Role of the sympathetic nervous system during the development of obesity-induced hypertension in rabbits

Animal Fats in Rabbit Feeding – A Review

The purpose of this article is to overview the history of feeding rabbits with different types of animal fats, and to discuss their effects on rabbit performance and quality of their products. Other aspects of the inclusion of various animal fats in rabbit diets are also described. This article is based on the analysis of relevant scientific literature and presents animal fats fed to rabbits, such as beef tallow, butter, pork lard, poultry fat, fish oil, krill oil, oil extracted from insect larvae, mixtures of various animal fats, and mixtures of animal and vegetable fats. The reported papers describe the effect of fats on growth performance, lactation, rearing performance, meat quality, and health status of rabbits. It is notable that in many cases, various animal fats were often an integral part of numerous diets or were included in control diets. The presented information demonstrates that animal fat can be fed to rabbits at 2–4% of the diet without negative effects on reproductive performance, growth performance and quality of meat obtained. Rabbits were used as model animals in many studies in which fat was added to balance the diets and to increase their energy value, especially when investigating various cardiovascular and obesity-related diseases.

The importance of bone marrow and the immune system in driving increases in blood pressure and sympathetic nerve activity in hypertension

NEW FINDINGS

What is the topic of this review? This manuscript provides a review of the current understanding of the role of the sympathetic nervous system in regulation of bone marrow-derived immune cells and the effect that the infiltrating bone marrow cells may have on perpetuation of the sympathetic over-activation in hypertension. What advances does it highlight? We highlight the recent advances in understanding of the neuroimmune interactions both peripherally and centrally as they relate to blood pressure control.

ABSTRACT

The sympathetic nervous system (SNS) plays a crucial role in maintaining physiological homeostasis, in part by regulating, integrating and orchestrating processes between many physiological systems, including the immune system. Sympathetic nerves innervate all primary and secondary immune organs, and all cells of the immune system express β-adrenoreceptors. In turn, immune cells can produce cytokines, chemokines and neurotransmitters capable of modulating neuronal activity and, ultimately, SNS activity. Thus, the essential role of the SNS in the regulation of innate and adaptive immune functions is mediated, in part, via β-adrenoreceptor-induced activation of bone marrow cells by noradrenaline. Interestingly, both central and systemic inflammation are well-established hallmarks of hypertension and its co-morbidities, including an inflammatory process involving the transmigration and infiltration of immune cells into tissues. We propose that physiological states that prolong β-adrenoreceptor activation in bone marrow can disrupt neuroimmune homeostasis and impair communication between the immune system and SNS, leading to immune dysregulation, which, in turn, is sustained via a central mechanism involving neuroinflammation.

Central proopiomelanocortin but not neuropeptide Y mediates sympathoexcitation and hypertension in fat fed conscious rabbits

OBJECTIVE

High-fat diet (HFD)-induced hypertension in rabbits is neurogenic because of the central sympathoexcitatory actions of leptin. Hypothalamic melanocortin and neuropeptide Y (NPY) neurons are recognized as the major signalling pathways through which leptin exerts its central effects. In this study, we assessed the effects of specific antagonists and agonists to melanocortin and NPY receptors on HFD-induced sympathoexcitation and hypertension.

METHODS

Rabbits were instrumented with intracerebroventricular cannula, renal sympathetic nerve activity (RSNA) electrode, and blood pressure telemetry transmitter.

RESULTS

After 3 weeks HFD (13.5% fat, n = 12) conscious rabbits had higher RSNA (+3.8  nu, P = 0.02), blood pressure (+8.6  mmHg, P < 0.001) and heart rate (+15  b/min, P = 0.01), and brain-derived neurotrophic factor levels in the hypothalamus compared with rabbits fed a control diet (4.2% fat, n = 11). Intracerebroventricular administration of the melanocortin receptor antagonist SHU9119 reduced RSNA (-2.7  nu) and blood pressure (-8.5  mmHg) in HFD but not control rabbits, thus reversing 100% of the hypertension and 70% of the sympathoexcitation induced by a HFD. By contrast, blocking central NPY Y1 receptors with BVD10 increased RSNA only in HFD rabbits. Intracerebroventricular α-melanocortin stimulating hormone increased RSNA and heart rate (P < 0.001) in HFD rabbits but had no effect in control rabbits.

CONCLUSION

These findings suggest that obesity-induced hypertension and increased RSNA are dependent on the balance between greater activation of melanocortin signalling through melanocortin receptors and lesser activation of NPY sympathoinhibitory signalling. The amplification of the sympathoexcitatory effects of α-melanocortin stimulating hormone also indicates that the underlying mechanism is related to facilitation of leptin-melanocortin signalling, possibly involving chronic activation of brain-derived neurotrophic factor.

