| | Reversal of impaired myocardial β-adrenergic receptor signaling by continuous-flow left ventricular assist device support published online 08 March 2010. BackgroundMyocardial β-adrenergic receptor (β-AR) signaling is severely impaired in chronic heart failure (HF). This study was conducted to determine if left ventricular (LV) β-AR signaling could be restored after continuous-flow LV assist device (LVAD) support. MethodsTwelve patients received LVADs as a bridge to transplant. Paired LV biopsy specimens were obtained at the time of LVAD implant (HF group) and transplant (LVAD group). The mean duration of LVAD support was 152 ± 34 days. Myocardial β-AR signaling was assessed by measuring adenylyl cyclase (AC) activity, total β-AR density (Bmax), and G protein-coupled receptor kinase-2 (GRK2) expression and activity. LV specimens from 8 non-failing hearts (NF) were used as controls. ResultsBasal and isoproterenol-stimulated AC activity was significantly lower in HF vs NF, indicative of β-AR uncoupling. Continuous-flow LVAD support restored basal and isoproterenol-stimulated AC activity to levels similar to NF. Bmax was decreased in HF vs NF and increased to nearly normal in the LVAD group. GRK2 expression was increased 2.6-fold in HF vs NF and was similar to NF after LVAD support. GRK2 activity was 3.2-fold greater in HF vs NF and decreased to NF levels in the LVAD group. ConclusionsMyocardial β-AR signaling can be restored to nearly normal after continuous-flow LVAD support. This is similar to previous data for volume-displacement pulsatile LVADs. Decreased GRK2 activity is an important mechanism and indicates that normalization of the neurohormonal milieu associated with HF is similar between continuous-flow and pulsatile LVADs. This may have important implications for myocardial recovery. Left ventricular assist devices (LVADs) are becoming an increasingly used treatment for patients with end-stage heart failure (HF). The most common indication is as a bridge to transplant; however, there is significant interest in implanting these devices as a bridge to recovery in patients with chronic heart failure (CHF). Although this has been reported for a very small percentage of patients, novel multimodality therapies are being investigated. CHF is characterized by severely impaired myocardial β-adrenergic receptor (β-AR) signaling.1 These signaling defects include a decrease in total β-AR density, known as receptor down-regulation, and impaired signaling through the remaining receptors, a process known as homologous desensitization.2 These abnormalities are thought to be partly caused by phosphorylation of agonist-occupied β-ARs by G protein-coupled receptor kinase-2 (GRK2), a member of the GRK family of serine-threonine kinases.3 After phosphorylation by GRK2, receptors are targeted for binding by β-arrestins, which sterically interdict further receptor coupling, leading to receptor internalization.4 In the setting of CHF in humans and animal models, myocardial expression and activity of GRK2 is up-regulated approximately 3-fold,5, 6 and this is thought to be a major mechanism of impaired β-AR signaling as circulating levels of catecholamines are significantly increased in an attempt to increase cardiac output. Inhibition of GRK2 has been shown to reverse the impaired β-AR signaling in animal models of HF and restore ventricular function.7, 8, 9 Our laboratory and others have shown that myocardial β-AR signaling can be restored to nearly normal after support with the pulsatile HeartMate XVE LVAD (Thoratec Corp, Pleasanton, CA).10, 11, 12 There was normalization of circulating catecholamine levels and, importantly, a decrease in myocardial GRK2 expression and activity. These studies are particularly important with regard to myocardial recovery, because the β-AR signaling system is the most critical pathway in the regulation of cardiac systolic and diastolic function. The primary goal of this study was to investigate whether the abnormal β-AR signaling characteristic of HF could be