Ngoc Nguyen

Ischemia induces translocation of the insulin-responsive glucose transporter GLUT4 to the plasma membrane of cardiac myocytes.

BackgroundAcute myocardial ischemia is accompanied by an increase in glucose uptake and metabolism, which appears to be important in protecting myocardial cells from irreversible ischemic injury. Because insulin augments myocardial glucose uptake by inducing the translocation of glucose transporters from an intracellular compartment to the plasma membrane, we hypothesized that acute ischemia would trigger a similar translocation. Methods and ResultsWe used a subcellular fractionation method to separate intracellular membranes and plasma membranes from control, ischemic, and hypoxic Langendorff-isolated perfused rat hearts and determined the expression of the major myocardial glucose transporter, GLUT4, in these separated membrane fractions. We found that translocation of GLUT4 molecules occurred in ischemic, hypoxic, and insulintreated hearts and in hearts that underwent ischemia plus insulin treatment. The percentages of GLUT4 molecules present on the plasma membrane in the different conditions were as follows: control. 18.0±2.8%; ischemia, 41.3±9.4%; hypoxia, 31.1±2.9%; insulin, 61.1±2.6%; and ischemia plus insulin, 66.8±5.7%. Among the statistically significant differences in these values were the difference between control and ischemia and the difference between ischemia alone and insulin plus ischemia. ConclusionsIschemia causes substantial translocation of GLUT4 molecules to the plasma membrane of cardiac myocytes. A combination of insulin plus ischemia stimulates an even greater degree of GLUT4 translocation. GLUT4 translocation is likely to mediate at least part of the increased glucose uptake of ischemic myocardium and may be a mechanism for the cardioprotective effect of insulin during acute myocardial ischemia.

Effects of wortmannin on insulin- and ischemia-induced stimulation of GLUT4 translocation and FDG uptake in perfused rat hearts

OBJECTIVE Myocardial glucose transport is enhanced by hormonal and other stimuli such as ischemia and hypoxia which induce glucose transporter 4 (GLUT4) translocation. Whether insulin and ischemia share a common signaling mechanism is not yet known. This study investigated whether phosphatidylinositol 3-kinase (PI3K), a signaling intermediate of the insulin-responsible pathway, also participates in the ischemia-induced stimulation of glucose. METHODS Isolated Langendorff-perfused Sprague-Dawley rat hearts were subjected to 100 nmol/l insulin or 15 min of no-flow ischemia with/without 1 mumol/l wortmannin, an inhibitor of PI3K. After perfusion, relative subcellular glucose transporter GLUT4 distribution was assessed by membrane fractionation and immunoblotting and compared to controls. Uptake kinetics of the glucose analog [18F]fluoro-deoxyglucose (FDG) were also studied during perfusion of rat hearts. RESULTS GLUT4 translocation to the plasma membrane (PM) was increased by insulin 1.8-fold and by ischemia 2.4-fold (P < 0.05). FDG uptake was increased by insulin 6.0-fold and by ischemia 6.2-fold (P < 0.05). Wortmannin 1 mumol/l inhibited insulin-mediated translocation of GLUT4 and increase in FDG uptake completely. However, it did not show any effect on ischemia-stimulated GLUT4 translocation or on ischemia-induced increase in FDG utilization. A significant correlation was found between relative GLUT4 translocation and FDG uptake in hearts of the insulin series (r = 0.9, P < 0.05) and of the ischemia series (r = 0.8, P < 0.05). CONCLUSIONS Our results demonstrate that wortmannin did not inhibit ischemia-induced stimulation of myocardial glucose transport, supporting the hypothesis of different signaling pathways for ischemia and insulin.

Comparison of technetium-99m sestamibi and thallium-201 retention characteristics in canine myocardium

OBJECTIVES The aim of this study was to compare the myocardial retention of technetium-99m (Tc-99m) sestamibi and thallium-201 over a wide range of blood flow at different time points after tracer injection. BACKGROUND Technetium-99m sestamibi has been proposed as a new perfusion tracer with better physical characteristics than those of thallium-201 for scintigraphic imaging. However, no studies have simultaneously compared the ability of both tracers to assess myocardial blood flow during pharmacologic vasodilation. METHODS The myocardial retention of Tc-99m sestamibi and thallium-201 were compared over a wide range of blood flow induced by regional coronary occlusion and dipyridamole infusion in an open chest dog model. Myocardial retention of both tracers was determined by in vitro tissue counting at 2, 5, and 20 min after tracer injection and was correlated with microsphere-determined blood flow. RESULTS Thallium-201 demonstrated greater absolute tissue retention than did Tc-99m sestamibi. At 2 min after tracer injection, there was an almost linear relation between the retention of both tracers and myocardial blood flow over a wide flow range. However, this relation was not maintained over time. At 20 min after injection, the retention of both tracers underestimated myocardial blood flow at higher flow rates. At 2, 5 and 20 min after injection, increments of relative tracer retention between the different levels of flow were always greater for thallium-201 than for Tc-99m sestamibi. CONCLUSIONS Thallium-201 displays more suitable physiologic characteristics as a flow tracer and may allow better differentiation of myocardial regions with different levels of coronary flow reserve. For both tracers, early cardiac imaging may minimize underestimation of blood flow at higher flow rates.