Origin of Aberrant Blood Pressure and Sympathetic Regulation in Diet-Induced Obesity

High fat diet (HFD)–induced hypertension in rabbits is neurogenic and caused by the central action of leptin, which is thought to be dependent on activation of α-melanocortin–stimulating hormone (α-MSH) and neuropeptide Y–positive neurons projecting to the dorsomedial hypothalamus (DMH) and ventromedial hypothalamus (VMH). However, leptin may act directly in these nuclei. Here, we assessed the contribution of leptin, α-MSH, and neuropeptide Y signaling in the DMH and VMH to diet-induced hypertension. Male New Zealand white rabbits were instrumented with a cannula for drug injections into the DMH or VMH and a renal sympathetic nerve activity (RSNA) electrode. After 3 weeks of an HFD (13.3% fat; n=19), rabbits exhibited higher RSNA, mean arterial pressure (MAP), and heart rate compared with control diet–fed animals (4.2% fat; n=15). Intra-VMH injections of a leptin receptor antagonist or SHU9119, a melanocortin 3/4 receptor antagonist, decreased MAP, heart rate, and RSNA compared with vehicle in HFD rabbits ( P <0.05) but not in control diet–fed animals. By contrast, α-MSH or neuropeptide Y injected into the VMH had no effect on MAP but produced sympathoexcitation in HFD rabbits ( P <0.05) but not in control diet–fed rabbits. The effects of the leptin antagonist, α-MSH, or neuropeptide Y injections into the DMH on MAP or RSNA of HFD rabbits were not different from those after vehicle injection. α-MSH into the DMH of control diet–fed animals did increase MAP, heart rate, and RSNA. We conclude that the VMH is the likely origin of leptin-mediated sympathoexcitation and α-MSH hypersensitivity that contribute to obesity-related hypertension.

Progression of cardiovascular and endocrine dysfunction in a rabbit model of obesity

In rabbits, mean arterial pressure (MAP) increases in response to fat feeding, but does not increase further with progressive weight gain. We documented the progression of adiposity and the alterations in endocrine/cardiovascular function in response to fat feeding in rabbits, to determine whether stabilization of MAP after 3 weeks could be explained by stabilization of neurohormonal factors. Rabbits were fed a control diet or high-fat diet for 9 weeks (n=23). Fat feeding progressively increased body mass and adiposity. Heart rate (HR) was elevated by week 3 (15±3%) but changed little thereafter. The effects of fat feeding on MAP were dependent on baseline MAP and peaked at 3 weeks. From baseline, MAP 80 mm Hg, MAP had increased by 8.1±1.3, 4.7±1.7 and 5.6±1.2 mm Hg, respectively, 3, 6 and 9 weeks after commencing the high-fat diet, but by only 2.6±1.5, 3.0±1.7 and 3.9±1.4 mm Hg, respectively, in control rabbits. Fat feeding did not increase MAP from a baseline >80 mm Hg. Plasma concentrations of leptin and insulin increased during the first 3-6 weeks of fat feeding and then stabilized (increasing by 111±17% and 731±302% by week 9, respectively), coinciding with the pattern of changes in MAP and HR. Plasma total cholesterol, triglycerides, renin activity, aldosterone and atrial natriuretic peptide were not significantly altered by fat feeding. Given that the changes in plasma leptin and insulin mirrored the changes in MAP and HR, leptin and insulin may be important factors in the development of hypertension and tachycardia in the rabbit model of obesity.

Vascular hyporeactivity to angiotensin II and noradrenaline in a rabbit model of obesity

This study was conducted to explore the vascular reactivity of angiotensin II and noradrenaline and their relationship with endothelial function in rabbits fed a high-fat diet (HFD). The animals were fed either an HFD or regular chow [control diet (CD)]. After 12 weeks, the rabbits fed the HFD showed higher blood pressure, body weight, and insulin levels. Glucose tolerance was impaired and positively related to blood pressure. An endothelium-independent decrease of the sensitivity to angiotensin II [pD2 endothelium-intact aortic rings (E+) in CD: 8.02 ± 0.07 vs. HFD: 7.60 ± 0.01; pD2 endothelium-removed aortic rings (E-) in CD: 8.16 ± 0.11 vs. HFD: 7.83 ± 0.16] and noradrenaline (pD2 E+ in CD: 6.36 ± 0.06 vs. HFD: 5.29 ± 0.06; pD2 E- in CD: 6.11 ± 0.08 vs. HFD: 5.80 ± 0.08) was found. Noradrenaline desensitized the angiotensin II response (pD2 with noradrenaline pretreatment in E+: 7.03 ± 0.16; in E-: 7.10 ± 0.02), but angiotensin II did not change the noradrenaline response. Acetylcholine maximal relaxation and basal nitric oxide (NO) release were comparable in both diet groups. The efficacy of angiotensin II (Rmax CD: 4604 ± 574 mg vs. HFD: 3251 ± 533 mg) and noradrenaline (Rmax CD: 11,675 ± 804 mg vs. HFD: 7975 ± 960 mg) was reduced in E+. L-N-nitroarginine methyl ester (L-NAME) recovered the efficacy of noradrenaline (Rmax L-NAME: 12,015 ± 317 mg). In contrast, L-NAME had no effect on the angiotensin II response. Noradrenaline enhanced NO levels, but angiotensin II did not. Therefore, NO was associated with hyporeactivity to noradrenaline. The resting potential was more negative in E+, and the endothelium diminished the angiotensin II-induced depolarization. These findings demonstrated that the crosstalk and the endothelium may induce hyporeactivity to angiotensin II and noradrenaline as a mechanism to compensate the increase in the blood pressure in HFD-induced obesity.