reversed by a continuous-flow LVAD, specifically, the HeartMate II LVAD. These data could be important in determining if continuous-flow devices can be used as a platform for achieving myocardial recovery in patients with long-standing HF in combination with novel pharmacologic, cell-based, or genetic therapies. Methods  All procedures for tissue procurement in this study were performed in compliance with institutional guidelines for human research and an approved Institutional Review Board protocol at the University of Chicago Medical Center. Study population We collected myocardial tissue and blood samples from 12 consecutive patients who underwent LVAD implantation and subsequent orthotopic heart transplantation (HTx). The indication for a LVAD in all patients was end-stage HF, defined as New York Heart Association functional class IV, with deterioration of cardiac function despite maximal medical therapy. The clinical characteristics of this study population are presented in Table 1. Right heart catheterization had been performed within 8 weeks before HTx. Myocardial tissue collection The LV apical core excised during implantation of the HeartMate II LVAD for each patient was snap frozen in liquid nitrogen and stored at −80°C. A section of the apex of the LV was excised and stored in identical fashion after LVAD explant and cardiectomy at the time of HTx. The 12 samples were paired from LVAD implant to HTx. Non-failing control LV apical tissue was obtained from 8 organ donors whose hearts were unsuitable for HTx but who had normal ventricular function and no structural heart disease. Lymphocyte samples For both pre-LVAD (HF) and post-LVAD samples, blood was collected intraoperatively and anti-coagulated with ethylenediaminetetraacetic acid (EDTA). Lymphocytes were isolated by Ficoll gradient using Histopague-1077 (Sigma, St Louis, MO), frozen, and stored at −80°C. All blood samples were paired from LVAD implant to explant (n = 10). Blood samples could not be obtained from non-HF organ donors. Protein immunoblotting Tissue was homogenized in lysis buffer (25 mmol/liter Tris-hydrogen chloride [pH 7.5], 5 mmol/liter EDTA, 5 mmol/liter ethyleneglycotetraacetic acid [EGTA]). Nuclei and tissue were separated by centrifugation at 800g for 20 minutes, and the crude supernatant was centrifuged at 20,000g for 20 minutes. Sedimented proteins (membrane fraction) were resuspended in 50 mmol/liter N-2-hydroxyethylpiperazine-N′-2-ethanesulfonate (HEPES; pH 7.3) and 5 mmol/liter magnesium chloride. The immunodetection of myocardial levels of GRK2 (Santa Cruz Biotechnology Inc, Santa Cruz, CA) was performed on cytosolic and membrane extracts (80 μg), electrophoresed through 12% Tris/glycine gels, and transferred to nitrocellulose. Membranes were blocked in 5% nonfat dried milk for 1 hour at room temperature. The protein was visualized using a horseradish peroxidase-linked secondary antibody and enhanced chemiluminescence detection (Amersham, Princeton, NJ). Measurement of GRK activity The membrane fractions of the myocardial extracts were used to determine GRK activity. Extracts (100 μg of protein) were incubated with rhodopsin-enriched rod outer-segment membranes, as previously described.13 After being incubated in white light for 15 minutes at room temperature, reactions were quenched with ice-cold lysis buffer and centrifuged for 15 minutes at 13,000g. Sedimented proteins were resuspended in protein-gel-loading dye and treated with 12% sodium dodecyl sulfate polyacrylamide gel electrophoresis. Phosphorylated rhodopsin was visualized by autoradiography and quantified using a Molecular Dynamics PhosphorImager (Piscataway, NJ). Radioligand binding assays Total β-AR density (Bmax) was determined by incubating 25 μg of cardiac sarcolemmal membranes with a saturating concentration of [125I] cyanopindolol and 20 μmol/liter alprenolol to define non-specific binding. Sarcolemmal membrane samples were studied in triplicate with 80 pmol/liter [125I] cyanopindolol and 10–4 mol/liter isoproterenol (ISO) in 