Number 35, 2007
Cardiac toxicity
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Reduction of hyperacute rejection and protection of metabolism and function in hearts of human decay accelerating factor (h-DAF)-expressing pigs
Smolenski RT, Forni M, Maccherini M, et al. Cardiovasc Res. 2007;73:143–152.
Heart failure is currently one of the major causes of mortality and factors affecting the quality of life in humans. Despite advances in pharmacological treatment and recent developments in mechanical circulatory support devices, cardiac transplantation remains the most effective treatment of endstage heart failure. However, the availability of human donor hearts is limited, and only a small percentage of those who would benefit from this treatment could actually receive a transplant. It is expected that the disparity between organ availability and demand will increase even further in the future. One possible solution of the donor organ shortage is the use of animal organs. To date, pigs seem to be the best source of organs for xenotransplantation because of their physiological compatibility and ethical acceptability. However, after transplantation into primates, pig hearts undergo a rapid and vigorous reaction termed hyperacute rejection, causing total dysfunction within minutes. An important challenge is therefore the development of effective and inexpensive procedures for the generation of transgenic animals. Sperm-mediated gene transfer (SMGT), developed by Lavitrano and colleagues several years ago [1], appears to be an efficient and cost-effective procedure for that purpose. Lavitrano et al successfully established a pig strain expressing human decay-accelerating factor (hDAF) [2]. They evaluated in the present paper whether hearts of hDAF transgenic pigs generated using SMGT were protected from structural damage, metabolic changes, and mechanical dysfunction during perfusion with human blood.
Commentary
Hearts from control or transgenic pigs were perfused ex vivo for 4 h with fresh human blood using an ex-vivo working model system allowing monitoring of function, metabolism, and structure.
Cardiac output remained constant in transgenic animals throughout the experiment, whereas it decreased in control pigs after 30 min of perfusion (P < 0.01 compared with transgenic animals). The maximum increase in coronary perfusion pressure was reduced to 154 ± 16% in transgenic animals, and to 237 ± 10% in control pigs (P < 0.001). After 4 h, myocardial ATP was 21.1 ± 1.1 nmol/mg dry weight (similar to the baseline value) in transgenic pigs, whereas it decreased to 17.2 ± 1.4 nmol/mg dry weight in control animals (P < 0.05).
Deposition of complement factors C3 and C5b9 was present in control but not transgenic animals after perfusion. Attenuation of hyperacute rejection was further confirmed by microscopic analysis of cardiac specimens: there was no structural damage in transgenic hearts.
There are several limitations to the system used in this study. First, it allows for only several hours of perfusion, essentially limiting its application to the study of hyperacute rejection. Secondly, not all mechanisms of rejection could be reproduced; for example, platelet-mediated rejection that was blunted by high-dose heparin. The ex-vivo system also neglects the effect of other organs on blood homeostasis. However, this study has shown that hearts from transgenic pigs produced by SMGT were protected from hyperacute rejection after exposure to human blood, and that those hearts, in a human blood environment, were relatively metabolically stable and maintained mechanical function above the threshold for life support. Further application of this method for the generation of multigene transgenic pigs, and in particular its combination with knockout techniques, could be a significant step towards clinical xenotransplantation.
REFERENCES
1. Lavitrano M, Camaioni A, Fazio VM, Dolci S, Farace MG, Spadafora C.
Sperm cells as vectors for introducing foreign DNA into eggs: genetic transformation of mice.
Cell. 1989;57:717–723.
PMID: 2720785 [PubMed - indexed for MEDLINE]
2. Lavitrano M, Bacci ML, Forni M, et al.
Efficient production by sperm-mediated gene transfer of human decay accelerating factor (hDAF) transgenic pigs for xenotransplantation.
Proc Natl Acad Sci U S A. 2002;99:14230–14235.
