Relationship Between Increasing Body Weight, Insulin Resistance, Inflammation, Adipocytokine Leptin, and Coronary Circulatory Function

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1 Journal of the American College of Cardiology Vol. 47, No. 6, by the American College of Cardiology Foundation ISSN /06/$32.00 Published by Elsevier Inc. doi: /j.jacc Relationship Between Increasing Body Weight, Insulin Resistance, Inflammation, Adipocytokine Leptin, and Coronary Circulatory Function Thomas H. Schindler, MD,* Jerson Cardenas, MD,* John O. Prior, MD, PHD,* Alvaro D. Facta, MD,* Michael C. Kreissl, MD,* Xiao-Li Zhang, MD, PHD,* James Sayre, PHD,* Magnus Dahlbom, PHD,* Julio Licinio, MD, Heinrich R. Schelbert, MD, PHD* Los Angeles, California OBJECTIVES BACKGROUND METHODS RESULTS CONCLUSIONS We sought to evaluate effects of obesity, insulin resistance, and inflammation on coronary circulatory function and its relationship to leptin plasma levels. It is not known whether obesity, commonly paralleled by insulin resistance, inflammation, and leptin, is independently associated with coronary circulatory dysfunction. Myocardial blood flow (MBF) responses to cold pressor test (CPT) and pharmacologic vasodilation was measured with positron emission tomography and 13 N-ammonia. Study participants were divided into three groups based on their body mass index (BMI, kg/m 2 ): control, 20 BMI 25 (n 19); overweight, 25 BMI 30 (n 21); and obese, BMI 30 (n 32). Body mass index was significantly correlated to the Homeostasis Model Assessment Index of insulin resistance and C-reactive protein levels (r 0.60 and r 0.47, p ). Compared with control subjects, endothelium-related change in MBF ( MBF) to CPT progressively declined in overweight and obese groups ( vs and ml/g/min; p 0.03 and p ). The dipyridamole-induced total vasodilator capacity was significantly lower in obese than in control subjects ( vs ml/g/min, p 0.02). On multivariate analysis, BMI (p 0.012) and age (p 0.035) were significant independent predictors of MBF. Finally, only in the obese group leptin plasma levels significantly correlated with MBF (r 0.37, p 0.036). Increased body weight is independently associated with abnormal coronary circulatory function that progresses from an impairment in endothelium-related coronary vasomotion in overweight individuals to an impairment of the total vasodilator capacity in obese individuals. The findings that elevated leptin plasma levels in patients that are obese might exert beneficial effects on the coronary endothelium to counterbalance the adverse effects of increases in body weight on coronary circulatory function should be tested. (J Am Coll Cardiol 2006;47: ) 2006 by the American College of Cardiology Foundation The prevalence of obesity in the U.S. has increased between 1994 and 2000 from 22.9% to 30.5%. Because obesity is recognized as a risk factor of cardiovascular morbidity and mortality, the increasing incidence of obesity is of considerable public health concern (1). The mechanisms by which obesity initiates and accelerates vascular disease are still poorly understood. The functional integrity of the vascular endothelium exerts antiatherosclerotic and antithrombotic effects (2). Conversely, impaired endothelium-dependent coronary vasomotor function describes a proatherosclerotic state with substantial diagnostic and prognostic implications (3,4). Previous studies have reported an association between obesity and endothelial dysfunction of the peripheral circulation (5,6). A similar association between obesity and coronary endothelial dysfunction also has been observed in From the Departments of *Molecular and Medical Pharmacology and Psychiatry and Biobehavioral Science and Medicine/Endocrinology, David Geffen School of Medicine at UCLA, University of California at Los Angeles, Los Angeles, California. This work was supported by Research Grant HL 33177, National Heart, Lung and Blood Institute, Bethesda, Maryland; Grant DK058851, DK063240, National Institute of Diabetes, Digestive and Kidney Diseases; and Grant RR from the National Center of Research Resources. Manuscript received August 6, 2005; revised manuscript received September 30, 2005, accepted October 10, patients with angiographically normal or mildly diseased coronary arteries and with traditional coronary risk factors (7). Thus, it is tempting to speculate that