250 μliter of binding buffer (50 mmol/liter HEPES [pH 7.3], 5 mmol/liter magnesium chloride, and 0.1 mmol/liter ascorbic acid). Assays were done at 37°C for 1 hour and then filtered over GF/C glass fiber filters (Whatman; Piscataway, NJ) that were washed twice and counted in a gamma counter. Data were analyzed by nonlinear least-square curve fit (GraphPad Prism, San Diego, CA). Sarcolemmal membrane adenylyl cyclase activity Cardiac sarcolemmal membranes (20 μg of protein) were incubated for 15 minutes at 37°C with [α-32P] adenosine triphosphate under basal conditions, with 10–4 mol/liter isoproterenol, or 10 mmol/liter sodium fluoride. Sodium fluoride directly stimulates the G protein subunit, Gαs, which activates adenylyl cyclase. This determines whether the G protein and cyclase moiety are intact and establishes whether uncoupling is occurring at the level of the receptor or downstream. Cyclic adenosine monophosphate (cAMP) production was quantified by standard methods described previously.14 Semi-quantitative real-time reverse transcription polymerase chain reaction RNA was extracted from tissue samples, converted to complimentary DNA, and used as the template in a polymerase chain reaction (PCR) that included the fluorophore SYBR Green. Increased fluorescence was detected by use of a real-time PCR machine (Applied Biosystems). PCR primers for GRK2 and GRK5 were designed to span introns that characterize their genomic DNA and prevent genomic DNA contamination of the RNA and subsequent reverse transcription PCR. GRK2 messenger RNA (mRNA) was determined (forward, 5′-GAACACATGCACAATCG-3′; reverse, 5′-CCAGGGAGAACCAGTC-3′). GRK5 mRNA was determined (forward, 5′-GAAGGTTAAGCGGGAAAGAGG-3′; reverse, 5′-TCCAGGCGCTTAAAGTTCAT-3′). Statistical analysis Repeated-measures analysis of variance (ANOVA) was used to analyze serial data over time within treatment groups. Analyses were conducted using StatView 4.01 software (SAS Institute, Cary, NC). Experimental groups were compared using the t-test or 1-way ANOVA, as appropriate. The Bonferroni test was applied to all significant ANOVA results using SigmaStat software (Systat Software Inc, San Jose, CA). Values of p < 0.05 were considered statistically significant. All results are expressed as mean ± standard error of the mean. Results Patient population Patient demographics are summarized in Table 1. All patients underwent HeartMate II LVAD implant for bridge to transplant. They were all at NYHA functional class IV at the time of implant and class I or II by the time of LVAD explant and HTx as a result of mechanical circulatory support. The study population included 8 men and 4 women. Four patients had ischemic cardiomyopathy and 8 had non-ischemic dilated cardiomyopathy. Ten patients were receiving dobutamine before LVAD implant. All patients received dobutamine or epinephrine, or both, after LVAD implant for a range of 3 to 10 days. None were receiving inotropic therapy at the time of HTx, and β-blockade was not resumed after LVAD implantation. The mean duration of LVAD support was 152 ± 34 days, after which all patients underwent LVAD explant and HTx. The mean LVAD flow was 5.0 ± 0.7 liters/min at a speed of 9,450 ± 55 rpm. Cardiac hemodynamics were measured by right heart catheterization before LVAD implant and within 2 months of HTx (Table 2). After LVAD implant, there was a significant decrease in right atrial pressure, pulmonary artery wedge pressure, mean pulmonary artery pressure, and a significant increase in mean diastolic blood pressure and cardiac index. For all tissue studies, non-failing (NF) left ventricular myocardium was obtained from organ donors with normal ventricular function when the heart was not procured because of non-cardiac issues. | ⁎ p < 0.05 vs heart failure. |