PMID: 12393815 [PubMed - indexed for MEDLINE]
Danielle Feuvray
Free fatty acid depletion acutely decreases cardiac work and efficiency in cardiomyopathic heart failure
Tuunanen H, Engblom E, Naum A, et al. Circulation. 2006;114:2130–2137
Metabolic modulators that enhance myocardial glucose metabolism by inhibiting free fatty acid (FFA) metabolism may improve cardiac function in patients with heart failure. The effect of acute FFA withdrawal on cardiac function was studied in 18 fasting non diabetic patients with heart failure caused by idiopathic dilated cardiomyopathy (IDCM) (14 men, four women, ages 58.8 ± 8.0 years, ejection fraction 33 ± 8.8%) and eight matched healthy controls. They underwent examination of myocardial perfusion and oxidative and FFA metabolism before and after acute reduction of serum FFA concentrations by acipimox, an inhibitor of lipolysis. Metabolism was monitored by positron emission tomography (PET) and [15O]H2O, [11C]acetate, and [11C]palmitate. Left ventricular function and myocardial work were measured by echocardiography and the efficiency of forward work was calculated. Acipimox decreased myocardial FFA uptake by >80% in both groups. The rate–pressure product and myocardial perfusion remained unchanged, but stroke volume decreased similarly in both groups. In the healthy controls, reduced cardiac work was accompanied by a decrease in oxidative metabolism (from 0.071 ± 0.019 ?/min to 0.055 ± 0.016 ?/min; P < 0.01). In patients with IDCM, cardiac work decreased, whereas oxidative metabolism remained unchanged and efficiency decreased (from 35.4 ± 12.6 mm Hg/μL per μg to 31.6 ± 13.3 mm Hg/μL per μg; P < 0.05). It was concluded that acutely decreased serum FFA depresses cardiac work. In healthy hearts, this was accompanied by a parallel decrease in oxidative metabolism, and myocardial efficiency was preserved. In failing hearts, FFA depletion did not downregulate oxidative metabolism, and myocardial efficiency deteriorated. Thus failing hearts are, unexpectedly, more dependent than healthy hearts on the availability of FFA. It is proposed that both glucose and fatty acid oxidation are required for optimal function of the failing heart.
Commentary
In this interesting study, both patients with IDCM and healthy volunteers underwent [11C]acetate PET imaging for measurement of oxidative metabolism, [11C]palmitate PET imaging for measurement of the free fatty acid uptake and the rate of β-oxidation, [15O]H2O PET imaging for myocardial perfusion, and echocardiography to determine left ventricular stroke volume and left ventricular mass. The same measurements were repeated after administration of acipimox, a drug that decreases free fatty acid concentrations in blood.
The most significant part of the study concerned the measurement of cardiac efficiency in patients and volunteers. This parameter is determined by combining the echocardiographic data with the [11C]acetate data:efficiency = (LV work power/LV mass)/LV Kmono
where LV (left ventricular) work power is ‘systolic blood pressure × stroke volume × heart rate’ and Kmono is a value derived from [11C]acetate in which the decline in tracer activity of the heart is fitted with a monoexponential curve. (For an overview of cardiac efficiency, see [1].)
As expected, acipimox decreased fatty acid concentrations in both patients and volunteers. Also expected was that left ventricular work power was decreased in patients with IDCM compared with that in healthy volunteers, both before and after the administration of acipimox. In both groups, administration of acipimox led to a small but non significant reduction of left ventricular work power. Oxidative metabolism (Kmono value) was similar between patients and healthy volunteers at baseline but, after the administration of acipimox, Kmono values decreased significantly in the healthy volunteers alone. By combining these work power data and Kmono values in the calculation of efficiency, the authors showed that decreasing the concentration of fatty acids by the use of acipimox resulted in a small but significant decrease (of 11%) in cardiac efficiency in patients with IDCM and an increase (of 18%) in the healthy volunteers.
These findings are quite surprising, and in contrast with what was to be expected. A wealth of experimental and clinical data have shown that a metabolic intervention that increases glucose metabolism and inhibits fatty acid metabolism in heart failure (eg, trimetazidine or glucose–insulin–potassium) improves cardiac function.