dysfunctional endothelium of the coronary circulation may reflect a mechanistic link that confers an increased risk of adverse cardiovascular outcome in obesity. Metabolic abnormalities within the insulin resistance syndrome (insulin resistance, hyperinsulinemia, hypertension, hypertriglyceridemia, and decreased high-density lipoprotein [HDL] cholesterol) and chronic inflammation commonly parallel the excess of body weight and may directly affect the development and progression of coronary artery disease (CAD) (8,9). To date, the effect of increased body weight, insulin resistance, and of chronic inflammation on the coronary circulatory function has not been investigated. Adipocytokines such as leptin have been implicated as direct participants in the regulation of coronary vasomotor function in obesity (10). For example, the administration of leptin may stimulate increases in oxidative stress in in vitro cultured human endothelial cells (11). Increases in oxidative stress in the vascular endothelium may interact with nitric oxide to form peroxynitrite and, thereby, decrease the bioavailability of nitric oxide, which is associated with an

2 JACC Vol. 47, No. 6, 2006 March 21, 2006: Schindler et al. Body Weight and Microvascular Dysfunction 1189 Abbreviations and Acronyms ANOVA analysis of variance BMI body mass index CAD coronary artery disease CPT cold pressor test CRP C-reactive protein CVR coronary vascular resistance ECG electrocardiogram HDL high-density lipoprotein HOMA Homeostasis Model Assessment LDL low-density lipoprotein MBF myocardial blood flow PET positron emission tomography ROS reactive oxygen species RPP rate-pressure product WHR waist-hip ratio impairment of endothelium-dependent vasodilation (10). This theory is in contrast to findings in leptin-deficient mice, in whom the administration of leptin restored endotheliumdependent vasodilation by receptor-mediated endothelial release of nitric oxide (12). These findings suggest a direct relationship between leptin and the coronary vasomotor tone that, however, has not been examined in humans. The aims of the current study were therefore to evaluate in normal, overweight, and obese individuals without traditional coronary risk factors: 1) possible effects of increased body weight, insulin resistance, and chronic inflammation on the coronary circulatory function, and 2) a possible relationship between leptin plasma levels and coronary circulatory function. METHODS Study population and design. A total of 72 persons (35 men and 37 women; mean age years) without traditional coronary risk factors for CAD was studied prospectively. All participants underwent an initial screening visit that included a physical examination, electrocardiogram (ECG), blood pressure measurements, and routine blood chemistry. Study participants were only included if traditional coronary risk factors such as hypercholesterolemia (total serum cholesterol 240 mg/dl and/or lowdensity lipoprotein [LDL] cholesterol 155 mg/dl), arterial hypertension (blood pressure 140/90 mm Hg), smoking, and diabetes mellitus (fasting plasma glucose obtained on more than two occasions 126 mg/dl) were excluded. Furthermore, study applicants were recruited if they were without a history of variant angina, a family history of premature CAD, or clinically manifest cardiovascular or any other disease. All were normal on physical examination and had normal resting ECGs. No study participant was on any cardiac or vasoactive medication, such as angiotensinconverting enzyme inhibitors, calcium-channel blockers, betablockers, or statins. Finally, enrolled study participants were then grouped by the body mass index (BMI, kg/m 2 ): control group, 20 BMI 25 (n 19); overweight group, 25 BMI 30 (n 21); and obese group, BMI 30 (n 32) (7). In addition, the waist-hip ratio (WHR) was measured and defined as the minimal abdominal circumference between the xiphoid process and the iliac crest (waist) divided by the circumference determined over the femoral heads (hip). Laboratory tests included plasma glucose, hemoglobin A1c, insulin, free fatty acids, total cholesterol, HDL and LDL cholesterol, triglycerides levels, and high sensitive C-reactive protein (CRP). Assessment of insulin resistance using the Homeostasis Model Assessment (HOMA) was determined as described previously (13). In each patient, leptin plasma levels (Radioimmunoassay, Quest Diagnostic, Los Angeles, California) were determined from peripheral arm vein blood samples. All study participants had a low probability of significant obstructive CAD, as evidenced by a normal homogenous 13 N-ammonia tracer uptake on visual inspection and polar map analysis of the 13 N-ammonia positron emission tomography (PET) images at rest and during dipyridamole stimulated hyperemia (14). Study participants refrained from consuming caffeine-containing food or beverages for 24 h before the PET baseline examination to determine coronary circulatory function. The study was approved by the UCLA Institutional Review Board, and each study participant signed the approved informed consent form. Noninvasive quantification of myocardial blood flow (MBF) with PET. Myocardial blood flow was determined noninvasively with intravenous 13 N-ammonia, serial image acquisition by PET (ECAT EXACT HR, Siemens/CTI model 931/08-12, Siemens AG, Munich, Germany), and a two-compartment tracer kinetic model (15). The relative myocardial perfusion was assessed by visual inspection and polar map analysis of the 13 N-ammonia PET images. Time-activity curves derived from the first 12 dynamic frames were used to determine absolute estimates of mean MBF in ml/g/min (15). Measurements of MBF using PET were performed at baseline, during cold pressor test (CPT; reflecting predominantly endothelium-dependent vasomotion), and during dipyridamole-induced hyperemia with standard dose dipyridamole (140 g/kg/min; reflecting predominantly endothelium-independent vasomotion) as described previously (16,17). The change in the MBF from rest to CPT was defined as MBF. Heart rate, blood pressure, and a 12-lead ECG were recorded continuously during each MBF measurement. From the average of heart rate and blood pressure during the first 2 min of each image acquisition, the rate-pressure product (RPP) was determined as an index of cardiac work. An index of coronary vascular resistance (CVR) was determined as the ratio of mean arterial blood pressure to MBF (mm Hg/ml/min/g). Statistical analysis. Data are presented as mean SD for quantitative and absolute frequencies for qualitative variables. For comparison of differences, appropriate t tests for independent or paired samples were used (Statistical Anal-

3 1190 Schindler et al. JACC Vol. 47, No. 6, 2006 Body Weight and Microvascular Dysfunction March 21, 2006: ysis Software Institute, Cary, North Carolina). A comparison of CPT-induced change in MBF and dipyridamole MBFs between the different groups was performed by two-way analysis of variance (ANOVA), followed by Scheffe s multiple comparison test. Pearson s correlation coefficient (r), assuming a linear regression, was calculated to investigate the associations between CPT- and dipyridamole-induced changes in MBFs and laboratory parameters. Multivariate analysis was performed with the linear regression model. Statistical significance was assumed if the null hypothesis could be rejected at p RESULTS Patient characteristics and metabolic profile. The clinical characteristics of the three study groups are given in Table 1. The increase in BMI in the three study groups was paralleled by the WHR so that BMI and WHR significantly correlated (r 0.60, p ). Overweight and obese groups demonstrated significantly lower levels of HDL cholesterol compared with the control group, whereas total and LDL cholesterol levels were similar among groups. Fasting plasma glucose, triglycerides, and leptin plasma levels were significantly higher in the obese than in the control group but did not differ significantly between the control and overweight groups. Furthermore, insulin, the HOMA as index for insulin resistance, and high-sensitive CRP were significantly elevated in the overweight and obese groups compared with the control group. Although the differences in plasma insulin and HOMA index were significant between the overweight and obese groups, statistically there was no difference in high-sensitive CRP levels. Finally, no difference among all three groups with regard to free fatty acids and HbA1c levels was found. Correlations between metabolic parameters. In the entire study population, Pearson s regression analysis revealed significant correlations between BMI and insulin resistance (HOMA) (r 0.60, p ), BMI and leptin (r 0.52, p ), and leptin and insulin resistance (HOMA) (r 0.40, p ), indicating an interrelation between these metabolic parameters. High-density lipoprotein cholesterol also showed significant and inverse correlations with BMI (r 0.34, p 0.004) and with insulin resistance (HOMA, r 0.33, p 0.006) but none with leptin (r 0.02, p NS). Furthermore, the regression analysis demonstrated also significant correlations between BMI and highsensitive CRP plasma levels (r 0.47, p ), insulin resistance (HOMA) and high-sensitive CRP (r 0.34, p 0.009), HDL and high-sensitive CRP (r 0.26, p 0.04), and leptin and high-sensitive CRP (r 0.40, p ), relating metabolic and inflammatory cardiovascular disease mechanism together. Hemodynamic parameters. At baseline, heart rate and blood pressures were comparable among the study groups (Table 2). Sympathetic stimulation with CPT lead to significant increases in heart rate and systolic and diastolic blood pressure (p 0.05), that did not differ between the three groups. Consequently, RPPs were similar between the three groups either at baseline or during CPT (Table 2). Furthermore, the percent change of RPP to CPT in the control, overweight, and obese group was similar (% RPP: 39 20% vs % vs %, respectively). Pharmacologic vasodilation with dipyridamole produced significant increases in heart rate in the three study groups (p 0.05), whereas systolic and diastolic blood pressures remained essentially unchanged from baseline. No significant differences in heart rate or RPP existed between the three study groups during pharmacologic vasodilation. Myocardial blood flow. At baseline, MBF did not differ among the three groups (Table 2, Fig. 1). Cold pressor test-induced MBFs were significantly less in the obese Table 1. Characteristics of Study Population (n 72) Controls (n 19) Overweight (n 21) Obesity (n 32) BMI, kg/m * * Waist-hip ratio * * Age, yrs Gender, F/M 11/8 10/11 16/16 Lipid status Serum cholesterol levels, mg/dl Serum LDL level, mg/dl Serum HDL level, mg/dl * 41 8* Triglyceride level, mg/dl * Glucose level, mg/dl * Insulin, mcu/ml * * HOMA * * Free fatty acids, mmol/l CRP levels, mg/l * * HbA lc,% Leptin, ng/ml * Values are mean SD. *p 0.05 vs. control. BMI body mass index; CRP C-reactive protein; HbA lc hemoglobin A lc ; HDL high-density lipoprotein; HOMA Homeostasis Model Assessment; LDL low-density lipoprotein.

4 JACC Vol. 47, No. 6, 2006 March 21, 2006: Schindler et al. Body Weight and Microvascular Dysfunction 1191 Table 2. Hemodynamic Results and Myocardial Blood Flow (MBF) During Positron Emission Tomography Group Control Overweight Obesity Hemodynamics at rest Heart rate (beats/min) Blood pressure (mm Hg) Systolic Diastolic RPP 7,366 1,380 7, ,494 1,071 CPT Heart rate (beats/min) Blood pressure (mm Hg) Systolic Diastolic RPP 10,194 1,380 9,548 1,221 10,314 1,703 Pharmacologic vasodilation Heart rate (beats/min) Blood pressure (mm Hg) Systolic Diastolic RPP 11,015 1,966 10,354 1,771 11,487 1,934 MBF (ml/min/g myocardium) At rest During CPT * change to CPT * * Pharmacologic vasodilation * CVR (mm Hg/ml/min/g) At rest During CPT * change to CPT * 17 40* Pharmacologic vasodilation * Values are mean SD. *p 0.05 vs. control group. CPT cold pressor test; CVR coronary vascular resistance; MBF myocardial blood flow; RPP rate-pressure product. group than in the control group (Table 2, Fig. 1). Furthermore, MBFs during CPT in the obese group were significantly lower than in the overweight group (Table 2, Fig. 1). Although the MBF during CPT in the overweight group was lower than in the control group, statistically it did not reach significance (Table 2, Fig. 1). When looking at the MBF to CPT, it was significantly less in the overweight and obese groups than in the control group, respectively (Table 2, Fig. 2). Furthermore, MBF was significantly more reduced in the obese group than in the overweight group (Table 2, Fig. 2). The group comparison between the MBF in the control group was significant as compared with the overweight and obese groups (p by ANOVA). Dipyridamole MBFs and, thus, the total vaso- Figure 1. Myocardial blood flow (MBF) at rest, during cold pressor testing (CPT), and during pharmacologic vasodilation with dipyridamole for the three study groups (significant difference by t test). Figure 2. Change of myocardial blood flow ( MBF) during cold pressor testing (CPT) for the three study groups (significant difference by t test).