Myocardial β-adrenergic signaling β-AR signaling was assessed in LV tissue samples from NF, failing (HF), and LVAD-supported (LVAD) hearts by measurement of total β-AR density (Bmax) and β-AR-coupled adenylyl cyclase activity. These assays were performed after preparation of sarcolemmal membranes from the LV tissue specimens. Consistent with previously reported data, total β-AR density was significantly decreased in the HF group compared with the NF group (36.2 ± 3.7 vs 76.1 ± 6.2 fmol/mg protein, p < 0.05). LVAD support led to a significant increase in β-AR density in these failing hearts (71.4 ± 9.5 vs 36.2 ± 3.7 fmol/mg protein, p < 0.05). Bmax in the LVAD group was similar to the NF control group (Figure 1). β-AR-effector coupling was studied by measuring sarcolemmal membrane adenylyl cyclase activity under basal conditions and after stimulation with the β-agonist isoproterenol (Figure 2). Basal cyclase activity was lower in the HF group compared with NF, although this did not reach statistical significance (data not shown). Isoproterenol-stimulated activity was severely attenuated in the HF group (40.3% ± 4.6% vs 89.2% ± 7.2% increase of ISO-stimulated cyclase activity over basal, p < 0.05) consistent with uncoupling of β-AR signaling. ISO-stimulated adenylyl cyclase activity was significantly increased after LVAD support (81.5% ± 9.9% vs 40.3% ± 4.6% increase of ISO-stimulated cyclase activity over basal; p < 0.05). There was no difference in sodium fluoride-stimulated cyclase activity among the 3 groups (data not shown), indicating that the uncoupling is at the level of the β-AR and not downstream. This normalization of β-AR signaling after continuous-flow LVAD support is similar to that seen after long-term pulsatile LVAD support.10 Myocardial GRK expression and activity To investigate potential mechanisms by which β-AR signaling is restored in failing hearts by continuous-flow LVAD support, we measured the expression and activity of GRK2 and GRK5, the 2 GRKs known to be present in the human myocardium (Figure 3). In the HF group, LV protein levels of GRK2 were increased greater than 2-fold compared with NF controls (22.3 ± 4.1 vs 7.8 ± 2.8 arbitrary densitometry units, p < 0.05). In contrast, after mechanical unloading, GRK2 protein expression was decreased to levels similar to the NF group (9.9 ± 2.5 vs 7.8 ± 2.8 arbitrary densitometry units, p > 0.05). This decrease in GRK2 protein expression correlated with a decrease in GRK2 mRNA in the LVAD group relative to HF (0.45 ± 0.03 vs 0.78 ± 0.10 pg of RNA; p < 0.05; Figure 4). There was no significant difference in LV GRK2 mRNA expression between the NF and LVAD groups (Figure 4). In contrast to GRK2, there was no increase in GRK5 protein or mRNA expression in the HF group compared with NF, and this was unchanged after LVAD support (data not shown). This is not unexpected considering that GRK5 is expressed in the heart at very low levels and its role in cardiovascular pathology is unclear. To assess the functional significance of increased GRK2 protein expression, GRK2 activity was measured in sarcolemmal membrane preparations using an in vitro rhodopsin phosphorylation assay (Figure 5). GRK2 activity was increased in the HF group nearly 3-fold compared with NF group (30.8 ± 4.0 vs 12.1 ± 2.3 densitometry units, p < 0.05). After continuous-flow LVAD support, GRK2 activity was decreased to levels similar to the NF group and significantly lower than the HF group (15.4 ± 2.5 vs 30.8 ± 4.0 densitometry units, p < 0.05). These data support the concept that GRK2 is important in the regulation of β-AR signaling in human myocardium. Lymphocyte GRK expression Lymphocyte GRK2 protein expression was measured by immunoprecipitation and immunoblotting analysis of paired blood samples from the time of LVAD implant and at HTx. Similar to the myocardium, GRK2 protein levels were significantly decreased in lymphocytes after mechanical unloading with a continuous-flow LVAD (LVAD 58.6 ± 6.1 vs HF 101.7 ± 12.3 densitometry units, p < 0.05; Figure 6). These data are consistent with a recent study showing a strong correlation between GRK2 expression in the myocardium and the peripheral lymphocytes in patients with HF.15 Discussion The myocardial β-AR signaling pathway is critical in the regulation