In the editorial accompanying the paper by Tuunanen and colleagues, Taegtmeyer and Ballal [2] offered some explanations for the observed phenomena. First, recent data in lipase-deficient mice have shown that fatty acids are needed for normal cardiac contraction. This is in line with the findings discussed here. Secondly, acipimox itself may have direct hemodynamic effects. This was not measured in the study, and may have altered left ventricular work power. Thirdly, acipimox itself may decrease insulin secretion. As the patients were insulin-resistant, the intervention to decrease fatty acids, combined with the insulin resistance, may have depleted the heart of the required energy nutrients.
Another point worthy of consideration is that, in this study by Tuunanen et al, Kmono values before the acipimox intervention were similar between IDCM patients and volunteers. Other studies [3] have shown that Kmono values were lower in patients with cardiomyopathy, suggesting a patient bias in the present study. Finally, earlier experimental studies have clearly shown that the clearance of radioactivity from the heart is biexponential: the fast part of the time–activity curve is related to turnover of [11C]acetate in the Krebs’ cycle, and the slower part is related to clearance of the radioactivity from Krebs’ cycle intermediates such as glutamate [4]. As is usual in human studies, in the present study oxidative metabolism was measured by fitting the time–activity curve solely with a monoexponential curve. It is therefore unknown to what extent the second, slow, component of the tracer influenced the values of the first. This may be different in healthy volunteers and in patients.
Despite some points of criticism, the findings of Tuunanen and colleagues show that the relationship between cardiac function and metabolism is complex, and that there remain many details that need to be studied. In conclusion, I will quote a statement by Taegtmeyer and Ballal: ‘When it comes to energy substrate metabolism of the heart, extremes are never good’.
REFERENCES
1. Knaapen P, Germans T, Knuuti J, et al.
Myocardial energetics and efficiency: current status of the noninvasive approach.
Circulation. 2007;115:918–927.
PMID: 17309938 [PubMed - indexed for MEDLINE]
2. Taegtmeyer H, Ballal K.
No low-fat diet for the failing heart?
Circulation. 2006;114:2092–2093.
PMID: 17101867 [PubMed - indexed for MEDLINE]
3. Bengel FM, Permanetter B, Ungerer M, Nekolla S, Schwaiger M.
Non-invasive estimation of myocardial efficiency using positron emission tomography and carbon-11 acetate – comparison between the normal and failing human heart.
Eur J Nucl Med. 2000;27:319–326.
PMID: 10774885 [PubMed - indexed for MEDLINE]
4. Klein LJ, Visser FC, Knaapen P, et al.
Carbon-11 acetate as a tracer of myocardial oxygen consumption.
Eur J Nucl Med. 2001;28:651–668.
PMID: 11383873 [PubMed - indexed for MEDLINE]
Frans Visser
Effect of ramipril on the incidence of diabetes
Bosch J, Yusuf S, Gerstein HC, et al; the DREAM Trial Investigators. N Engl J Med. 2006;355:1551–1562.
