How to assess sympathetic activity in humans

The role of the nervous system in hypertension

The central nervous system plays an important role in the minute-to-minute regulation of arterial pressure, but its contribution to chronic regulation of arterial pressure is less clear. A nervous system role in essential hypertension in humans has been postulated for decades, but conclusive data on the relationship has been lacking. However, several lines of evidence in animal models and in humans suggest that the sympathetic nervous system is a primary contributor to the development and maintenance of some forms of essential hypertension. The primary final common pathway for the nervous system's contribution to hypertension is the sympathetic nervous system. Sympathetic nervous system overactivity may result from either inappropriately elevated sympathetic drive from brain centers, an increase in synaptically released neurotransmitters in the periphery, or amplification of the neurotransmitter signal at the target tissue. This review examines recent evidence for the central and peripheral nervous systems' roles in hypertension, and considers recent findings in this area that suggest that sex steroids and circadian rhythms are important considerations in the nervous system's regulation of arterial pressure.

Exercise and autonomic function in health and cardiovascular disease

Autonomic nervous system activity contributes to the regulation of cardiac output during rest, exercise, and cardiovascular disease. Measurement of HRV has been particularly useful in assessing parasympathetic activity, while its utility for assessing sympathetic function and overall sympathovagal balance remains controversial. Studies have revealed that parasympathetic tone dominates the resting state, while exercise is associated with prompt withdrawal of vagal tone and subsequent sympathetic activation. Conversely, recovery is characterized by parasympathetic activation followed by sympathetic withdrawal, although clarification of the normal trajectory and autonomic basis of heart rate decay following exercise is needed. Abnormalities in autonomic physiology--especially increased sympathetic activity, attenuated vagal tone, and delayed heart rate recovery--have been associated with increased mortality. Exercise training is associated with a relative enhancement of vagal tone, improved heart rate recovery after exercise, and reduced morbidity in patients with cardiovascular disease. However, whether exercise training leads to reduced mortality in this population because of its ability to specifically modulate autonomic function is unknown at the present time. Although the results of a recent randomized study in patients with CHF and a meta-analysis in the setting of a recent myocardial infarction determined that exercise training leads to improved outcomes in these populations, neither study measured autonomic function. Improved autonomic function due to exercise training is a promising rationale for explaining improvements in outcome, although more research is needed to confirm this hypothesis.

Central sympathoinhibitory effects of calcium channel blockers

It is generally assumed that the arterial vasodilation induced by inhibition of Ca(2+) influx into vascular smooth muscle cells represents the main mechanism for the hypotensive effect of dihydropyridine calcium channel blockers. Increases in sympathetic tone have been related to activation of the arterial baroreflex by rapid lowering of blood pressure. This review highlights new findings in two areas. First, in animal studies, direct central administration of dihydropyridines such as nifedipine or amlodipine lowers sympathetic nerve activity and thereby blood pressure. Peripheral administration of nifedipine or amlodipine at low rates appears to result in gradual accumulation of drug in the central nervous system, and also causes lowering of sympathetic nerve activity and thereby lowering of blood pressure (rather than by arterial vasodilation). Second, in hypertensive humans treated with long-acting dihydropyridines and presumably little activation of the arterial baroreflex, some studies have demonstrated lowering of sympathetic activity (as assessed by plasma norepinephrine), but others reported increases (as assessed by plasma norepinephrine or microneurography). This sympathoexcitatory response may be due to activation of the renin-angiotensin system, particularly at higher doses.

Assessment of the left ventricular diastolic reserve in essential hypertension: the acute saline load test

OBJECTIVE

The aim of this study was to evaluate the significance of the development of a restrictive response to an acute saline load, defined as an increase in the ratio of peak early to peak late diastolic transmitral flow velocity (E/A ratio) associated with a decrease in the deceleration time, in patients with mild to moderate untreated hypertension.

BACKGROUND

Recognised abnormal patterns of transmitral diastolic flow include, from 'best' to 'worst': prolonged relaxation, pseudonormalisation, and restrictive physiology. The common denominator of these transitions is the constellation of an increase in the E/A ratio associated with a decrease in deceleration time.

PATIENTS AND METHODS

Sixteen normal control subjects (6 males, 10 females, age 51.6 +/- 6.9 years) and 24 patients with mild to moderate untreated hypertension (12 males, 12 females, age 46.8 +/- 7.5 years) underwent supine blood pressure measurement with sphygmomanometry, biochemical studies, and transthoracic M-mode, 2D, and Doppler echocardiography before and after an acute saline load (7 mL kg(-1), maximum 500 mL, NaCl 0.9% within 15 min IV).