of cardiac contractility. β-ARs (β1 and β2 sub-types) are the primary myocardial targets of the sympathetic neurotransmitter norepinephrine and the adrenal hormone epinephrine. Activation of β-ARs in the heart by these 2 catecholamines leads to positive chronotropic and inotropic action by stimulation of adenylyl cyclase and subsequent increases in cAMP and intracellular Ca2+ release.16 Continued exposure of β-ARs to agonists results in a rapid decrease in responsiveness, known as desensitization. Agonist-dependent desensitization can be initiated by the phosphorylation of activated receptors by members of the family of GRKs.17 GRK2 specifically phosphorylates activated β1- β2-ARs, leading to desensitization in vitro and in vivo.18, 19 In addition, cardiac-specific overexpression of GRK2 (3-fold) in genetically engineered animal models leads to a decrease in baseline and β-agonist-stimulated contractility.20, 21 In contrast, inhibition of myocardial GRK2 in transgenic mice or through adenoviral-mediated gene transfer significantly enhances cardiac function and can rescue several models of heart failure.22, 23, 24 These studies demonstrated that GRK2 is a critical mediator of ventricular function and remodeling. HF in humans is characterized by specific alterations in the β-AR signaling system. These include selective down-regulation of β1-Rs by approximately 50% and desensitization of remaining β-ARs, which leads to the blunting of agonist-mediated stimulation.1 The enhanced desensitization of myocardial β-ARs is most likely due to the elevated expression and activity of GRK2 (about 3-fold) present in human HF.5 It is generally thought that these changes in the β-AR system in HF are triggered by increased sympathetic stimulation of the heart in this disease state.25 The dysfunctional β-AR signaling, including increased GRK2 expression and activity, is a contributing factor to the impaired myocardial contractility present in CHF. This study demonstrates that impaired myocardial β-AR signaling, which is a hallmark of CHF, can be reversed by mechanical unloading with a continuous-flow rotary pump. This finding was similar to the effect of a pulsatile volume-displacement pump on restoration of this critical signaling pathway.10 More important, the improvement in myocardial β-AR signaling is likely a result of decreased GRK2 expression and activity and appears to be due to relative normalization of the neurohormonal milieu associated with end-stage HF. In addition to the LV, right ventricular response to catecholamine stimulation should also be improved. Our data show that peripheral lymphocyte GRK expression mirrors what is seen in the myocardium and may serve as a novel biomarker for the status of cardiac β-AR signaling. Although limited by the number of patients in this study, the restoration of β-AR signaling in the heart and lymphocytes correlates with the hemodynamic improvement. Thus, measurement of lymphocyte GRK2 expression may represent a novel non-invasive method to assess the status of cardiac β-AR signaling. Haft et al26 recently compared hemodynamic and exercise performance between pulsatile and continuous-flow LVADs and found no difference in pressure unloading and cardiopulmonary exercise testing at 3 months after implant, despite a greater degree of LV volume unloading with the pulsatile pumps.26 It is unknown to what extent the differences observed in LV volume unloading have on the potential for LV recovery. Interestingly, Thohan et al27 observed similar changes in the degree of regression of myocyte hypertrophy between these 2 pump designs. In conjunction with our data, this may indicate that differences in pump design (pulsatile vs continuous-flow) may have little influence on the degree of LV reverse remodeling and β-AR signaling. In conclusion, this study demonstrates that the dysfunctional LV β-AR signaling characteristic of CHF can be restored to normal levels with a continuous-flow LVAD. This study also provides further evidence that myocardial GRK2 