The cannabinoid receptor agonist THC attenuates weight loss in a rodent model of activity-based anorexia

Anorexia nervosa (AN) is characterized by anhedonia whereby patients experience little pleasure or reward in many aspects of their lives. Reward pathways and the endocannabionid system have been implicated in the mediation of food intake. The potential to exploit these systems to reverse weight loss is investigated in a rodent model of activity-based anorexia (ABA). The effect of subchronic (6 days) Δ(9)-tetrahydrocannabinol (THC) treatment (0.1, 0.5, or 2.0 mg/kg/day) was assessed on chow and high-fat diet (HFD) intake, body weight, running wheel activity (RWA) as well as thermogenesis in brown adipose tissue (BAT) and lipid metabolism in white adipose tissue (WAT). Limited time availability of food and continuous access to running wheels led to anorexia and significantly reduced body weight. THC treatment (0.5 and 2.0 mg/kg/day) transiently stimulated chow intake with a moderate effect on RWA. THC (2.0 mg/kg/day) significantly reduced body weight loss and shifted markers of thermogenesis in BAT and lipid metabolism in WAT in directions consistent with reduced energy expenditure and lipolysis. THC (2.0 mg/kg/day) combined with HFD, produced a transient increase in food intake, reduction in RWA, attenuation of body weight loss, and changes in markers of thermogensis in BAT and lipolysis in the WAT. These changes were significantly greater than those seen in vehicle (HFD), vehicle (chow), and THC (chow)-treated animals. These data show for the first time the effectiveness of the endocannabinoid system in attenuating the weight loss associated with the development of ABA via a mechanism involving reduced energy expenditure.

Weight cycling during growth and beyond as a risk factor for later cardiovascular diseases: the 'repeated overshoot' theory

In people trying to lose weight, there are often repeated cycles of weight loss and regain. Weight cycling is, however, not limited to obese adults but affects people of normal weight, particularly young women, who are unhappy with their appearance. Furthermore, the onset of a pattern of weight cycling is shifting towards younger ages, owing to the increasing prevalence of overweight and obesity in children and adolescents, and the pressure from the media and society for a slim image even for normal weight children. Although there is still controversy whether weight cycling promotes body fat accumulation and obesity, there is mounting evidence from large population studies for increased cardiovascular risks in response to a behavior of weight cycling. Potential mechanisms by which weight cycling contributes to cardiovascular morbidity include hypertension, visceral fat accumulation, changes in adipose tissue fatty acid composition, insulin resistance and dyslipidemia. Moreover, fluctuations in blood pressure, heart rate, sympathetic activity, glomerular filtration rate, blood glucose and lipids that may occur during weight cycling--with overshoots above normal values during weight regain periods--put an additional load on the cardiovascular system, and may be easily overlooked if humans or animals are studied during a state of relatively stable weight. Overshoot of those risks factors, when repeated over time, will stress the cardiovascular system and probably contribute to the overall cardiovascular morbidity of weight cycling.

Exposure to a high-fat diet alters leptin sensitivity and elevates renal sympathetic nerve activity and arterial pressure in rabbits

The activation of the sympathetic nervous system through the central actions of the adipokine leptin has been suggested as a major mechanism by which obesity contributes to the development of hypertension. However, direct evidence for elevated sympathetic activity in obesity has been limited to muscle. The present study examined the renal sympathetic nerve activity and cardiovascular effects of a high-fat diet (HFD), as well as the changes in the sensitivity to intracerebroventricular leptin. New Zealand white rabbits fed a 13.5% HFD for 4 weeks showed modest weight gain but a 2- to 3-fold greater accumulation of visceral fat compared with control rabbits. Mean arterial pressure, heart rate, and plasma norepinephrine concentration increased by 8%, 26%, and 87%, respectively (P<0.05), after 3 weeks of HFD. Renal sympathetic nerve activity was 48% higher (P<0.05) in HFD compared with control diet rabbits and was correlated to plasma leptin (r=0.87; P<0.01). Intracerebroventricular leptin administration (5 to 100 microg) increased mean arterial pressure similarly in both groups, but renal sympathetic nerve activity increased more in HFD-fed rabbits. By contrast, intracerebroventricular leptin produced less neurons expressing c-Fos in HFD compared with control rabbits in regions important for appetite and sympathetic actions of leptin (arcuate: -54%, paraventricular: -69%, and dorsomedial hypothalamus: -65%). These results suggest that visceral fat accumulation through consumption of a HFD leads to marked sympathetic activation, which is related to increased responsiveness to central sympathoexcitatory effects of leptin. The paradoxical reduction in hypothalamic neuronal activation by leptin suggests a marked "selective leptin resistance" in these animals.

