Ronald G. Victor

Cardiac Steatosis in Diabetes Mellitus A 1H-Magnetic Resonance Spectroscopy Study

Background— The risk of heart failure in type 2 diabetes mellitus is greater than can be accounted for by hypertension and coronary artery disease. Rodent studies indicate that in obesity and type 2 diabetes mellitus, lipid overstorage in cardiac myocytes produces lipotoxic intermediates that cause apoptosis, which leads to heart failure. In humans with diabetes mellitus, cardiac steatosis previously has been demonstrated in explanted hearts of patients with end-stage nonischemic cardiomyopathy. Whether cardiac steatosis precedes the onset of cardiomyopathy in individuals with impaired glucose tolerance or in patients with type 2 diabetes mellitus is unknown. Methods and Results— To represent the progressive stages in the natural history of type 2 diabetes mellitus, we stratified 134 individuals (age 45±12 years) into 1 of 4 groups: (1) lean normoglycemic (lean), (2) overweight and obese normoglycemic (obese), (3) impaired glucose tolerance, and (4) type 2 diabetes mellitus. Localized 1H magnetic resonance spectroscopy and cardiac magnetic resonance imaging were used to quantify myocardial triglyceride content and left ventricular function, respectively. Compared with lean subjects, myocardial triglyceride content was 2.3-fold higher in those with impaired glucose tolerance and 2.1-fold higher in those with type 2 diabetes mellitus (P<0.05). Left ventricular ejection fraction was normal and comparable across all groups. Conclusions— In humans, impaired glucose tolerance is accompanied by cardiac steatosis, which precedes the onset of type 2 diabetes mellitus and left ventricular systolic dysfunction. Thus, lipid overstorage in human cardiac myocytes is an early manifestation in the pathogenesis of type 2 diabetes mellitus and is evident in the absence of heart failure.

Cyclosporine-induced sympathetic activation and hypertension after heart transplantation

BACKGROUND Hypertension is a frequent complication of cyclosporine-induced immunosuppression, but the underlying mechanism is unknown. In anesthetized animals, the administration of cyclosporine increases sympathetic-nerve discharge, which may contribute to hypertension. METHODS To determine whether cyclosporine-induced hypertension is accompanied by sustained sympathetic neural activation in patients, we recorded sympathetic action potentials using intraneural microelectrodes (in the peroneal nerve) in heart-transplant recipients receiving azathioprine and prednisone alone (n = 5) or in combination with cyclosporine (n = 14). We performed the same studies in eight patients with myasthenia gravis who were receiving cyclosporine and eight who were not, in five patients with essential hypertension, and in nine normal controls. RESULTS Heart-transplant recipients receiving cyclosporine had higher mean arterial blood pressure (+/- SE) than those not receiving cyclosporine (112 +/- 3 vs. 96 +/- 4 mm Hg; P less than 0.05) and a 2.7-fold higher rate of sympathetic-nerve firing (80 +/- 3 vs. 30 +/- 4 bursts per minute; P less than 0.05). For patients with myasthenia gravis, similar doses of cyclosporine were associated with smaller elevations in mean arterial blood pressure (100 +/- 2 mm Hg, as compared with 91 +/- 4 mm Hg in those not receiving cyclosporine; P less than 0.05) and in the rate of sympathetic-nerve firing (46 +/- 3 bursts per minute, as compared with 25 +/- 4 bursts per minute; P less than 0.05). Sympathetic activity in patients with heart transplants or myasthenia gravis who were not being treated with cyclosporine was no different from that in patients with essential hypertension or in normal controls. CONCLUSIONS Cyclosporine-induced hypertension is associated with sympathetic neural activation, which may be accentuated by the cardiac denervation that results from heart transplantation.

Myocardial triglycerides and systolic function in humans: in vivo evaluation by localized proton spectroscopy and cardiac imaging.

Recent experimental data suggest that adiposity directly damages the heart by promoting ectopic deposition of triglyceride, a process known as myocardial steatosis. The goal of this study was to develop and validate proton magnetic resonance spectroscopy (1H MRS) as an in vivo tool to measure myocardial lipid content. Complementary studies in rat tissue ex vivo and in 15 healthy humans in vivo provided evidence that 1H MRS constitutes a reproducible technique for the measurement of myocardial triglyceride. In myocardial tissue from Zucker rats, the 1H MRS measurement of triglyceride matched that obtained by biochemical measurement (P < 0.001). In human subjects triglyceride was evident in the hearts of even the very lean individuals and was elevated in overweight and obese subjects. Increased myocardial triglyceride content was accompanied by elevated LV mass and suppressed septal wall thickening as measured by cardiac imaging. Magn Reson Med 49:417–423, 2003.