expression can be monitored in peripheral lymphocytes, which could provide a mechanism to follow changes in myocardial β-AR signaling after an intervention. Restoration of β-AR signaling alone clearly does not lead to recovery of normal ventricular function allowing LVAD explant, because this is a very infrequent clinical outcome and the basic science of CHF is extremely complex. However, this may provide a platform for adjunctive therapies (novel pharmacologic, cell-based, or, potentially, gene therapy) in restoring and maintaining long-term cardiac function because this signaling pathway is fundamental to the regulation of myocardial contractility. Disclosure statement This work was supported, in part, by the National Institutes of Health (S.A.A.) and the Thoracic Surgery Foundation for Research and Education (S.A.A.). None of the authors has a financial relationship with a commercial entity that has an interest in the subject of the presented manuscript or other conflicts of interest to disclose. References 1. 1Bristow MR, Ginsburg R, Minobe W, et al. Decreased catecholamine sensitivity and beta-adrenergic-receptor density in failing human hearts. N Engl J Med. 1982;307:205–211. MEDLINE |
CrossRef
2. 2Brodde OE, Michel MC, Zerkowski HR. Signal transduction mechanisms controlling cardiac contractility and their alterations in chronic heart failure. Cardiovasc Res. 1995;30:570–584. MEDLINE |
CrossRef
3. 3Inglese J, Freedman NJ, Koch WJ, et al. Structure and mechanism of the G protein-coupled receptor kinases. J Biol Chem. 1993;268:23735–23738. MEDLINE 4. 4Ungerer M, Parruti G, Bohm M, et al. Expression of beta-arrestins and beta-adrenergic receptor kinases in the failing human heart. Circ Res. 1994;74:206–213. MEDLINE 5. 5Ungerer M, Bohm M, Elce JS, et al. Altered expression of beta-adrenergic receptor kinase and beta 1-adrenergic receptors in the failing human heart. Circulation. 1993;87:454–463. MEDLINE 6. 6Akhter SA, Skaer CA, Kypson AP, et al. Restoration of beta-adrenergic signaling in failing cardiac ventricular myocytes via adenoviral-mediated gene transfer. Proc Natl Acad Sci U S A. 1997;94:12100–12105. MEDLINE |
CrossRef
7. 7Rengo G, Lymperopoulos A, Zincarelli C, et al. Myocardial adeno-associated virus serotype 6-betaARKct gene therapy improves cardiac function and normalizes the neurohormonal axis in chronic heart failure. Circulation. 2009;119:89–98.
CrossRef
8. 8Raake PW, Vinge LE, Gao E, et al. G protein-coupled receptor kinase 2 ablation in cardiac myocytes before or after myocardial infarction prevents heart failure. Circ Res. 2008;103:413–422.
CrossRef
9. 9Akhter SA, Eckhart AD, Rockman HA, et al. In vivo inhibition of elevated myocardial beta-adrenergic receptor kinase activity in hybrid transgenic mice restores normal beta-adrenergic signaling and function. Circulation. 1999;100:648–653. 10. 10Pandalai PK, Bulcao CF, Merrill WH, et al. Restoration of myocardial β-adrenergic receptor signaling following left ventricular assist device support. J Thorac Cardiovasc Surg. 2006;131:975–980. Abstract | Full Text |
Full-Text PDF (188 KB)
|
CrossRef
11. 11Heerdt PM, Holmes JW, Cai B, et al. Chronic unloading by left ventricular assist device reverses contractile dysfunction and alters gene expression in end-stage heart failure. Circulation. 2000;102:2713–2719. 12. 12James KB, McCarthy PM, Thomas JD, et al. Effect of the implantable left ventricular assist device on neuroendocrine activation in heart failure. Circulation. 1995;92:II191–II195. MEDLINE 13. 13Akhter SA, D'Souza KM, Petrashevskaya NN, et al. Myocardial β1-adrenergic receptor polymorphisms affect functional recovery following ischemic injury. Am J Physiol Heart Circ Physiol. 2006;290:H1427–H1432. MEDLINE |
CrossRef
14. 14Salomon Y, Londos C, Rodbell M. A highly sensitive adenylate cyclase assay. Anal Biochem. 1974;58:541–548. MEDLINE |
CrossRef
15. 15Iaccarino G, Barbato E, Cipolletta E, et al. Elevated myocardial and lymphocyte GRK2 expression and activity in human heart failure. Eur Heart J. 2005;26:1752–1758.