Altered responsiveness of the kidney to activation of the renal nerves in fat-fed rabbits

We tested whether mild adiposity alters responsiveness of the kidney to activation of the renal sympathetic nerves. After rabbits were fed a high-fat or control diet for 9 wk, responses to reflex activation of renal sympathetic nerve activity (RSNA) with hypoxia and electrical stimulation of the renal nerves (RNS) were examined under pentobarbital anesthesia. Fat pad mass and body weight were, respectively, 74% and 6% greater in fat-fed rabbits than controls. RNS produced frequency-dependent reductions in renal blood flow, cortical and medullary perfusion, glomerular filtration rate, urine flow, and sodium excretion and increased renal plasma renin activity (PRA) overflow. Responses of sodium excretion and medullary perfusion were significantly enhanced by fat feeding. For example, 1 Hz RNS reduced sodium excretion by 79 +/- 4% in fat-fed rabbits and 46 +/- 13% in controls. RNS (2 Hz) reduced medullary perfusion by 38 +/- 11% in fat-fed rabbits and 9 +/- 4% in controls. Hypoxia doubled RSNA, increased renal PRA overflow and medullary perfusion, and reduced urine flow and sodium excretion, without significantly altering mean arterial pressure (MAP) or cortical perfusion. These effects were indistinguishable in fat-fed and control rabbits. Neither MAP nor PRA were significantly greater in conscious fat-fed than control rabbits. These observations suggest that mild excess adiposity can augment the antinatriuretic response to renal nerve activation by RNS, possibly through altered neural control of medullary perfusion. Thus, sodium retention in obesity might be driven not only by increased RSNA, but also by increased responsiveness of the kidney to RSNA.

Multiple mechanisms involved in obesity-induced hypertension

Obesity and hypertension are two major cardiovascular risk factors gaining epidemic proportions in our modern world. The interplay between hypertension, obesity and their major detrimental outcome, cardiovascular disease, is multifaceted and can be represented as the three corners of a triangle. Obesity and hypertension both lead to cardiovascular complications. In addition, obesity per se can promote hypertension. In turn, cardiovascular diseases can also predispose to obesity and hypertension. Low levels of physical activity due to a weakened heart promote weight gain. Endothelial, vascular and renal dysfunctions, all consequences of high blood pressure, further worsen hypertension. The loop of mutually amplifying detrimental effects is thus closed: a 'vicious triangle' is established. The association between obesity and hypertension was recognised and described almost a century ago, but the mechanisms that underlie this connection are still not fully understood. Vasoconstriction and sodium retention seem to be the cornerstones of the obesity-hypertension puzzle. However, pathways possibly leading to vasoconstriction and sodium retention are numerous. Evidence has been gathered that hyperleptinaemia, hyperinsulinaemia and elevated free fatty acids may induce sympathetic activation and vasoconstriction. The latter is further potentiated by insulin resistance and endothelial dysfunction. Positive sodium balance and ensuing volume expansion may be due to increased renal tubular sodium reabsorption induced by sympathetic stimulation, insulin or by a hyperactive renin-angiotensin system. All enumerated factors act together toward a state of permanently elevated blood pressure.

Lipid metabolism in diet-induced obese rabbits is similar to that of obese humans

Whereas diet-induced obese rabbits have been used to study various aspects of obesity, alterations of lipid metabolism in this model have not been clarified. This study aimed to compare plasma nonesterified fatty acid (NEFA) and triglyceride (TG) kinetics in obese and lean rabbits by means of U-(13)C16-palmitate infusion. Young female rabbits consumed either a high-fat diet (49% energy from fat) ad libitum to develop obesity (n = 6) or a normal diet (7.9% energy from fat) as lean control (n = 5). After 10 wk of feeding, the body weight of obese rabbits (5.33 +/- 0.05 kg) was greater (P < 0.001) than that of lean rabbits (3.89 +/- 0.07 kg). The obese rabbits had higher concentrations of plasma NEFA and TG and a greater rate of fatty acid (FA) turnover. Whereas the fractional secretion rates of hepatic TG did not differ, 100% of hepatic secretory TG was synthesized from plasma NEFA in the lean rabbits compared to 59% in the obese rabbits (P < 0.001). In the lean rabbits, hepatic lipase-mediated hydrolysis of lipoprotein TG did not contribute to the FA pool for synthesis of secretory TG, consistent with the naturally occurring deficit in hepatic lipase in this species. We conclude that lipid metabolism in diet-induced obese rabbits is similar to that in obese humans. The deficiency in hepatic lipase in rabbits simplifies the quantitation of hepatic lipid kinetics.