The relationship between myocardial retention of technetium-99m teboroxime and myocardial blood flow

OBJECTIVES The aim of this study was to define the temporal changes in the relationship between technetium-99m teboroxime tissue retention and myocardial blood flow in a canine model. BACKGROUND Technetium-99m teboroxime is a new neutral lipophilic myocardial perfusion agent. It is known to be highly extracted by the myocardium but to have a rapid clearance rate. METHODS A wide range of myocardial blood flow was induced in each experiment by regional coronary occlusion and dipyridamole infusion. Myocardial retention of technetium-99m teboroxime was determined by in vitro tissue counting at 1, 2 or 5 min after injection of the tracer. Tracer retention was correlated with microsphere-determined blood flow and the data were fitted to nonlinear functions. RESULTS Correlation coefficients for these functions were 0.92, 0.95 and 0.95 at 1, 2, and 5 min, respectively. At 1 min after injection, the relationship of technetium-99m teboroxime retention to blood flow was linear over a wide flow range, becoming nonlinear at flow rates greater than 4.5 ml/min per g. After 5 min the retention-flow relationship was linear only to 2.5 ml/min per g, above which little change in retention was noted. Normalized myocardial retention, expressed as a percent of the retention at 1 ml/min per g, was also calculated. At flow rates of 1, 2, 3, 4 and 5 ml/min per g, normalized retention was 100, 169, 228, 277 and 317% at 1 min and 100, 171, 217, 239 and 237% at 5 min after injection. CONCLUSIONS At 1 min after injection, the relationship of technetium-99m teboroxime myocardial retention to blood flow is well maintained over a wide range of flow. However, after only 5 min, tracer retention underestimates flow changes at moderate and high flow rates. Thus, rapid acquisition protocols are necessary to fully exploit the potential of this promising new tracer in the evaluation of myocardial perfusion.

Extrarenal ETB Plays a Significant Role in Controlling Cardiovascular Responses to High Dietary Sodium in Rats

Endothelin-B receptor (ETB)-deficient rats have low-renin, salt-sensitive hypertension. We hypothesized this was caused by an absence of renal ETB signaling and performed a series of experiments to examine the effect of dietary sodium (Na) on endothelin-1 (ET1) expression and renal function in wild-type (WT) and ETB-deficient rats. We found that ETB deficiency, but not dietary Na, increases circulating and tissue (kidney and aorta) ET1 levels. Quantitative reverse-transcription polymerase chain reaction reveals that aortic and renal ET1 and endothelin-A receptor (ETA) mRNA, however, are similarly increased by dietary Na in ETB-WT and ETB-deficient rats. We then determined the effect of chronic ETA blockade on blood pressure (direct conscious measurements), urinary protein excretion, and creatinine clearance (Crcl). On a Na-deficient diet, ETB-deficient rats have mild proteinuria and impaired Crcl. On a high-Na diet, severe hypertension and renal dysfunction develop in ETB-deficient rats. Chronic ETA blockade prevents hypertension and renal injury. To determine the role of the renal versus the extrarenal endothelin system, we performed renal cross-transplantation. We found that ETB deficiency in the body is associated with renal injury and an impaired ability to excrete an Na load. We also found that ETB deficiency in the body affects blood pressure response to dietary Na. Expression of ET1 and ETA are regulated by dietary Na. ETB receptors outside of the kidney, likely by functioning as a clearance receptor for ET1, limit salt-sensitivity in rats.

An approach to ventricular efficiency by use of carbon 11-labeled acetate and positron emission tomography

BackgroundPositron emission tomography-derived11C-labeled acetate kinetics have been shown to reflect myocardial oxidative metabolism. The objective of the study was to use this metabolic imaging technique in combination with an evaluation of left ventricular work as an index of ventricular mechanical efficiency.Methods and ResultsThe effects of ventricular ejection fraction and loading on this index were studied quantitatively in a canine experimental model. There was a curvilinear relationship beween efficiency and the end-diastolic volume per unit mass (r=0.84), which appeared to integrate the main determinants of left ventricular mechanical performance successfully and allowed the detection of a decreased ventricular efficiency in acute experimental heart failure.ConclusionsThis approach appears to have the potential to assess the energetic working point of the ventricle in clinical heart disease and follow the effects of therapy. The data demonstrate the feasibility of an estimate of ventricular efficiency that relies on noninvasive data-acquisition techniques.