Adiposity of the Heart*, Revisited

The unrelenting obesity epidemic is one likely explanation for the recent adverse secular trends in cardiovascular morbidity and mortality rates in the United States (1, 2). Hospitalizations for congestive heart failure have increased, and the steady decline in coronary heart diseaserelated deaths since the 1950s has leveled off (3). The recent obesity epidemic poses a major threat to human health in the United States because these persons will be predisposed to a burden of major chronic disease (1, 2). Obesity has both metabolic and cardiovascular health consequences; in particular, obese individuals are at much greater risk for type 2 diabetes and cardiovascular disease (3, 4). Obesity is traditionally considered to be an indirect cause of heart disease. Obese persons typically present with several Framingham risk factors, including hypertension, dyslipidemia, and diabetes mellitus. These risk factors predispose the patient to myocardial infarction that, in severe cases, results in ischemic cardiomyopathy (4). In addition to an elevated Framingham risk score, the hemodynamic hallmarks of obesity are increased heart rate and stroke volume (5). This hyperdynamic circulation is thought to be a compensatory adaptation to increased adipose tissue mass at the expense of eccentric left ventricular remodeling. In extreme obesity, this condition can progress to nonischemic dilated cardiomyopathy (2, 6). In contrast to these 2 rather traditional concepts, an emerging body of basic research is revisiting a previous hypothesis (7, 8): that fat is a direct cardiotoxin (9, 10). In 1933, the original autopsy studies of Smith and Willius (8) suggested that fatty degeneration of the heart is a common consequence of obesity and a possible cause of dilated cardiomyopathy in humans. After Alexander and colleagues (11) called this theory into question in the 1960s, the issue lay dormant for the next several decades (9). Now a growing body of evidence is revisiting the hypothesis that excessive deposits of lipids within myocardial tissue (that is, cardiac lipotoxicity) is an important but forgotten cause of nonischemic dilated cardiomyopathy in humans (12, 13). Under healthy conditions, most triglyceride is stored in adipocytes; the amount of triglyceride stored in nonadipocyte tissues (such as the pancreas, liver, and myocardium) is minimal and very tightly regulated. Various genetic rodent models of obesity have shown that cytosolic triglyceride accumulates excessively in these organs (termed steatosis) when this regulation is disrupted. This accumulation has been implicated in activating adverse signaling cascades that culminate in irreversible cell death (termed lipotoxicity) and lead to several well-recognized clinical syndromes (13). These include nonalcoholic hepatic steatosis; pancreatic -cell failure in type 2 diabetes; and most recently, dilated cardiomyopathy (Figure 1). Figure 1. Concept of lipotoxicity. bottom The purposes of this article are to review recent basic animal research that demonstrates direct toxic effects of lipid accumulation on the myocardium and to highlight emerging efforts to translate this work into the clinical setting by using novel cardiac magnetic resonance imaging and spectroscopy technology. The results of this research could provide insight into the pathogenesis of heart disease in obese humans and guide the development of a novel biomarker and drug target for the prevention of heart failure in these persons. Steatosis in Rodents The seminal research that showed a role for steatosis in obesity-related organ dysfunction was performed with the Zucker diabetic fatty rat, which is a genetic model of progressive type 2 diabetes (14-16). In this obese rodent, type 2 diabetes developed secondary to a loss-of-function mutation in tissue receptors for leptin, the adipocyte-derived hormone that regulates appetite and body weight (17). This model of genetic obesity is more extreme than the milder leptin resistance that commonly accompanies dietary obesity in humans (17). Initial studies demonstrated that pancreatic steatosis directly caused islet cell failure and the subsequent hyperglycemia that characterized this model (14-16). Although leptin was generally thought to act centrally to regulate caloric intake and energy expenditure (17), a series of studies provided experimental evidence that leptin also acts directly on the pancreatic islet cells to stimulate fatty acid oxidation, thereby limiting cellular triglyceride accumulation (18). These findings suggested that leptin signaling is also essential in regulating peripheral lipid stores. Furthermore, the investigators described a pathway whereby failure of the leptin receptor led to excessive cytosolic accumulation of triglyceride and its by-product, ceramide, within islet cells. This accumulation activated the inducible form of nitric oxide synthase, which accelerated cell death (apoptosis) and failure of the -cell (14, 