Dysfunction of purinergic regulation of sympathetic neurotransmission in SHR/NDmcr-cp (SHR-cp) rat

1. The effect of 2-chloroadenosine (2CA), a P1 receptor agonist and beta,gamma-methylene ATP (betagamma mATP), a P2 receptor agonist, on the overflow of endogenous noradrenaline (NE) and the contractile response were examined in the electrically field-stimulated (EFS) (1 Hz) caudal artery obtained from Wistar-Kyoto (WKY) and SHR/NDmcr-cp (SHR-cp) rats. 2. Both 2CA and betagamma mATP reduced the EFS-evoked release of NE from the arteries of WKY. Also, 2CA significantly reduced the EFS-evoked contractile response in WKY, while it had no effect at all in SHR-cp. Betagamma mATP significantly reduced the EFS-evoked contractile response in both WKY and SHR-cp. Both 2CA and betagamma mATP did not affect the contractile response induced by NE at 1 micromol/L. 3. These results indicate that in the caudal arteries of SHR-cp, the P2 agonist but not the P1 agonist is functional in the prejunctional inhibitory regulation of adrenergic neurotransmission. This P1 dysfunction may play a role in the sympathetic hyperinnervation in metabolic syndrome.

Effect of peripheral administration of leptin on the renal sympathetic nerve activity in high-fat diet-related hypertensive rats

A previous study of ours demonstrated that a high-fat diet (FAT) causes body fat accumulation, as well as elevation of plasma leptin level, renal sympathetic nerve activity (RSNA), and blood pressure (BP). In the study reported here, we analyzed the role of leptin in these elevations of the RSNA and BP due to FAT feeding by assessing sympathetic and cardiovascular responses to intravenous (IV) administration of leptin in rats fed either a FAT or a high-carbohydrate diet (CHO). The results showed that baseline body fat, plasma leptin level, RSNA and BP were significantly higher in the FAT group than in the CHO group, and that IV administration of leptin elevated RSNA and plasma leptin levels but lowered BP in the CHO group. However, these effects of leptin were eliminated in the FAT group. These findings suggest that FAT-fed rats which expose basal elevation of plasma leptin levels, RSNA and BP might be hyposensitive to endogenous leptin. Therefore, leptin resistance appeared obviously in FAT-induced hypertension might indicate that leptin is implicated in generating the elevation of RSNA and BP induced by long-term FAT feeding.

Relationship between left ventricular mass and heart sympathetic activity in male obese subjects

BACKGROUND

Pathogenesis of left ventricular hypertrophy (LVH) is multifactorial, but evidence that the sympathetic system is involved in progression of cardiovascular structural alterations in hypertension is substantial. However, the relationship between heart sympathetic activity and left ventricular mass in normotensive obese subjects has not been studied.

METHODS

We studied 103 male normotensive subjects (53 obese and 50 non-obese). Left ventricular mass (LVM), heart sympathetic activity by 60 min electrocardiograph (EKG) monitoring, leptin, insulin, lipid profile, and blood pressure were measured.

RESULTS

Obese subjects showed higher LVM (p=0.0002), low frequency/high frequency ratio (LF/HF) (p=0.004), systolic and diastolic blood pressure (p <0.00001 for both), insulin (p=0.00001), and leptin levels (p <0.00001) than non-obese subjects. In contrast, standard deviation (SD) of all rate-rhythm (R-R) intervals (SDNN) (p=0.0002) and total power (TP) (p=0.03) were lower in obese than in non-obese subjects. In multiple regression analysis for factors associated with LVM, body mass index (BMI) (R=0.24), systolic blood pressure (R=0.22), and LF/HF (R=0.21) were the only factors included in the model (R2=0.32, p <0.0001).

CONCLUSIONS

LVM is increased in normotensive obese subjects and correlates with systolic blood pressure and heart sympathetic activity.

Association analysis of the Gln223Arg polymorphism in the human leptin receptor gene, and traits related to obesity in Mexican adolescents

Polymorphisms of leptin receptor (LEPR) may contribute to a common form of obesity and, as a consequence, obesity-related diseases. We evaluated the potential role of genetic variation at the LEPR gene in heart sympathetic activity and other traits related to obesity in Mexican adolescents. Adolescents aged between 12 and 17 years, with steady body weight for the last 3 months were included. We evaluated anthropometric measurements, blood pressure, seric glucose, insulin, leptin levels, heart sympathetic activity (by electrocardiograph monitoring at rest), and the Gln223Arg and Pro1019Pro LEPR polymorphisms in each subject. In total, 103 adolescents (55 obese and 48 nonobese) were included. The group of obese adolescents showed higher sympathetic activity, blood pressure, glucose, insulin, and leptin levels. The genotype frequencies for the two polymorphisms were found to be in Hardy-Weinberg equilibrium. There was no difference in the genotype frequencies for Gln223Arg or Pro1019Pro polymorphisms between obese and nonobese adolescents. However, there was a higher prevalence of Gln223 allele among subjects with higher insulin levels (0.72 vs 0.57; P = 0.04 for adolescents with insulin levels higher and lower than 100 pmol/l, respectively). According to Gln223Arg polymorphism, those with Gln allele (Gln/Gln and Gln/Arg) had higher heart sympathetic activity, body fat percentage, and leptin levels. To conclude, our results support the hypothesis that Gln223Arg polymorphism of LEPR in Mexican adolescents is associated with haemodynamic and metabolic disturbances related to obesity.