RESULTS

The baseline E/A ratio was lower (0.90 +/- 0.14 vs. 1.04 +/- 0.18; P < 0.01) and the deceleration time was longer (158.8 +/- 19.4 vs. 135 +/- 8.9 ms; P < 0.01) in patients with hypertension compared with normotensive controls. However, no patient with hypertension exhibited a transmitral flow velocity pattern compatible with typical prolonged relaxation. A restrictive response to the acute saline load was observed in 12 (50%) of the hypertensive and none of the control subjects. Hypertensive patients with a restrictive response to the acute saline load had a lower baseline E velocity (54.8 +/- 8.7 cm s(-1) vs. 66 +/- 6.4 cm s(-1); P = 0.003), a lower baseline E/A ratio (0.83 +/- 0.13 vs. 0.97 +/- 0.12; P = 0.015), and a longer deceleration time (167.5 +/- 15.4 ms vs. 150 +/- 19.5; P = 0.03) than hypertensive patients without such a response.

CONCLUSION

A restrictive response to an acute saline load is indicative of a limited diastolic reserve in patients with mild to moderate untreated hypertension. Further studies are required in order to evaluate the significance of such a response with regards to risk stratification and efficacy of medical treatment in this patient population.

Assessment of cardiac sympathetic nerve integrity with positron emission tomography

The autonomic nervous system plays a critical role in the regulation of cardiac function. Abnormalities of cardiac innervation have been implicated in the pathophysiology of many heart diseases, including sudden cardiac death and congestive heart failure. In an effort to provide clinicians with the ability to regionally map cardiac innervation, several radiotracers for imaging cardiac sympathetic neurons have been developed. This paper reviews the development of neuronal imaging agents and discusses their emerging role in the noninvasive assessment of cardiac sympathetic innervation.

Effect of essential hypertension on cardiac autonomic function in type 2 diabetic patients

OBJECTIVES

The aim of this study was to examine the effects of essential hypertension on cardiac autonomic function in type 2 diabetic patients.

BACKGROUND

Hypertension is common in type 2 diabetic patients and is associated with a high mortality. However, the combined effects of type 2 diabetes and essential hypertension on cardiac autonomic function have not been fully elucidated.

METHODS

Thirty-three patients with type 2 diabetes were assigned to a hypertensive diabetic group (n = 15; age: 56 +/- 8 years, mean +/- SD) or an age-matched normotensive diabetic group (n = 18, 56 +/- 6 years). Cardiac autonomic function was assessed by baroreflex sensitivity (BRS), heart rate variability (HRV), plasma norepinephrine concentration and cardiac 123I-metaiodobenzylguanidine (MIBG) scintigraphic findings.

RESULTS

Baroreflex sensitivity was lower in the hypertensive diabetic group than it was in the normotensive diabetic group (p < 0.05). The early and delayed myocardial uptake of 123I-MIBG was lower (p < 0.01 and p < 0.05, respectively), and the percent washout rate of 123I-MIBG was higher (p < 0.05) in the hypertensive diabetic group. However, the high frequency (HF) power and the ratio of low frequency (LF) power to HF power (LF/HF) of HRV and plasma norepinephrine concentration were not significantly different. The homeostasis model assessment index was higher in the hypertensive diabetic group than it was in the normotensive diabetic group (p < 0.01).

CONCLUSIONS

Our results indicate that essential hypertension acts synergistically with type 2 diabetes to depress cardiac reflex vagal and sympathetic function, and the results also suggest that insulin resistance may play a pathogenic role in these processes.

Leptin interacts with heart rate but not sympathetic nerve traffic in healthy male subjects

OBJECTIVE

Administration of leptin to animals increases sympathetic nerve activity and heart rate. We therefore tested the hypothesis that plasma leptin is linked independently to muscle sympathetic nerve activity (MSNA) and heart rate in healthy humans.

METHODS

We measured plasma leptin, plasma insulin, body mass index (BMI), percent body fat, waist: hip ratio, MSNA, heart rate and blood pressure in 88 healthy individuals (50 men and 38 women).