15). Interventions that stimulated free-fatty acid oxidation, like restoration of leptin signaling or thiazolidinedione therapy, effectively attenuated triglyceride accumulation in islet cells and prevented the onset of type 2 diabetes (19). These findings provided evidence that steatosis is an integral determinant of -cell failure in the pathogenesis of obesity-associated type 2 diabetes. In addition to pancreatic -cell failure, the Zucker diabetic fatty rat experienced age-related cardiac dysfunction that was characterized by eccentric left ventricular remodeling, increased left ventricular pressure, and decreased systolic performance (9, 20). The abnormalities in cardiac structure and function are accompanied by a 2-fold increase in myocardial triglyceride content and ceramide that is similar to the accumulation seen in islet cells. Myocardial DNA laddering, which is a marker of apoptosis, is also increased (9). Of note, early administration of thiazolidinedione therapy is effective in attenuating myocardial triglyceride accumulation and normalizing left ventricular contractile performance (9, 20), as shown in Figure 2. Because reduced myocardial lipid content and improved cardiac structure and function were observed independent of changes in body weight, they strongly suggest a role for myocardial steatosis in obesity-related cardiomyopathy. Figure 2. Myocardial lipotoxicity in the Zucker diabetic fatty rat. Top panel. white bars light gray bars dark gray bars Bottom panel. The extreme obesity in the Zucker rat model makes it difficult to determine whether the cardiac maladaptations are related to excessive myocardial lipid accumulation or to increased expression of conventional risk factors for cardiovascular disease. To address this limitation, various lean genetic mouse models of cardiac-restricted steatosis have recently been developed (10, 21-29). These animals display diffuse myocardial lipid content in the absence of obesity or any other traditional cardiovascular risk factors, thereby allowing researchers to study the acute effects of myocardial steatosis on left ventricular structure and function. Overexpression of long-chain acyl-CoA synthetase, a key enzyme involved in triglyceride synthesis, produces an example of cardiac-restricted steatosis. Increased protein expression of acyl-CoA synthetase in the myocardium disrupts the balance between lipid import and export in the myocardium (Figure 3), which results in diffuse lipid accumulation and a greater than 2-fold increase in heart mass (10). The severe myocardial steatosis that is observed in this animal is associated with substantial left ventricular hypertrophy by 4 weeks of age that coincides with left ventricular dilatation and eventually progresses to heart failure. Of importance, the changes in cardiac lipid content, structure, and function develop without any change in lipid profile or body weight of the animal. This pattern of steatosis-induced heart failure has been reproduced by targeted overexpression of genes that are involved in lipid delivery (24, 26) and synthesis (10, 25) and by targeted deletion of genes that are involved in lipoprotein secretion (21) from the myocardium. Taken together, these data demonstrate that cardiac-specific steatosis, independent of systemic obesity, is a direct cause of dilated cardiomyopathy. Figure 3. Myocardial-specific lipotoxicity. Top panel. Middle panel. gray bars white bars Bottom panel. The development of cardiac-restricted transgenic murine models have also shown the therapeutic potential of several countermeasures, including adenoviral administration of leptin (Figure 3) and apolipoprotein B (26, 28), dietary replacement of long-chain triglycerides with medium-chain triglycerides (22), and blockade of production of reactive oxygen species (29). Each of these interventions has effectively ameliorated the myocardial steatosis in these mouse models and has rescued the myocardium from progression to dilated cardiomyopathy. These data reinforce the observations in the Zucker diabetic fatty rat that lipid accumulation is toxic in the myocardium. It is important to note that current thinking suggests that the cardiomyopathy is not a direct consequence of triglyceride accumulation alone, but that cardiomyopathy develops secondary to an accumulation of by-products of lipid metabolism, such as ceramide or other fatty acid derivatives that are known to interfere with intracellular signaling pathways (9, 30). This research provides convincing evidence for an acute role of steatosis in the development of left ventricular hypertrophy and dysfunction in animal models of obesity; until recently, however, few data from human research were available to support this theory. Quantification of Lipids in Human Tissues To study the role of steatosis in the clinical setting, we and others have developed a magnetic resonance imaging and spectroscopy technique that permits the precise and reproducible quantification of intracellular trig