Imaging of cardiac neuronal function after cocaine exposure using Carbon-11 hydroxyephedrine and positron emission tomography

OBJECTIVES The aim of the study was to define the effect of cocaine on the myocardial uptake and retention of C-11 hydroxyephedrine in the anesthetized dog model. BACKGROUND Cardiac toxicity of cocaine has been linked to its inhibitory effect on norepinephrine reuptake by the sympathetic nerve terminals of the heart. Carbon-11 hydroxyephedrine is a C-11-labeled norepinephrine analog that has high specific affinity for uptake-1 and thus makes possible the assessment of the effect of cocaine on norepinephrine reuptake by cardiac sympathetic nerve terminals. METHODS The cardiac kinetics of C-11 hydroxyephedrine as assessed by dynamic positron emission tomographic imaging were used to characterize norepinephrine reuptake by the sympathetic nerve terminals. Carbon-11 hydroxyephedrine was injected intravenously before, as well as at 5 min and 2.5 h after, intravenous administration of 2 mg/kg body weight of cocaine in anesthetized dogs. Hemodynamic variables and microsphere-determined cardiac blood flow were also measured before and after cocaine exposure. RESULTS Intravenous injection of cocaine did not significantly affect hemodynamic variables and myocardial blood flow in the anesthetized animals. Compared with baseline, myocardial retention of C-11 hydroxyephedrine was significantly reduced by 78 +/- 3% (mean +/- SD) at 5 min and remained significantly reduced (28 +/- 17%) at 2.5 h after cocaine injection. Cocaine administration after C-11 hydroxyephedrine injection (30 min) resulted in rapid biexponential clearance of C-11 hydroxyephedrine from myocardium. CONCLUSIONS These results suggest prolonged effects of cocaine on the sympathetic nerve terminals of the heart. Positron emission tomography provides a noninvasive and sensitive means to objectively assess the cardiac pharmacokinetics of drugs such as cocaine.

Scintigraphic evaluation of cardiac autonomic innervation

Alterations in cardiac autonomic neuronal function have become the focus of intense research in various cardiovascular diseases. Both single-photon emission tomography (SPECT) and the positron emission tomography (PET) imaging techniques in combination with radiolabled neurotransmitters and receptor ligands have become available for the scintigraphic visualization of presynaptic and postsynaptic neuronal function. Several clinical studies have shown changes in tracer distribution in different clinical conditions, such as ischemic heart disease, congestive heart failure, malignant arrhythmias, heart transplantation, and in patients with diabetes. In patients with congestive heart failure, previous in vitro investigations have concentrated on the postsynaptic level of the sympathetic innervation. However, alterations in presynaptic nerve function have been demonstrated with scintigraphic investigations by decreased presynaptic tracer retention. Moreover, correlation between scintigraphic findings and clinical outcome was shown in patients with heart failure, providing important prognostic information superior to conventional risk assessment. In conclusion, scintigraphic evaluation by SPECT and PET allows functional characterization of cardiac presynaptic and postsynaptic neurons. Regional tracer uptake can be used as an index for the integrity of innervation in various diseases. Newer tracer approaches may allow the noninvasive quantification of neuronal function by PET.

Effect on coronary artery flow reserve and resistance in the remote area after acute coronary artery occlusion in the pig model

BackgroundIt has been reported that vasodilator function in remote myocardial regions supplied by “angiographically normal” coronary arteries is impaired in patients after acute myocardial infarction (MI). The aim of this study was to determine whether coronary artery flow reserve and coronary artery resistance in remote, nonischemic areas are also altered in experimental MI.MethodsExperiments were performed in anesthetized pigs. In group 1 infarction was induced by ligation of the left-anterior descending artery (LAD); group 2 consisted of sham-operated animals. Hemodynamic parameters, coronary artery resistance, and myocardial blood flow (MBF) were measured before and 4 hours after MI under rest and during infusion of adenosine.ResultsCoronary artery dilation by adenosine caused a similar increase in MBF before and 4 hours after coronary artery occlusion. Resting MBF after acute MI was not altered, although a significant reduction (15%; P<.04) in mean aortic pressure was observed compared with base-line. Coronary artery resistance was significantly reduced by adenosine (P<.04) before MI, as well as at 4 hours after MI (P<.03). Coronary artery flow reserve was not adversely affected. The sham-operated animals showed similar results without any significant differences between the two study groups.ConclusionThis study indicates that an acute MI in pigs did not increase coronary artery resistance in the remote area after MI and therefore did not adversely affect coronary artery flow reserve in the nonischemic vascular bed. Further studies are necessary to fully understand the exact mechanism of the alterations in remote flow reserve of patients after MI.

Imaging of myocardial autonomic innervation in patients with congestive heart failure: Methods and clinical implications

The pathophysiologic role of the autonomic nervous system in the development of heart failure was recently recognized. Noninvasive imaging of cardiac autonomic innervation has become available for routine clinical diagnostic using SPECT in combination with 123I-metaiodobenzylguanidine. Positron emission tomography represents an alternative imaging modality with superior spatial resolution, allowing for sophisticated tracer approaches to various presynaptic and postsynaptic binding sites but is limited however, to few academic centers. The present review addresses the experimental and clinical experience with neuronal imaging in congestive heart failure. The clinical implications of these imaging findings are discussed. Present developments and future directions concerning the clinical application of available tracers, as well as the evaluation of new radiolabeled compounds for different sites of the autonomic nervous system and their significance for heart failure research, are presented.