RESULTS

In men, plasma leptin concentration correlated significantly with BMI (r = 0.75, P < 0.001), percent body fat (r = 0.70, P< 0.001), waist: hip ratio (r = 0.69, P < 0.001), insulin (r = 0.37, P = 0.009), and age (r = 0.38, P = 0.006). Only BMI and waist: hip ratio were linked independently to plasma leptin concentration (r = 0.78, P < 0.001). Plasma leptin concentrations also correlated with heart rate (r = 0.39, P = 0.006) and mean arterial pressure (MAP; r = 0.38, P = 0.007), but not with MSNA (r = 0.17, P = 0.24). After adjustment for BMI and waist: hip ratio, plasma leptin concentration correlated significantly only with heart rate (r = 0.29, P = 0.04), and not with MAP (r = 0.21, P = 0.14). Individuals were divided into high-leptin and low-leptin subgroups on the basis of plasma leptin concentrations adjusted for BMI and waist: hip ratio. Those with high leptin concentrations had significantly faster heart rates than those with low leptin. MAP and MSNA were similar in both subgroups. No relationship between leptin and either heart rate or MSNA was evident in women.

CONCLUSIONS

In normal men, heart rate, but not MSNA, is linked to plasma leptin concentration. This sex-specific relationship between heart rate and plasma leptin is independent of plasma insulin, BMI, waist:hip ratio and percentage body fat.

Sympathetic nervous system activation in essential hypertension, cardiac failure and psychosomatic heart disease

Regional sympathetic activity can be studied in humans using electrophysiological methods measuring sympathetic nerve firing rates and neurochemical techniques providing quantification of noradrenaline spillover to plasma from sympathetic nerves in individual organs. Essential hypertension: Such measurements in patients with essential hypertension disclose activation of the sympathetic outflows to skeletal muscle blood vessels, the heart and kidneys, particularly in younger patients. This sympathetic activation, in addition to underpinning the blood pressure elevation, most likely also contributes to left ventricular hypertrophy, and to the commonly associated metabolic abnormalities of insulin resistance and hyperlipidaemia. Antihypertensive drugs, such as moxonidine, which act primarily by inhibiting the sympathetic nervous system, should have additional clinical benefits beyond those attributable to blood pressure reduction, in protecting against hypertensive complications. Obesity-related hypertension: Understanding the neural pathophysiology of hypertension in the obese has been difficult. In normotensive obesity, renal sympathetic tone is doubled, but cardiac noradrenaline spillover (a measure of sympathetic activity in the heart) is only 50% of normal. In obesity-related hypertension, there is a comparable elevation of renal noradrenaline spillover, but without suppression of cardiac sympathetics (cardiac sympathetic activity being more than double that of normotensive obese and 25% higher than in healthy volunteers). Increased renal sympathetic activity in obesity may be a 'necessary' cause for the development of hypertension (and predisposes to hypertension development), but apparently is not a 'sufficient' cause. The discriminating feature of the obese who develop hypertension is the absence of the adaptive suppression of cardiac sympathetic tone seen in the normotensive obese. Heart failure: In cardiac failure, the sympathetic nerves of the heart are preferentially stimulated. Noradrenaline release from the failing heart at rest in untreated patients is increased as much as 50-fold, similar to the level seen in the healthy heart during near-maximal exercise. Activation of the cardiac sympathetic outflow provides adrenergic support to the failing myocardium, but at a cost of arrhythmia development and progressive myocardial deterioration. Psychosomatic heart disease: No more than 50% of clinical coronary heart disease is explicable in terms of classical cardiac risk factors. There is gathering evidence that psychological abnormalities, particularly depressive illness, anxiety states, including panic disorder and mental stress, are involved here, 'triggering' clinical cardiovascular events, and possibly also contributing to atherosclerosis development. The mechanisms of increased cardiac risk attributable to mental stress and psychiatric illness are not entirely clear, but activation of the sympathetic nervous system seems to be of prime importance.