Ectopic fat storage in heart, blood vessels and kidneys in the pathogenesis of cardiovascular diseases

In humans and most animal models, the development of obesity leads not only to increased fat depots in classical adipose tissue locations but also to significant lipid deposits within and around other tissues and organs, a phenomenon known as ectopic fat storage. The purpose of this review is to explore the possible locations of ectopic fat in key target-organs of cardiovascular control (heart, blood vessels and kidneys) and to propose how ectopic fat storage can play a role in the pathogenesis of cardiovascular diseases associated with obesity. In animals fed a high-fat diet, cardiac fat depots within and around the heart impair both systolic and diastolic functions, and may in the long-term promote heart failure. Accumulation of fat around blood vessels (perivascular fat) may affect vascular function in a paracrine manner, as perivascular fat cells secrete vascular relaxing factors, proatherogenic cytokines and smooth muscle cell growth factors. Furthermore, high amounts of perivascular fat could mechanically contribute to the increased vascular stiffness seen in obesity. Finally, accumulation of fat in the renal sinus may limit the outflow of blood and lymph from the kidney, which would alter intrarenal physical forces and promote sodium reabsorption and arterial hypertension. Taken together, ectopic fat storage in key target-organs of cardiovascular control may impair their functions, contributing to the increased prevalence of cardiovascular diseases in obese subjects.

Diet-induced obesity and cardiovascular regulation in C57BL/6J mice

1. In the present study, we determined the effect of diet-induced obesity on cardiovascular and metabolic regulation in mice at standard laboratory temperatures (ambient temperature (Ta) = 22 degrees C) and during exposure to thermoneutrality (Ta = 30 degrees C). 2. Male C57BL/6J (B6) mice fed a high-fat diet (HFF; n = 17) or chow (CHW; n = 14) for 15 weeks were surgically instrumented with telemetry devices, housed in metabolic chambers and assigned to either control or atenolol treatment (25 mg/kg per day in drinking water) to determine the effects of obesity on baseline cardiovascular function and on the responses to thermoneutrality and 24 h fasting. Mean arterial pressure (MAP), heart rate (HR), arterial pressure and HR variability (time and frequency domain), oxygen consumption (VO2) and locomotor activity were determined. 3. The HFF mice exhibited increased bodyweight (+10.6 +/- 4.1 g), moderate light period hypertension (+8.6 +/- 2.6 mmHg), no difference in HR and increased HR variability at standard laboratory temperature compared with CHW controls. Atenolol produced less of a decrease in HR in HFF mice (-42 +/- 10 b.p.m.) compared with CHW controls (-73 +/- 15 b.p.m.). Acute exposure to thermoneutrality (Ta = 30 degrees C) reduced HR similarly in both HFF and CHW mice (approximately 175 b.p.m.), but reduced MAP less in HFF than in CHW mice (-7.3 +/- 2.5 and -15.2 +/- 1.0 mmHg), respectively. Atenolol treatment had only minor effects on the HR response to thermonuetrality (-114 +/- 13 and -129 +/- 8 b.p.m. in HFF and CHW mice, respectively). The HFF mice displayed greater fasting-induced reductions in light period MAP than did CHW mice (-10.0 +/- 1.1 vs-3.1 +/- 3.5 mmHg, respectively), whereas HR was decreased equally in both groups. Fasting-induced increases in HR variability were attenuated in HFF mice. 4. We conclude that diet-induced obesity produced generally minor changes in cardiovascular regulation in B6 mice at baseline, some of which are distinct from the effects of diet-induced obesity in larger animal models. In contrast, acute variations in Ta or caloric availability produce pronounced alterations in cardiovascular function in either lean or obese mice, which are generally evident after atenolol and, thus, presumably not due exclusively to variation in cardiac sympathetic activity. Interestingly, the degree of obesity induced hypertension was augmented when mice were studied at thermonuetrality. The results suggest an important unrecognized role for vagal tone in the regulation of cardiovascular function in mice and support the need for considerable caution when using mouse models of obesity to examine regulation of cardiovascular function. We argue that mouse physiology studies should be performed in thermoneutral conditions.