5 1192 Schindler et al. JACC Vol. 47, No. 6, 2006 Body Weight and Microvascular Dysfunction March 21, 2006: Figure 3. Correlation between leptin plasma levels and change of myocardial blood flow ( MBF) during cold pressor testing (CPT) in the entire study group. dilator capacity, were statistically not significantly lower in the overweight group compared with the control group but were significantly less in the obese group (Table 2, Fig. 1). Dipyridamole MBFs did not differ significantly between the overweight and obese group (Table 2, Fig. 1). The group comparison of dipyridamole MBFs was not significant (p NS by ANOVA). To account for interindividual variations in the coronary driving pressure, an index of CVR was calculated as ratio of mean arterial blood pressure to MBF. As depicted in Table 2, the CVR at rest, during CPT, and pharmacologic vasodilation reflected those of MBF for each study group. Changes in MBF related to leptin plasma levels. Pearson regression analysis did not show a significant correlation between leptin plasma levels and MBF to CPT in the entire study population (Fig. 3). Also, leptin plasma levels did not correlate with dipyridamole MBFs (r 0.07, p NS). In the overweight group, the regression analysis between leptin plasma levels and MBF did not correlate (Fig. 4), whereas in the obese group a significant association Figure 5. Correlation between leptin plasma levels and change of myocardial blood flow ( MBF) during cold pressor testing (CPT) in the obese group. was found (Fig. 5). Thus, in the obese group higher leptin plasma levels were associated with relatively higher MBFs. In regard to leptin plasma levels and dipyridamole MBFs, the regression analysis did not show an association for either the overweight group (r 0.10, p NS) or the obese group (r 0.15, p NS). Determinants of MBF. In the entire study group, on univariate analysis only BMI, HDL cholesterol, HOMA, and CRP levels were significantly associated with MBF (Table 3). As demonstrated in Table 3, by multivariate analysis only BMI and age remained statistically independently predictors of MBF. The univariate analysis in the overweight group (Table 4) demonstrated age to be significantly associated with MBF. By multivariate analysis none of the parameters was an independent predictor of the MBF (p NS). Finally, on univariate analysis in the obese group (Table 5), only leptin plasma levels were significantly associated with MBF. By multivariate analysis leptin plasma levels and HDL were independently Table 3. Entire Study Group MBF to CPT Multivariate Analysis Univariate p Value Standardized Coefficient p Value Leptin, ng/ml BMI, kg/m * * Total cholesterol, mg/dl LDL cholesterol, mg/dl HDL cholesterol, mg/dl 0.009* Triglyceride, mg/dl Glucose, mg/dl HOMA 0.02* CRP, mg/l 0.048* Age, yrs * Gender Figure 4. Correlation between leptin plasma levels and change of myocardial blood flow ( MBF) during cold pressor testing (CPT) in the overweight group. *Significant difference by analysis of variance. BMI body mass index; CPT cold pressor test; CRP C-reactive protein; HDL high-density lipoprotein; HOMA Homeostasis Model Assessment; LDL low-density lipoprotein; MBF myocardial blood flow.

6 JACC Vol. 47, No. 6, 2006 March 21, 2006: Schindler et al. Body Weight and Microvascular Dysfunction 1193 Table 4. Overweight Group associated with the MBF. Further, regarding the attenuated dipyridamole MBFs in the obese group, on univariate analysis only age showed a significant association with the MBFs during dipyridamole (p 0.029), whereas on multivariate analysis none of the parameters were independently predictive for the dipyridamole MBFs. DISCUSSION MBF to CPT Multivariate Analysis Univariate p Value Standardized Coefficient p Value Leptin, ng/ml BMI, kg/m Total cholesterol, mg/dl LDL cholesterol, mg/dl HDL cholesterol, mg/dl Triglyceride, mg/dl Glucose, mg/dl HOMA CRP, mg/l Age, yrs 0.002* Gender *Significant difference by analysis of variance. Abbreviations as in Table 3. The results of the present study provide several new findings. First, increased body weight, paralleled by an