CrossRef
16. 16Rockman HA, Koch WJ, Lefkowitz RJ. Seven-transmembrane-spanning receptors and heart function. Nature. 2002;415:206–212. MEDLINE |
CrossRef
17. 17Freedman NJ, Liggett SB, Drachman DE, et al. Phosphorylation and desensitization of the human β1-adrenergic receptor: involvement of G protein-coupled receptor kinases and cAMP-dependent protein kinase. J Biol Chem. 1995;270:17953–17961. MEDLINE |
CrossRef
18. 18Koch WJ, Inglese J, Stone WC, et al. The binding site for the βγ subunits of heterotrimeric G proteins on the beta-adrenergic receptor kinase. J Biol Chem. 1993;268:8256–8260. MEDLINE 19. 19Eckhart AD, Duncan SJ, Penn RB, et al. Hybrid transgenic mice reveal in vivo specificities of G protein-coupled receptor kinases in the heart. Circ Res. 2000;86:43–50. MEDLINE 20. 20Koch WJ, Rockman HA, Samama P, et al. Cardiac function in mice overexpressing the β-adrenergic receptor kinase or a βARK inhibitor. Science. 1995;268:1350–1353. MEDLINE 21. 21White DC, Hata JA, Shah AS, et al. Preservation of myocardial beta-adrenergic receptor signaling delays the development of heart failure after myocardial infarction. Proc Natl Acad Sci U S A. 2000;97:5428–5433. MEDLINE |
CrossRef
22. 22Rockman HA, Chien KR, Choi DJ, et al. Expression of a beta-adrenergic receptor kinase 1 inhibitor prevents the development of myocardial failure in gene-targeted mice. Proc Natl Acad Sci U S A. 1998;95:7000–7005. MEDLINE |
CrossRef
23. 23Pleger ST, Boucher M, Most P, Koch WJ. Targeting myocardial β-adrenergic receptor signaling and calcium cycling for heart failure gene therapy. J Cardiac Fail. 2007;13:401–414. 24. 24Kypson AP, Peppel K, Akhter SA, et al. Ex Vivo adenovirus-mediated gene transfer to the adult rat heart. J Thorac Cardiovasc Surg. 1998;115:623–630. Abstract | Full Text |
Full-Text PDF (780 KB)
|
CrossRef
25. 25Brodde OE. Beta-adrenoceptors in cardiac disease. Pharmac Ther. 1993;60:405–430. 26. 26Haft J, Armstrong W, Dyke DB, et al. Hemodynamic and exercise performance with pulsatile and continuous-flow left ventricular assist devices. Circulation. 2007;116(suppl I):8–15. 27. 27Thohan V, Stetson SJ, Nagueh SF, et al. Cellular and hemodynamic responses of failing myocardium to continuous flow mechanical circulatory support using the DeBakey-Noon left ventricular assist device: a comparative analysis with pulsatile-type devices. J Heart Lung Transplant. 2005;24:566–575. Abstract | Full Text |
Full-Text PDF (265 KB)
|
CrossRef
a Department of Surgery, Section of Cardiac and Thoracic Surgery, University of Chicago Medical Center, Chicago, Illinois b Department of Medicine, Section of Cardiology, University of Chicago Medical Center, Chicago, Illinois  Reprint requests: Shahab A. Akhter, MD, University of Chicago Medical Center, 5841 S Maryland Ave, MC 5040, Chicago, IL 60637. Telephone: 773-702-2500; Fax: 773-702-4187
PII: S1053-2498(10)00042-2 doi:10.1016/j.healun.2010.01.010 © 2010 International Society for Heart and Lung Transplantation. Published by Elsevier Inc. All rights reserved. | |
|
FULL TEXT