The peripheral sympathetic nervous system in human obesity

The peripheral sympathetic nervous system is a key factor in the regulation of energy balance in humans. Differences in sympathetic nervous system activity may contribute to variations in 24 h energy expenditure between individuals. beta-Adrenoceptors play a more important role than alpha-adrenoceptors in this regulation. The involvement of both beta 1- and beta 2-adrenoceptor subtypes has been demonstrated, the role of the beta 3-adrenoceptor subtype is not yet clear. Normal or increased levels of sympathetic nervous system activity and reduced reactivity appear to be present in established obesity. Furthermore, the sensitivity for beta-adrenoceptor stimulation is impaired in obesity. The blunted reactivity and sensitivity may contribute to the maintenance of the obese state. There are data to suggest that they may also play a role in the aetiology of obesity, because the impairments often remain after weight reduction. Furthermore, a negative correlation between baseline sympathetic nervous system activity and weight gain during follow-up has been found in Pima Indians. Recently, genetic evidence about the involvement of adrenoceptors in obesity has become available. Although the results of association and linkage studies on polymorphisms in the beta 2-, beta 3- and alpha 2-adrenoceptor genes are inconsistent, the functional correlates of some of these polymorphisms (changes in agonist-promoted down-regulation, protein expression levels, lipolytic sensitivity, basal metabolic rate, sympathetic nervous system activity) suggest that they may be important in the aetiology of obesity.

Antihypertensive drugs and sympathetic nervous system

Several studies have demonstrated that essential hypertension is accompanied by sympathetic activation, which contributes to blood pressure elevation. Sympathetic activation also has adverse consequences in hypertensive patients beyond initiating blood pressure elevation. There is evidence that neural vasoconstriction has metabolic effects in skeletal muscle, impairing glucose delivery to muscles. In the liver, retarding of post prandial clearance of lipids contributes to hyperlipidemia. Cardiac sympathetic activation is a probable cause of sudden death in hearth failure. A trophic effect of sympathetic activation on cardiovascular growth is also likely, contributing to the development of left ventricular hypertrophy. Consequently, one of the major aims of antihypertensive therapy should be to attenuate sympathetic tone. It is possible that, among the antihypertensive drugs available, those inhibiting the sympathetic nervous system might best reduce cardiovascular risk.

Peripheral adrenergic system and hypertension

Hypertension is a condition where adrenergic responsiveness, sympathetic activity and adrenoceptors are somewhat altered. Many techniques are available to assess human sympathetic nervous system activity. They each present limitations and disadvantages. Characterization and subdivision of the alpha and beta-adrenoceptors, according to their localization and answer to different agonists, was facilitated in recent years by the extensive use of pharmacological and molecular biology techniques. Some adrenoceptor studies were conducted on animal models, human tissues and peripheral blood cells to assess their changes in various forms and stages of hypertension. Our group has pointed out that alpha1-adrenergic receptors expressed by human peripheral blood lymphocytes underwent changes of density in essential hypertensives, compared to normotensive control subjects. The importance of these findings could provide an assessment of alpha1-peripheral receptors with possible future clinical implications in the pathophysiology and treatment of hypertension.

Interpreting oscillations of muscle sympathetic nerve activity and heart rate variability

Computer analysis of spontaneous cardiovascular beat-by-beat variability has gained wide credibility as a means of inferring disturbances of autonomic cardiovascular regulation in a variety of cardiovascular conditions, including hypertension, myocardial infarction and heart failure. Recent applications of spectral analysis to muscle sympathetic nerve activity (MSNA) offer a new approach to a better understanding of the relationship between cardiovascular oscillations and autonomic regulation. However, areas of uncertainty and unresolved debates remain, mostly concerning different methodologies and interpretative models that we will consider in this article. Perusal of all available literature suggests that average sympathetic nerve activity and its oscillatory components, although correlated to some extent, are likely to provide different types of information. In addition, the specific experimental context is of paramount importance, as the rules that seem to govern the relationship between average and oscillatory properties of MSNA appear to be different in usual conditions and in conditions of extremes of activation or disease. In general, dynamic experiments, such as with graded tilt or with vasoactive drugs, are more suited to investigations of the complexity of autonomic regulation than are static comparisons. In addition, because the information is spread across variables and is affected by a potentially large error, it appears that several different techniques should be perceived as complementary rather than as mutually exclusive. Available evidence suggests that low-frequency and high-frequency oscillations in peripheral signals of variability might have a predominantly central, rather than a peripheral, origin and that this applies in particular to low-frequency oscillations. A crucial point in the assessment of the meaning of spectral components relates to consideration of the varying level of very-low-frequency noise, and the mathematical manipulation of derived indices, particularly using a normalization procedure. This appears easier to obtain with auto-regressive than with fast Fourier techniques. With this approach, discrepant interpretations seem to be resolved, provided adequate care is taken in separating direct physiological data from derived meaning, which relates to hidden information and neural codes; in the case of sympathetic discharge, the latter display greater complexity than simple average spike activity per unit time. Accordingly we believe, in conclusion, that the judicious use of spectral methodology, in addition to other techniques, might provide unprecedented, useful insights into autonomic cardiovascular regulation, in both physiopathological and clinical circumstances.