Sympathetic nerve discharge is coupled to muscle cell pH during exercise in humans.

We used phosphorus nuclear magnetic resonance spectroscopy (31P-NMR) to probe the cellular events in contracting muscle that initiate the reflex stimulation of sympathetic outflow during exercise. In conscious humans, we performed 31P-NMR on exercising forearm muscle and simultaneously recorded muscle sympathetic nerve activity (MSNA) with microelectrodes in the peroneal nerve to determine if the activation of MSNA is coupled to muscle pH, an index of glycolysis, or to the concentrations (II) of inorganic phosphate (Pi) and adenosine diphosphate (ADP) which are modulators of mitochondrial respiration. During both static and rhythmic handgrip, the onset of sympathetic activation in resting muscle coincided with the development of cellular acidification in active muscle. Furthermore, increases in MSNA were correlated closely with decreases in intracellular pH but dissociated from changes in phosphocreatine [( PCr]), [Pi], and [ADP]. The principal new conclusion is that activation of muscle sympathetic outflow during exercise in humans is coupled to the cellular accumulation of protons in contracting muscle.

Left Ventricular Hypertrophy Is More Prevalent in Blacks Than Whites in the General Population The Dallas Heart Study

Although recent studies have suggested that blacks compared with whites have an increased prevalence of left ventricular hypertrophy, it remains uncertain whether this is true despite adjustment for body composition (fat mass and fat-free mass) and when assessed by cardiac MRI in the general population. The Dallas Heart Study is a population-based study of Dallas County in which 1335 black and 858 white participants 30 to 67 years of age underwent detailed assessment including dual-energy x-ray absorptiometry scan to measure body composition and cardiac MRI. Left ventricular hypertrophy, whether defined by indexation to body surface area (P<0.001), fat-free mass (P=0.002), or height2.7 (P<0.001) was 2- to 3-fold more common in black versus white women. Similar results were seen when comparing black and white men (P<0.001 when left ventricular hypertrophy was indexed to body surface area or height2.7 and P=0.05 when indexed to fat-free mass). Ethnic disparities in left ventricular mass persisted in multivariable models despite adjustment for fat mass, fat-free mass, systolic blood pressure, age, gender, and measures of socioeconomic status. We conclude that blacks compared with whites have increased left ventricular mass and a 2- to 3-fold higher prevalence of left ventricular hypertrophy in the general population, as assessed by cardiac MRI. The ethnic differences in left ventricular mass are independent of differences in body composition.

Paradoxical withdrawal of reflex vasoconstriction as a cause of hemodialysis-induced hypotension.

Acute hypotension is an important complication of hemodialysis, but the underlying mechanisms remain poorly understood. Because hemorrhage-induced hypovolemia can trigger a sudden decrease in sympathetic activity resulting in bradycardia and vasodilation, we hypothesized that hemodialysis-induced hypovolemia also can trigger the same type of vasodepressor reaction, which would exacerbate the volume-dependent fall in blood pressure. We therefore measured blood pressure, vascular resistance, and sympathetic nerve activity (intraneural microelectrodes) during sessions of maintenance hemodialysis in 7 patients with and 16 patients without a history of hemodialysis-induced hypotension. During hemodialysis, blood pressure at first remained unchanged as calf resistance increased in both hypotension-resistant (from 37 +/- 4 to 49 +/- 5 U, P < 0.05) and hypotension-prone (from 42 +/- 6 to 66 +/- 12 U, P < 0.05) patients; sympathetic activity increased comparably in the subset of patients in whom it could be measured. With continued hemodialysis, calf resistance and sympathetic activity increased further in the hypotension-resistant patients, but in the hypotension-prone patients the precipitous decrease in blood pressure was accompanied by decreases in sympathetic activity, vascular resistance, and heart rate as well as symptoms of vasodepressor syncope. On an interdialysis day, both groups of patients increased vascular resistance normally during unloading of cardiopulmonary baroreceptors with lower body negative pressure and increased heart rate normally during unloading of arterial baroreceptors with infusion of nitroprusside. These findings indicate that in a group of hemodialysis patients without diabetes or other conditions known to impair autonomic reflexes, hemodialysis-induced hypotension is not caused by chronic uremic impairment in arterial or cardiopulmonary baroreflexes but rather by acute, paradoxical withdrawal of sympathetic vasoconstrictor drive producing vasodepressor syncope.