increase in plasma markers of the insulin-resistance syndrome and chronic inflammation, is independently associated with abnormal coronary circulatory function that progresses from an impairment in endothelium-dependent coronary vasomotion in overweight individuals to an impairment of the total vasodilator capacity in obese individuals. This progressive worsening of the coronary circulatory function might provide a mechanistic link between abnormal coronary vasomotion and adverse cardiovascular outcome. Second, in the obese individuals, increased leptin plasma levels were significantly associated with relatively higher endotheliummediated MBF increases to CPT, which might reflect a beneficial effect of leptin and/or leptin-related but stillundetermined factors on the coronary vascular endothelium to counteract the adverse effects of increases in body weight on coronary vasomotor function. BMI and coronary vascular function. Steinberg et al. (5), in their observations, were first to demonstrate that obesity is associated with peripheral endothelial dysfunction, which may be related to insulin resistance. Recent findings support the Steinberg et al. (5) observations by adding (16) that endothelium-mediated coronary vasodilation is progressively impaired with increasing severity of insulin resistance and carbohydrate intolerance. Al Suwaidi et al. (7) studied the coronary vascular reactivity in a large group of 397 consecutive patients with angiographically normal or mildly diseased coronary arteries. These patients were mostly treated for coronary risk factors such as hypertension, hypercholesterolemia, and diabetes; 48% of them were also smokers. Notably, Al Suwaidi et al. (7) identified obesity as an independent predictor of coronary endothelial dysfunction, strongly suggesting obesity as a possible alternate mediator of coronary vascular disease rather than as an epiphenomenon related to other traditional coronary risk factors commonly associated with obesity. This observation appears to be consistent with recent ones in obese individuals and components of the metabolic syndrome (18), where the waist circumference (as an anthropomorphic index for body weight) and systolic blood pressure were independently related to peripheral endothelial dysfunction. However, adding HOMA index of insulin resistance removed the independent contribution of waist circumference in the modulation of endothelial dysfunction. Therefore, it is equally possible that the effects of obesity on endothelial dysfunction are primarily related to insulin resistance. In particular, this premise may hold true because obesity is a major cause of insulin resistance and chronic inflammation (8,9). Because previous assessment of coronary vasoreactivity (7) did not evaluate the effect of insulin resistance and chronic inflammation on coronary endothelial function, it remains uncertain whether obesity does indeed independently contribute to the determination of coronary endothelial dysfunction. The results of the current study in a relatively young study population provide first evidence that, indeed, increases in body weight exert a direct effect on coronary endothelial dysfunction independent of overweight- and obesity-related insulin resistance and chronic inflammation. Interestingly and also contrary to a previous investigation (7), impaired coronary vasodilator function was not confined to endothelium-mediated mechanism but extended to an impairment in predominantly endothelium-independent MBF increases to pharmacologic vasodilation in obesity. We also found that endotheliummediated MBF increases to CPT were not only impaired in obesity but also in overweight individuals. This observation may indicate that, even in otherwise-healthy individuals, Table 5. Obese Group MBF to CPT Multivariate Analysis Univariate p Value Standardized Coefficient p Value Leptin, ng/ml 0.036* * BMI, kg/m Total cholesterol, mg/dl LDL cholesterol, mg/dl HDL cholesterol, mg/dl * Triglyceride, mg/dl Glucose, mg/dl HOMA CRP, mg/l Age, yrs Gender *Significant difference by analysis of variance. Abbreviations as in Table 3.