Phenotypic evidence of faulty neuronal norepinephrine reuptake in essential hypertension

Previous reports suggest that neuronal norepinephrine (NE) reuptake may be impaired in essential hypertension, perhaps because of dysfunction of the NE transporter, although the evidence is inconclusive. To further test this proposition, we applied phenotypically relevant radiotracer methodology, infusion of tritiated NE and quantification of NE metabolites, to 34 healthy lean subjects (body mass index <27.0 kg/m(2)), 19 overweight (body mass index >28.0 kg/m(2)) but otherwise healthy normotensive subjects, 13 untreated lean patients with essential hypertension, and 14 obesity-related hypertensives. Spillover of NE from the heart was increased in lean hypertensives only (mean+/-SD 33.4+/-20.6 versus 16.1+/-11.7 ng/min in lean normotensives, P<0.05), but this could have resulted from high cardiac sympathetic nerve firing rates, faulty NE reuptake, or both. The arterial plasma concentration of 3-methoxy-4-hydroxylphenylglycol, an extraneuronal metabolite of NE, was elevated in lean hypertensives only (3942+/-1068 versus 3055+/-888 pg/mL in healthy subjects, P:<0.05). The fractional extraction of plasma tritiated NE in passage through the heart, determined on the basis of neuronal NE uptake, was reduced in lean essential hypertensives (0.65+/-0.19 versus 0.81+/-0.11 in healthy subjects, P<0.05). Cardiac release of the tritiated NE metabolite [(3)H]dihydroxylphenylglycol, produced intraneuronally by monoamine oxidase after uptake of [(3)H]NE by the transporter, was reduced in lean hypertensives only (992+/-1435 versus 4588+/-3189 dpm/min in healthy subjects, P<0.01) These findings suggest that neuronal reuptake of NE is impaired in essential hypertension. Through amplification of the neural signal, such a defect could constitute a neurogenic variant of essential hypertension. In obesity-related hypertension, there was no phenotypic evidence of NE transporter dysfunction.

Ethnic differences in insulinemia and sympathetic tone as links between obesity and blood pressure

Hyperinsulinemia and increased sympathetic nervous system (SNS) activity are thought to be pathophysiological links between obesity and hypertension. In the present study, we examined the relation among heart rate (HR), blood pressure (BP), and percent body fat (hydrodensitometry or DEXA), fasting plasma insulin concentration, and muscle sympathetic nerve activity (MSNA, microneurography) in male, normotensive whites (n=42) and Pima Indians (n=77). Pima Indians have a high prevalence of obesity and hyperinsulinemia but a relatively low prevalence of hypertension. Compared with whites, Pima Indian men had a higher percent body fat (28% versus 21%) and higher fasting insulin concentrations (210 versus 132 pmol/L) but lower MSNA (27 versus 33 bursts/min) (all P<0.001). In both ethnic groups, HR and BP were positively related to percent body fat and MSNA, and both were significant independent determinants of HR and BP in multiple regression analyses. However, MSNA was positively related to percent body fat and the fasting insulin concentration in whites (r=0.60 and r=0.47, both P<0.01) but not in Pima Indians (r=0.15 and r=0.03, NS) (P<0.01 for ethnic differences in the slope of the regression lines). These results confirm the physiological importance of the SNS in normal BP regulation but indicate that the roles of hyperinsulinemia and increased SNS activity as mediators for the relation between obesity and hypertension can differ between different ethnic groups. The lack of an increase in SNS activity with increasing adiposity and insulinemia in Pima Indians may contribute to the low prevalence of hypertension in this population.

The 'adrenaline hypothesis' of hypertension revisited: evidence for adrenaline release from the heart of patients with essential hypertension

OBJECTIVE

Whether adrenaline acts as a sympathetic nervous cotransmitter in humans and stimulates beta2-adrenoceptors to augment neuronal noradrenaline release remains a subject of considerable dispute. The aim of this study was to test if adrenaline is released from regional sympathetic nerves (in the heart) in patients with essential hypertension, and to investigate whether locally released adrenaline might enhance cardiac noradrenaline release.

