and heart, respectively. Heterologous genes consisting of skeletal and cardiac muscle-specific actin promoters fused to the genomic coding region of the B form of CK were used to create transgenic mice. Lines were established from the three highest expressing founders. Analysis of skeletal muscle extracts revealed that all three lines had an increase in total CK activity measured under maximal velocity conditions. The highest expressing line, 7001, had a CK activity 150% that of control muscle. Nuclear magnetic resonance saturation transfer was used to measure the in vivo rate of the CK reaction. In 7001 hindlimb muscles, the CK catalyzed reaction was 200% that of control muscle. The elevation in CK activity in transgenic muscle was accompanied by significant changes in the composition of the cytosolic isoenzyme ratio of CK. In control, 100% of CK was MM, whereas 7001 had 60 +/- 18% MM, 32 +/- 10% MB, and 8 +/- 2% BB. There were no changes in ATP, phosphocreatine, Pi, or creatine levels in transgenic muscle compared with control. Immunofluorescence of myofibrils isolated from control and transgenic muscle revealed specific association of CK to the M line. Small amounts of MB CK were detected on myofibrils from transgenic mice. Transgenic mice expressing the B subunit of CK in muscle represent a first step toward altering CK isoforms so as to elucidate the specific roles of these isoforms in energy metabolism.

Animals↗
PubMed1993

Phosphocreatine protects transgenic mouse liver expressing creatine kinase from hypoxia and ischemia.

Creatine kinase (CK) is normally found at high levels in muscle and brain and catalyzes the reaction phosphocreatine (PCr) + MgADP + H+<==>creatine (Cr) + MgATP. CK is not normally found at high levels in liver. A line of transgenic mice that express high levels of the BB-dimer of CK (CKB) in liver has allowed us to assess the role of CKB during periods of low oxygen stress. During 40 min of ischemia of normal perfused livers at 25 degrees C, ATP levels are depleted, and pH decreases to 6.6. pH recovers to a preischemic level after 30 min of reperfusion of normal livers; however, P(i) levels are significantly higher and ATP levels significantly lower than preischemic values. In transgenic liver with an initial PCr-to-ATP ratio of 4.5, ATP levels are maintained until PCr is markedly depleted. pH remains at preischemic levels for 16 min of ischemia of transgenic livers. During this length of ischemia in normal livers, pH has dropped to 6.9. pH, P(i), and ATP levels return to preischemic values within 30 min of reperfusion in transgenic livers containing PCr and CK. During 90 min of hypoxia of normal perfused livers at 37 degrees C, ATP is depleted. After 15 min of hypoxia of normal livers, there is a significant increase in the release of lactate dehydrogenase (LDH). In transgenic livers, ATP is maintained, and no increase in LDH release is observed for up to 90 min, depending on the level of PCr before hypoxia. These results demonstrate the role of CKB in buffering ATP levels and regulating intracellular pH during periods of low oxygen stress.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1992

2H-nuclear magnetic resonance imaging of tumor blood flow: spatial and temporal heterogeneity in a tissue-isolated mammary adenocarcinoma.

2H-Nuclear magnetic resonance imaging of deuteron accumulation in tissue following an i.v. bolus of deuterium oxide provides a noninvasive means of constructing maps of tissue perfusion. With a measured arterial input function and a simple model for tissue-capillary exchange, these data can provide quantitative estimates of local flow. This technique was tested in rat brain and then applied to the study of spatial heterogeneity and temporal variation of blood flow in the tissue-isolated R3230AC mammary adenocarcinoma. Global flow from the brain averaged 0.96 ml/min.g, in good agreement with results obtained from other methods; the perfusion of brain was relatively homogeneous. Global tumor blood flow averaged 0.32 ml/min.g, ranging from 0.11 to 0.96 ml/min.g. Imaging revealed variations in perfusion both within and between the tumors that far exceeded those expected from brain flow heterogeneity and uncertainty in the flow estimates. By obtaining repeated flow images at 30-min intervals, it was possible to show that the regional blood flow shifted with time in single pixels and in multipixel regions. These experiments show that 2H-nuclear magnetic resonance may be useful in obtaining noninvasive and quantitative measurement of temporal blood flow changes in a solid tumor in vivo.

Adenocarcinoma↗
PubMed1992

Magnetic resonance imaging of perfusion using spin inversion of arterial water.

A technique has been developed for proton magnetic resonance imaging (MRI) of perfusion, using water as a freely diffusable tracer, and its application to the measurement of cerebral blood flow (CBF) in the rat is demonstrated. The method involves labeling the inflowing water proton spins in the arterial blood by inverting them continuously at the neck region and observing the effects of inversion on the intensity of brain MRI. Solution to the Bloch equations, modified to include the effects of flow, allows regional perfusion rates to be measured from an image with spin inversion, a control image, and a T1 image. Continuous spin inversion labeling the arterial blood water was accomplished, using principles of adiabatic fast passage by applying continuous-wave radiofrequency power in the presence of a magnetic field gradient in the direction of arterial flow. In the detection slice used to measure perfusion, whole brain CBF averaged 1.39 +/- 0.19 ml.g-1.min-1 (mean +/- SEM, n = 5). The technique's sensitivity to changes in CBF was measured by using graded hypercarbia, a condition that is known to increase brain perfusion. CBF vs. pCO2 data yield a best-fit straight line described by CBF (ml.g-1.min-1) = 0.052pCO2 (mm Hg) - 0.173, in excellent agreement with values in the literature. Finally, perfusion images of a freeze-injured rat brain have been obtained, demonstrating the technique's ability to detect regional abnormalities in perfusion.

Animals↗
PubMed1992

Perfusion imaging.

Measurement of tissue perfusion is important for the functional assessment of organs in vivo. Here we report the use of 1H NMR imaging to generate perfusion maps in the rat brain at 4.7 T. Blood water flowing to the brain is saturated in the neck region with a slice-selective saturation imaging sequence, creating an endogenous tracer in the form of proximally saturated spins. Because proton T1 times are relatively long, particularly at high field strengths, saturated spins exchange with bulk water in the brain and a steady state is created where the regional concentration of saturated spins is determined by the regional blood flow and regional T1. Distal saturation applied equidistantly outside the brain serves as a control for effects of the saturation pulses. Average cerebral blood flow in normocapnic rat brain under halothane anesthesia was determined to be 105 +/- 16 cc.100 g-1.min-1 (mean +/- SEM, n = 3), in good agreement with values reported in the literature, and was sensitive to increases in arterial pCO2. This technique allows regional perfusion maps to be measured noninvasively, with the resolution of 1H MRI, and should be readily applicable to human studies.

Animals↗
PubMed1992

Measurement of brain perfusion by volume-localized NMR spectroscopy using inversion of arterial water spins: accounting for transit time and cross-relaxation.

The theoretical model for perfusion measurement by NMR using arterial labeling of endogenous water is extended to include the effects of transit time and cross-relaxation of tissue water with macromolecules. Water magnetization in rat brain is monitored using the STEAM method to simultaneously determine the transit time, magnetization transfer rate constant, and perfusion. The results show that the transit time in rat brain is quite short, and thus its effect on perfusion measurement is small. It is also demonstrated both theoretically and experimentally that the steady-state effects of cross-relaxation with macromolecules on perfusion measurement are accounted for by a proper control experiment.

Animals↗