7 1194 Schindler et al. JACC Vol. 47, No. 6, 2006 Body Weight and Microvascular Dysfunction March 21, 2006: being overweight may predispose an individual to an increased risk for future cardiovascular events (3,4). In the current study population, components of the metabolic syndrome such as BMI, HDL cholesterol, insulin resistance, and CRP plasma level were predictive of the alterations in MBF to sympathetic stimulation. By multivariate analysis, however, only BMI and age remained independent predictors of the CPT-induced changes in MBF. Because on regression analysis BMI revealed significant associations with components of the metabolic syndrome (low HDL cholesterol, insulin resistance, and CRP plasma levels), the independent predictive value of BMI for CPT-induced MBF alterations also suggests that a complex interplay of low HDL cholesterol, insulin resistance, and systemic inflammation affects endothelium-dependent coronary vasomotor function with increases in body weight (8,19). Thus, these results add further evidence for increased body weight as an important integrating determinant of coronary vascular homeostasis (8). Interestingly, the age of study participants also proved to independently predict changes in MBF to cold exposure, which may indicate the endothelium-related change in MBF is not only be related to the age of the study participants and the numbers of cardiovascular risk factors (2) but also to the duration of its exposure (20,21). The reason for the mechanistic link between increases in body weight and the progressive impairment of coronary circulatory function is not fully understood but is likely multifactorial in etiology, possibly being related to alterations in glucose tolerance, lipid profile, triglycerides, and microinflammation (9). Apart from insulin resistance, elevated plasma free fatty acid levels combined with increased plasma triglycerides and small, dense LDL bodies may alter intracellular signaling of the nitric oxide synthase activity (2,9). In addition, increases in reactive oxygen species (ROS) through activation of nicotinamide-adenine-dinucleotide phosphate oxidase may diminish the bioavailability of endothelium-derived nitric oxide (22,23). Beneficial effects of acute antioxidant vitamin C challenges on endothelial dysfunction of the brachial artery (24) indeed suggests the involvement of ROS underlying endothelial dysfunction in obesity. The contribution of ROS in mediating abnormal coronary vasomotion in obesity, however, remains to be determined. Adipocytokine leptin and coronary vasomotion. Leptin is primarily synthesized in adipocytes and its concentration has been shown to increase as a direct function of increasing percentage body fat, even though for the same amount of body fat there can be up to threefold differences in leptin levels (10,25). Notably, leptin specifically targets receptors on the vascular endothelium and smooth muscle cells (10) that in in vitro studies have been shown to lead to leptinstimulated prothatherosclerotic effects, such as increases in ROS in cultured human endothelial cells (11), acceleration of vascular cell calcification, smooth muscle cell proliferation, and migration (10). Furthermore, in a mice model elevated leptin plasma levels appear to directly mediate arterial thrombotic disease in vivo (26). On the other hand, leptin has been demonstrated to evoke both endotheliumdependent and -independent vasorelaxation (27,28). Intracoronary infusion of leptin in humans with angiographically normal coronary arteries also may directly lead to a coronary vasodilation of the conduit and arteriolar vessels (29). In addition, there is, in conclusion, a large body of in vivo evidence indicating that leptin may stimulate increases in the release of endothelium-derived nitric oxide in a rat model (12,28,30), suggesting important antiatherosclerotic and antithrombotic effects (2). In the current study, we found a significant association (r 0.37, p 0.036) between increases in leptin plasma levels and CPT-induced changes in MBF in the obese, whereas there was no such association in overweight and in the study populations as a whole. Thus, in obese individuals, increased leptin plasma levels were associated with relatively higher increases in MBF to CPT, which could possibly reflect a beneficial effect of leptin and/or leptin-related but still undetermined factors on the coronary endothelium. The latter observation is also consistent with the observed beneficial effects of leptin administration on endothelium-dependent vasomotion in obese leptin knockout mice (12). However, to provide more definite answers regarding the role of leptin in the regulation of coronary vasomotor control in obesity further studies are needed. Conclusions. This study demonstrates for the first time that increased body weight, paralleled by an increase in plasma markers of the insulin-resistance syndrome and chronic inflammation, is independently associated with abnormal coronary circulatory function that progresses from an impairment in endothelium-related coronary vasomotion in overweight individuals to an impairment of the total vasodilator capacity in obese individuals. Moreover, the finding that elevated plasma leptin levels in obesity might exert beneficial effects on the coronary endothelium to counterbalance the adverse effects of increases in body weight on coronary circulatory function warrants further investigations. Reprint requests and correspondence: Dr. Heinrich H. Schelbert, Department of Molecular and Medical Pharmacology, David Geffen School of Medicine at UCLA, Le Conte Avenue, CHS, Box , Los Angeles, California hschelbert@mednet.ucla.edu. REFERENCES 1. Eckel RH, Daniels SR, Jacobs AK, Robertson RM. America s children: a critical time for prevention. Circulation 2005;111: Widlansky ME, Gokce N, Keaney JF Jr., Vita JA. The clinical implications of endothelial dysfunction. J Am Coll Cardiol 2003;42: Lerman A, Zeiher AM. Endothelial function: cardiac events. Circulation 2005;111: Schindler TH, Nitzsche EU, Schelbert HR, et al. 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