Nitric oxide mediates contraction‐induced attenuation of sympathetic vasoconstriction in rat skeletal muscle

1 Sympathetic vasoconstriction is attenuated by metabolic events in contracting rat skeletal muscle, in part by activation of ATP‐sensitive potassium (KATP) channels. However, the specific metabolites in contracting muscle that open KATP channels are not known. We therefore asked if contraction‐induced attenuation of sympathetic vasoconstriction is mediated by the endogenous vasodilators nitric oxide (NO), adenosine, or prostaglandins PGI2 or PGE2, all of which are putative KATP channel openers. 2 In anaesthetized rats, hindlimb contraction alone significantly attenuated the vasoconstrictor responses to lumbar sympathetic nerve stimulation. Inhibition of NO synthase with N‐nitro‐L‐arginine methyl ester (L‐NAME, 5 mg kg−1, i.v.) partially reversed this effect of contraction, resulting in enhanced sympathetic vasoconstriction in contracting hindlimb. Subsequent treatment with the KATP channel blocker glibenclamide (20 mg kg−1, i.v.) had no further effect on sympathetic vasoconstriction in contracting hindlimb. 3 This effect of L‐NAME to partially reverse contraction‐induced attenuation of sympathetic vasoconstriction was not replicated by D‐NAME (5 mg kg−1, i.v.) or angiotensin II (12.5 ng kg−1 min−1, i.v.), the latter used as a hypertensive control. 4 Adenosine receptor blockade with 8‐(p‐sulphophenyl)theophylline (10 mg kg−1, i.v.) or cyclooxygenase inhibition with indomethacin (5 mg kg−1, i.v.) had no effect on contraction‐induced attenuation of sympathetic vasoconstriction. 5 These results suggest that NO plays an important role in the precise regulation of blood flow in exercising skeletal muscles by opposing sympathetic vasoconstriction. Although the underlying mechanism is not known, it may involve NO‐induced activation of vascular KATP channels.

Association between Chronic Kidney Disease and Coronary Artery Calcification: The Dallas Heart Study

The hypothesis that chronic kidney disease (CKD) is associated with increased coronary artery calcification (CAC) was tested using data from the Dallas Heart Study, a representative sample of Dallas County residents aged 30 to 65 yr. CKD was defined as presence of microalbuminuria and GFR > or =60 ml/min per 1.73 m(2) (stage 1 to 2), or GFR <60 ml/min per 1.73 m(2) (stage 3 to 5), excluding end-stage kidney disease. Logistic regression was used to examine the association between stages of CKD and CAC scores >10, >100, and >400 versus scores < or =10 compared with no CKD while adjusting for covariates. Analyses were repeated after stratifying by presence of diabetes. The mean age was 43.9 yr, and hypertension and diabetes were noted in 31.0 and 9.8%, respectively. No association was noted between stage 1 to 2 CKD and increased CAC scores. Compared with no CKD, stage 3 to 5 CKD was associated with CAC scores >100 (odds ratio, 2.85; 95% confidence interval, 0.92 to 8.80) and >400 (odds ratio, 8.35; 95% confidence interval, 1.94 to 35.95) in the total population after adjustment for covariates, but these associations were substantially reduced after exclusion of participants with diabetes. Participants with diabetes and stage 3 to 5 CKD had a ninefold increased odds of CAC scores >10 versus scores < or =10 compared with participants with diabetes and without CKD, whereas no association was noted between stage 3 to 5 CKD and CAC scores >10 in the nondiabetic population. In conclusion, stage 3 to 5 CKD is associated with increased CAC scores, but this association may be substantially stronger among adults with diabetes. These findings need to be confirmed in study populations that include adults >65 yr of age and a larger number of CKD cases.

Sympathetic overactivity as a cause of hypertension in chronic renal failure.

Objective To review the current literature on sympathetic mediation of hypertension in chronic renal failure. Background Hypertension is present in the vast majority of patients with chronic renal failure and constitutes a major risk factor for the excessive cardiovascular morbidity and mortality in this patient population. Although, traditionally, this hypertension is thought to be largely volume-dependent, an increasing body of literature suggests that there is an important sympathetic neural component. Microneurographic studies have demonstrated sympathetic overactivity without baroreflex impairment in both hypertensive chronic hemodialysis patients as well as in those with less advanced renal insufficiency. Sympathetic nerve activity was found to be normal in hemodialysis patients with bilateral nephrectomy, leading to the hypothesis that sympathetic overactivity in uremia is caused by a neurogenic signal (carried by renal afferents) arising in the failing kidney. This hypothesis is supported by rat studies showing that renal deafferentation abrogates hypertension in the 5/6 nephrectomy model of chronic renal insufficiency. In addition, in patients with chronic renal insufficiency and renin-dependent hypertension, sympathetic overactivity was normalized by chronic angiotensin converting enzyme inhibition but not by calcium channel blockade, implicating a major central neural action of angiotensin II. Conclusions Sympathetic overactivity in chronic renal failure is caused by neurohormonal mechanisms arising in the failing kidney. Future clinical studies are needed to determine whether normalization of sympathetic activity should constitute an important therapeutic goal in this high-risk patient population.