Hypertension and Insulin Resistance Are Not Directly Related in Obese Dogs

In dogs fed a high-fat diet, we determined whether there was a direct relation between obesity-induced insulin resistance and obesity-induced hypertension. Thirty-six adult mongrel dogs were chronically instrumented and assigned to receive either a high-fat diet alone (n=7) or a high-fat diet combined with a low-sodium diet plus furosemide (n=6), prazosin plus atenolol (n=7), clonidine (n=10), or aspirin (n=6). Blood pressure, heart rate, and body weight were measured daily. Insulin resistance was assessed with a single-dose euglycemic hyperinsulinemic clamp (2 mU · kg −1 · min −1 ) before and after 1, 3, and 6 weeks of the high-fat diet. The low-salt diet plus furosemide, prazosin plus atenolol, and clonidine treatments prevented the hypertension associated with feeding the dogs a high-fat diet. Only clonidine treatment totally prevented the development of insulin resistance, and high-dose aspirin, known to prevent insulin resistance by inhibition of the activity of IκB kinase-β, decreased the degree of insulin resistance by almost 70%. However, aspirin had no effect on the development of hypertension. We conclude that obesity-induced hypertension and obesity-induced insulin resistance are not directly related. In addition, there is a suggestion that insulin resistance in this experimental model is mediated through the central and or peripheral α 2 -adrenoceptors, whereas hypertension is mediated through the α 1 - and or β-adrenoceptors.

Obesity and hypertension

This article has discussed some of the mechanisms involved in the causal relation between obesity and hypertension. Obesity causes a constellation of maladaptive disorders that individually and synergistically contribute to hypertension, among other cardiovascular morbidities. Well-designed population-based studies are needed to assess the individual contribution of each of these disorders to the development of hypertension. In addition, because the control of obesity may eliminate 48% of the hypertension in whites and 28% in blacks, this article has offered an up-to-date on the management of this problem. It is hoped that this article will help scientists formulate a thorough understanding of obesity hypertension and form the basis for more research in this field, which has a huge impact on human life.

Emerging concepts in the pathophysiology and treatment of obesity-associated hypertension

The dramatic increase in the prevalence of obesity is a global phenomenon associated with increased risk of the development of cardiovascular and renal disease. Changes in renal structure and function that occur early in the development of obesity may lead to urine outflow obstruction and increased intrarenal pressure, mechanisms sufficient to shift the pressure-natriuresis relation to higher blood pressure levels. Another important alteration that may lead to hypertension with obesity is the increase in sympathetic nervous system activity. Several studies point to higher leptin levels associated with hypertension in humans, and animal data now convincingly suggest that leptin has direct central effects that increase sympathetic outflow to the kidneys, associated with increases in blood pressure. Although understanding of the pathophysiology of obesity-associated hypertension has made substantial progress during the past years, treatment of obese hypertensives remains largely empirical and clearly deserves to be addressed in larger randomized, controlled trials.

Exercise training in obesity lowers blood pressure independent of weight change

PURPOSE

We used the rabbit model of obesity and exercise training to determine effects of exercise training during the development of obesity on resting blood pressure and heart rate, ventricular hypertrophy, blood volume, and hormonal profile.

METHODS

Female New Zealand white rabbits were assigned to one of four groups: lean sedentary (L-S, N = 17), lean exercise-trained (L-EX, N = 16), obese sedentary (O-S, N = 18), and obese exercise-trained (O-EX, N = 15). Lean rabbits were fed a maintenance diet whereas obese rabbits were fed an ad libitum high fat (10% added fat) diet. Simultaneously, exercise-trained animals underwent a progressive treadmill exercise training protocol for 12 wk. After 12 wk of diet and exercise regimens, resting blood pressure and heart rate were measured from a central ear artery catheter. Ventricular hypertrophy was evaluated using wet ventricular weights. Blood volume was measured using the Evans blue dye procedure; hormonal profile was evaluated from arterial plasma/serum samples.

RESULTS

After 12 wk, O-S and O-EX had similar body weights and similar percentage increases in body weight. Despite similar body weights, O-EX had an approximate 6-mm Hg lower mean blood pressure compared with the elevated pressure seen in O-S (P < or = 0.05). Obese rabbits had greater resting heart rate, plasma cholesterol and triglycerides, and plasma renin activity compared with lean rabbits, and these values were unaffected by exercise training. Plasma and blood volumes, as well as plasma insulin, cortisol, and aldosterone were unaffected by exercise training.

CONCLUSION

These data suggest that exercise training, in the absence of differences in body weight, may be useful in the reduction of obesity-induced hypertension but that other therapies may be needed in order to control other cardiovascular risk factors.