Previous studies have suggested that blockade of the renin–angiotensin system may prevent diabetes in people with cardiovascular disease or hypertension. In a double-blind, randomized clinical trial with a 2-by-2 factorial design, we studied 5269 participants without cardiovascular disease but with impaired fasting glucose concentrations (after an 8 h fast) or with impaired glucose tolerance. They were randomly assigned to receive ramipril (up to 15 mg per day) or placebo (and rosiglitazone or placebo) and followed for a median of 3 years. We studied the effects of ramipril on the development of diabetes or death, whichever came first (the primary outcome), and on secondary outcomes, including regression to normoglycemia. The incidence of the primary outcome did not differ significantly between the ramipril group (18.1%) and the placebo group (19.5%; hazard ratio for the ramipril group 0.91, 95% confidence interval [CI] 0.81 to 1.03; P = 0.15). Participants receiving ramipril were more likely to have regression to normoglycemia than those receiving placebo (hazard ratio 1.16, 95% CI 1.07 to 1.27; P = 0.001). At the end of the study, the median fasting plasma glucose concentration was not significantly lower in the ramipril group (102.7 mg/dL [5.70 mmol/L]) than in the placebo group (103.4 mg/dL [5.74 mmol/L]; P = 0.07), although plasma glucose concentrations 2 h after an oral glucose load were significantly lower in the ramipril group (135.1 mg/dL [7.50 mmol/L] compared with 140.5 mg/dL [7.80 mmol/L]; P = 0.01). Among persons with impaired fasting glucose concentrations or impaired glucose tolerance, the use of ramipril for 3 years does not significantly reduce the incidence of diabetes or death, but does significantly increase regression to normoglycemia. (ClinicalTrials.gov number, NCT00095654 [ClinicalTrials.gov].)
Commentary
Angiotensin converting enzyme inhibitors (ACEIs) and angiotensin type I receptor blockers (ARBs) have been shown to reduce the incidence of diabetes in a number of trials. In the main, these trials were designed to examine other aspects of ACEI/ARB efficacy and the observed effect on diabetes was either an incidental finding or at best a prespecified, although self-reported, secondary endpoint. The lack of rigid determination of diabetes in all individuals before study initiation and at study end could have introduced systematic biases that resulted in the antidiabetic effect being spurious. For example, in most of these trials, ACEIs/ARBs reduced events, and those with events would have been under greater medical scrutiny than those without events. This may have increased the recognition, but not the true incidence, of diabetes. Further, in some studies it may not have been the effect of the ACEI or ARB being protective, but rather the increased likelihood of the use of diabetogenic antihypertensive drugs in the non ACEI/ARB group(s). For these, and other reasons, a prospective randomized trial was required.
The Diabetes Reduction Assessment with Ramipril and Rosiglitazone Medications (DREAM) trial investigators enrolled individuals with ‘pre-diabetes’, based on plasma glucose concentrations. The inclusion criteria necessitated a plasma glucose of either at least 110 mg/dL (6.1 mmol/L) but not more than 126 mg/dL (7.0 mmol/L) after an overnight fast, or at least 140 mg/dL (7.8 mmol/L) but not more than 200 mg/dL (11.1 mmol/L) 2 h after a 75 g oral glucose load. At the 2 year and final visits, the glucose tolerance tests were repeated in those who had not already developed overt diabetes. The primary outcome was newly diagnosed diabetes or death. Over the average of 3 years of follow-up, death occurred in only 1.2% of individuals and there was no difference by group. The predominant outcome driving the primary endpoint was therefore new diabetes. As can be seen from the abstract, this did not differ significantly between groups. However, a number of secondary endpoints such as reversion to normoglycemia and plasma glucose concentrations after oral glucose loading were modestly, but significantly, improved. When these are considered together with the 95% confidence interval of the primary endpoint (0.81 to 1.03), one is left with the distinct impression that ramipril probably does have an antidiabetic effect. However, the magnitude of this effect is modest and not as robust as that suggested in previous trials, such as Heart Outcomes Prevention Evaluation.
As the DREAM trial failed to reach its primary endpoint, and the results as a whole suggest the antidiabetic effect of ramipril at doses as high as 15 mg is modest, what should we do? First, the basic ‘reverse translational’ studies to find the molecular mechanism by which ACEI reduces diabetes are now less urgent. Secondly, there is no compelling reason to prescribe ACEI in those with pre-diabetes unless there is an alternative accepted indication such as hypertension. Obviously, once diabetes develops, an ACEI or ARB is indicated. Unfortunately, apart from oral hypoglycemic agents, the only interventions known to reduce the progression to overt diabetes are weight loss and exercise. Once again, healthy lifestyle wins through!