METHODS

Using dual isotope dilution methodology, adrenaline and noradrenaline plasma kinetics was measured for the whole body and in the heart in 13 untreated patients with essential hypertension and 27 healthy volunteers. All research participants underwent cardiac catheterization under resting conditions.

RESULTS

At rest, there was negligible adrenaline release from the sympathetic nerves of the heart in healthy subjects, 0.27 +/- 1.62 ng/min. In contrast, in patients with essential hypertension, adrenaline was released from the heart at a rate of 1.46 +/- 1.73 ng/min, equivalent on a molar basis to approximately 5% of the associated cardiac noradrenaline spillover value. Cardiac noradrenaline spillover was higher in hypertensive patients, 24.9 +/- 17.0 ng/min compared to 15.4 +/- 11.7 ng/min in healthy volunteers (P< 0.05). Among patients, rates of cardiac adrenaline and noradrenaline spillover correlated directly (r= 0.59, P< 0.05).

CONCLUSIONS

This study, in demonstrating release of adrenaline from the heart in patients with essential hypertension, and in disclosing a proportionality between rates of cardiac adrenaline and noradrenaline release, provides perhaps the most direct evidence to date in support of the 'adrenaline hypothesis' of essential hypertension.

Reducing cardiac filling pressure lowers norepinephrine spillover in patients with chronic heart failure

BACKGROUND

We studied the cardiac sympathetic response to selective unloading of cardiopulmonary baroreceptors in subjects with normal left ventricular (LV) function and congestive heart failure (CHF).

METHODS AND RESULTS

Eight patients with normal LV function (age 57+/-5 years, ejection fraction 58+/-2%) and 8 patients with CHF (age 60+/-2 years; ejection fraction 19+/-2%) were studied. Instrumentation consisted of an arterial line, a pulmonary artery catheter, and a coronary sinus thermodilution catheter. The radiotracer technique was used for measurement of cardiac norepinephrine spillover (CANESP) and total-body norepinephrine spillover. Lower-body negative pressure (LBNP) was applied at 2 levels: nonhypotensive and hypotensive LBNP. Nonhypotensive LBNP reduced filling pressures significantly in both groups. Arterial pressure did not change. This reduction in filling pressures caused a significant reduction in CANESP in the CHF group (from 167+/-53 to 125+/-37 pmol/min, P<0.05) but no change in the normal LV function group. Hypotensive LBNP caused a significant increase in CANESP in the normal group (73+/-13 vs 122+/-27 pmol/min, P<0.05) but no significant change in those with CHF.

CONCLUSIONS

We conclude that selective reduction in filling pressures lowers cardiac norepinephrine spillover in patients with CHF. These findings suggest that a goal of CHF management should be to reduce cardiac filling pressures while avoiding systemic hypotension.

Development of hypertension in a rat model of diet-induced obesity

Although obesity is a risk factor for hypertension, the relationship between these 2 conditions is not well understood. Therefore, we examined some parameters of hypertension and cardiovascular disease in a dietary model of obesity. Male Sprague-Dawley rats were provided either a control diet (C) or a diet containing 32% kcal as fat (similar to a Western diet) for 1, 3, or 10 weeks. Rats in the latter group diverged based on body weight gain into obesity-prone (OP) and obesity-resistant (OR) groups. Systolic blood pressure in OP rats was significantly higher after 10 weeks of the diet (149+/-4. 8 mm Hg) compared with both OR and C groups (131+/-3.7 and 129+/-4.5 mm Hg, respectively). The aortic wall area of OP rats was significantly increased, indicating arterial hypertrophy, and a 2-fold increase in plasma renin activity was found in OP rats compared with OR and C rats. The lipid profile showed a significant increase in plasma and VLDL triglycerides of OP versus OR and C groups as early as 3 weeks on the diet. Plasma and LDL-cholesterol levels were increased in the OP group versus the OR and C groups after 3 weeks of the diet, but the difference was blunted after 10 weeks. Lipid peroxidation (thiobarbituric acid-reactive substances) in OP rats was increased 2-fold in LDL and 1.5-fold in aortic wall compared with OR rats, suggesting an increased oxidative stress in these animals. Periodic acid-Schiff staining of the kidney showed mesangial expansion and focal sclerosis that were more prominent in OP rats than in OR rats. The results suggest that hypercholesterolemia, but not hypertriglyceridemia, is linked to the diet; that hypertension and renin-angiotensin system activation are associated with obesity; and that lipid peroxidation and renal damage are the results of both factors.