Loss of nocturnal dipping of blood pressure and heart rate in obesity-induced hypertension in rabbits

We have investigated in rabbits whether overfeeding and weight gain, which lead to hypertension, are associated with changes in circadian rhythm of blood pressure (BP) and heart rate, and whether the sympathetic nervous system is involved in these changes. In adult male rabbits, mean arterial pressure (MAP) and heart rate (HR) were monitored by telemetry 22 h a day. Daily MAP and HR records were divided into four equal intervals and used to calculate day-night differences. After a 1-week control period, animals were switched to a high-fat (HFD) ad libitum diet for 8 weeks. HFD increased whole day MAP and HR, and rapidly abolished the normal diurnal rhythm of MAP and HR. Since HFD abolished the nocturnal dip in MAP, but had little effect on daytime values, the loss of dipping appears to account for most of the hypertension in this model of obesity. In a separate set of rabbits, alpha- and beta-adrenergic blockade (terazosin + propranolol) prevented HFD-induced hypertension and attenuated the increase in HR by more than half. Adrenergic blockade alone abolished the diurnal rhythm of MAP, chiefly by preventing daytime elevation of MAP. The addition of HFD ad libitum did not further modify daily MAP or its circadian pattern. The diurnal rhythm of HR was relatively unaffected by alpha + beta blockade alone, but was abolished after switching to HFD. In conclusion, rabbits fed an HFD ad libitum develop hypertension and tachycardia associated with a loss of the normal diurnal rhythm of MAP and HR. The hypertension appears to be sympathetically mediated.

Hemodynamic effects of lipids in humans

Evidence suggests lipid abnormalities may contribute to elevated blood pressure, increased vascular resistance, and reduced arterial compliance among insulin-resistant subjects. In a study of 11 normal volunteers undergoing 4-h-long infusions of Intralipid and heparin to raise plasma nonesterified fatty acids (NEFAs), we observed increases of blood pressure. In contrast, blood pressure did not change in these same volunteers during a 4-h infusion of saline and heparin. To better characterize the hemodynamic responses to Intralipid and heparin, another group of 21 individuals, including both lean and obese volunteers, was studied after 3 wk on a controlled diet with 180 mmol sodium/day. Two and four hours after starting the infusions, plasma NEFAs increased by 134 and 111% in those receiving Intralipid and heparin, P < 0.01, whereas plasma NEFAs did not change in the first group of normal volunteers who received saline and heparin. The hemodynamic changes in lean and obese subjects in the second study were similar, and the results were combined. The infusion of Intralipid and heparin induced a significant increase in systolic (13.5 +/- 2.1 mmHg) and diastolic (8.0 +/- 1.5 mmHg) blood pressure as well as heart rate (9.4 +/- 1.4 beats/min). Small and large artery compliance decreased, and systemic vascular resistance rose. These data raise the possibility that lipid abnormalities associated with insulin resistance contribute to the elevated blood pressure and heart rate as well as the reduced vascular compliance observed in subjects with the cardiovascular risk factor cluster.

New developments in mechanisms of obesity-induced hypertension: role of adipose tissue

Hypertension develops in almost 60% of obese individuals. Apart from the recent observation of obesity-associated structural changes in kidney structure that may lead to enhanced tubular sodium reabsorbtion, reports of paracrine and hormonal factors derived from adipose tissue have prompted speculations about the role of adipose tissue in the pathophysiology of obesity-induced hypertension. We summarize recent data on leptin's sympathoexcitatory actions, the possible influence of adipose tissue on atrial natriuretic peptide levels, and the formation of vasoactive substances, such as angiotensin II and nonesterified fatty acids, by adipocytes. The mechanisms discussed herein may contribute to the typical findings in obesity-induced hypertension, including volume expansion, sodium retention, enhanced sympathetic nervous system activity, increased activity of the systemic renin-angiotensin system, low atrial natriuretic peptide levels, and disturbed glucose and insulin metabolism. Together, these data strengthen the hypothesis that adipose tissue is potentially a major regulator of cardiovascular-renal function.

Direct and indirect methods used to study arterial blood pressure

A number of different approaches exist for assessing blood pressure in experimental animals. Here, we briefly consider the traditional indirect (rodent tail-cuff) and direct (saline-filled catheter) methods of blood pressure measurement before going on to describe our experience with blood pressure telemetry in rabbits, rats, and mice. Blood pressure telemetry offers the ability to obtain a high-fidelity recording of blood pressure continuously, for relatively long periods of time, in conscious, freely moving animals, without the limitations of restraint or anaesthesia. Since some drift in telemeter offset and sensitivity are inevitable, recalibration of the telemeter devices immediately before implantation and following explantation is essential to ensure and document the accuracy of the blood pressure measurements. For long-term implantations, verification of the calibration can be performed in vivo, at least in the case of large animals, such as rabbits. Telemetry devices suitable for small animals, such as mice, are also available now, which will facilitate the accurate characterization of blood pressure in transgenic animals. Telemeter implantation methods in mice are presently difficult, with relatively low success rates being reported. However, validation of new methods, such as the insertion of the catheter tip via the carotid artery, may make the technique more widely accessible in the near future.