M. Marber
Acute impairment of regional myocardial glucose uptake in the apical ballooning (takotsubo) syndrome
Bybee KA, Murphy J, Prasad A, et al. J Nucl Cardiol. 2006;13:244–250.
Apical ballooning syndrome is a poorly understood clinical entity characterized by acute, transient systolic dysfunction of the left ventricular apex in the absence of epicardial coronary artery disease, and commonly associated with acute emotional stress. We report abnormal regional myocardial perfusion and glucose uptake in four consecutive patients with apical ballooning syndrome who were studied using positron emission tomography with [13N]ammonia and [18F]fluorodeoxyglucose within 72 h of presentation with the syndrome. All patients were postmenopausal women, three of whom had a major recent life stress event. Coronary angiography revealed no or minimal obstructive epicardial coronary artery disease. All patients exhibited reduced glucose uptake in the mid left ventricular and apical myocardial segments, which was out of proportion to perfusion abnormalities in 50% of the cases. In all four patients, affected regions recovered regional left ventricular systolic function within 6 weeks.
Commentary
Takostubo syndrome is a relatively new syndrome that was described for the first time in 1991 by Dote et al [1]. It is characterized by a transient systolic left ventricular dysfunction, typically localized in the distal and apical parts of the myocardium, by abnormalities on the electrocardiogram mimicking acute myocardial infarction, and by an increase in cardiac enzymes, in the absence of significant coronary artery disease. Its prognosis is relatively benign (see review [2]). The etiology is unknown but, because the onset of this syndrome is often preceded by emotional or physical stress and it is also observed in subarachnoid hemorrhage. Catecholamine-mediated multivessel epicardial spasm, microvascular coronary spasm, or possible direct catecholamine-mediated myocyte injury have been proposed as possible pathophysiological mechanisms.
Metabolic and perfusion studies in these patients reveal an interesting pattern. Studies have shown that perfusion is decreased in the affected regions. Kurisu et al [3] described a decrease in fatty acid uptake during the acute phase of the disease, which gradually improved over time. The observation of decreased perfusion and decreased fatty acid uptake can be explained by the presence of myocardial stunning, as has been described earlier in patients with reperfused myocardial infarction. In the present study by Bybee et al, a decrease in glucose uptake was also found. This is in contrast to the classical concept of myocardial stunning, because, in stunning, an increase in glucose uptake is observed. Thus a possibly new metabolic profile in Takotsubo cardiomyopathy may emerge: decreased perfusion in combination with decreased uptake of both fatty acid and glucose. Of course, the data are from two different studies, with different patients, and a combination of fatty acid and glucose imaging has not been done, thus the observations need further confirmation. Bybee et al suggest two possible mechanisms for the decreased glucose uptake: direct effect of cathecholamines on insulin resistance or a direct toxic effect of cathecholamines on glucose metabolism. The latter may also involve fatty acid metabolism. An additional explanation may be that flow and metabolism are adapted to the decreased function of the myocardial areas involved. Why and how function is primarily depressed is unknown, but it may also be related to the toxic effects of cathecholamines. As usual, further studies are needed!
REFERENCES
1. Dote K, Sato H, Tateishi H, Uchida T, Ishihara M.
Myocardial stunning due to simultaneous multivessel coronary spasms: a review of 5 cases.
J Cardiol. 1991;21:203–214.
PMID: 1841907 [PubMed - indexed for MEDLINE]
2. Gianni M, Dentali F, Grandi AM, Sumner G, Hiralal R, Lonn E.
Apical ballooning syndrome or takotsubo cardiomyopathy: a systematic review.
Eur Heart J. 2006;27:1523–1529.
PMID: 16720686 [PubMed - indexed for MEDLINE]
3. Kurisu S, Inoue I, Kawagoe T, et al.
Myocardial perfusion and fatty acid metabolism in patients with tako-tsubo-like left ventricular dysfunction.
J Am Coll Cardiol. 2003;41:743–748.
PMID: 12628716 [PubMed - indexed for MEDLINE]
F.C. Visser
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