Alexandre A. da Silva

Obesity-induced Hypertension: Role of Sympathetic Nervous System, Leptin, and Melanocortins

Excess weight gain contributes to increased blood pressure in most patients with essential hypertension. Although the mechanisms of obesity hypertension are not fully understood, increased renal sodium reabsorption and impaired pressure natriuresis play key roles. Several mechanisms contribute to altered kidney function and hypertension in obesity, including activation of the sympathetic nervous system, which appears to be mediated in part by increased levels of the adipocyte-derived hormone leptin, stimulation of pro-opiomelanocortin neurons, and subsequent activation of central nervous system melanocortin 4 receptors.

Obesity-Induced Hypertension Interaction of Neurohumoral and Renal Mechanisms

Excess weight gain, especially when associated with increased visceral adiposity, is a major cause of hypertension, accounting for 65% to 75% of the risk for human primary (essential) hypertension. Increased renal tubular sodium reabsorption impairs pressure natriuresis and plays an important role in initiating obesity hypertension. The mediators of abnormal kidney function and increased blood pressure during development of obesity hypertension include (1) physical compression of the kidneys by fat in and around the kidneys, (2) activation of the renin-angiotensin-aldosterone system, and (3) increased sympathetic nervous system activity. Activation of the renin-angiotensin-aldosterone system is likely due, in part, to renal compression, as well as sympathetic nervous system activation. However, obesity also causes mineralocorticoid receptor activation independent of aldosterone or angiotensin II. The mechanisms for sympathetic nervous system activation in obesity have not been fully elucidated but may require leptin and activation of the brain melanocortin system. With prolonged obesity and development of target organ injury, especially renal injury, obesity-associated hypertension becomes more difficult to control, often requiring multiple antihypertensive drugs and treatment of other risk factors, including dyslipidemia, insulin resistance and diabetes mellitus, and inflammation. Unless effective antiobesity drugs are developed, the effect of obesity on hypertension and related cardiovascular, renal and metabolic disorders is likely to become even more important in the future as the prevalence of obesity continues to increase.

Aldosterone Antagonism Attenuates Obesity-Induced Hypertension and Glomerular Hyperfiltration

Abstract—This study examined the importance of aldosterone (ALDO) in mediating changes in renal function and increased mean arterial pressure (MAP) during the development of dietary-induced obesity in chronically instrumented dogs. Mean arterial pressure, heart rate (HR), and cardiac output (CO) were recorded 24 hours per day in lean dogs (n=7) before and after administration of an ALDO antagonist, eplerenone (EP) (10 mg/kg twice daily), for 10 days. After 10 days of EP treatment, the dogs (n=7) were given a supplement of cooked beef fat for 5 weeks while EP was continued. An untreated group (n=6) was fed a high fat diet for 5 weeks and used as control (C). In lean dogs, EP decreased MAP from 89±4 to 84±4 mm Hg and glomerular filtration rate from 67.4±6.8 to 53.2±4.9 mL/min while inducing a small negative Na+ balance (−42±12 mEq). Plasma renin activity increased from 0.4±0.1 to 2.7±0.7 ng AI/mL per hour and plasma K+ increased from 4.8±0.1 to 6.1±0.3 mEq/L. After 5 weeks of a high fat diet, body weight increased 45% to 53% in EP and C obese dogs. In C dogs, MAP increased by 16±3 mm Hg, compared with only 7±1 mm Hg in EPLE dogs. Compared with untreated dogs, the EP dogs had smaller increases in CO (18±4.6% versus 43±1.5%), HR (33±5% versus 60±3%), glomerular filtration rate (19±5% versus 38±6%), and cumulative Na+ balance (138±35 mEq versus 472±110 mEq) after 5 weeks of a high fat diet. Thus, EP markedly attenuated glomerular hyperfiltration, sodium retention, and hypertension associated with chronic dietary-induced obesity. These observations indicate that ALDO plays an important role in the pathogenesis of obesity hypertension.

Obesity, hypertension, and chronic kidney disease

Obesity is a major risk factor for essential hypertension, diabetes, and other comorbid conditions that contribute to development of chronic kidney disease. Obesity raises blood pressure by increasing renal tubular sodium reabsorption, impairing pressure natriuresis, and causing volume expansion via activation of the sympathetic nervous system and renin–angiotensin–aldosterone system and by physical compression of the kidneys, especially when there is increased visceral adiposity. Other factors such as inflammation, oxidative stress, and lipotoxicity may also contribute to obesity-mediated hypertension and renal dysfunction. Initially, obesity causes renal vasodilation and glomerular hyperfiltration, which act as compensatory mechanisms to maintain sodium balance despite increased tubular reabsorption. However, these compensations, along with increased arterial pressure and metabolic abnormalities, may ultimately lead to glomerular injury and initiate a slowly developing vicious cycle that exacerbates hypertension and worsens renal injury. Body weight reduction, via caloric restriction and increased physical activity, is an important first step for management of obesity, hypertension, and chronic kidney disease. However, this strategy may not be effective in producing long-term weight loss or in preventing cardiorenal and metabolic consequences in many obese patients. The majority of obese patients require medical therapy for obesity-associated hypertension, metabolic disorders, and renal disease, and morbidly obese patients may require surgical interventions to produce sustained weight loss.

Melanocortin-4 Receptor Mediates Chronic Cardiovascular and Metabolic Actions of Leptin

This study tested whether the melanocortin 4-receptor (MC4R) is essential for the chronic cardiovascular and metabolic actions of leptin. Twenty- to 22-week–old male wild-type (WT) C57BL/6J and obese MC4R (−/−) mice (N=5 to 6 per group) were implanted with radiotelemetric transmitters and catheters for measuring mean arterial pressure (MAP) and heart rate 24 hours per day and intravenous infusions. After a 3-day stable control period, leptin was infused (2 &mgr;g/kg per minute IV) for 7 days in WT, obese ad libitum–fed MC4R (−/−), and nonobese pair–fed MC4R (−/−) mice. WT mice receiving vehicle for 7 days served as controls. MC4 (−/−) mice were 30% heavier and had 4- and 11-fold increases in plasma insulin and leptin levels, respectively, compared with WT mice. Despite obesity, MAP and heart rate tended to be lower in MC4R (−/−) mice compared with WT mice. Chronic leptin infusion in the different groups increased plasma leptin levels to 45 to 65 ng/mL. Seven-day leptin infusion in WT mice increased MAP by 12±3 mm Hg despite a 35% reduction in food intake and an 8% reduction in body weight. Leptin did not alter plasma glucose but reduced plasma insulin in WT mice (5.9±1.0 versus 3.0±0.5 &mgr;U/mL). These cardiovascular and metabolic actions of leptin were abolished in obese and nonobese MC4R (−/−) mice. These data suggest that MC4R deficiency, and not obesity-induced leptin resistance, abolished the cardiovascular and metabolic actions of leptin in obese MC4R (−/−) mice. Thus, a functional MC4R is essential for the chronic cardiovascular and metabolic actions of leptin.

Role of Hypothalamic Melanocortin 3/4-Receptors in Mediating Chronic Cardiovascular, Renal, and Metabolic Actions of Leptin

The present study examined whether blockade of melanocortin receptors subtypes 3 and 4 (MC3/4-R) inhibits chronic cardiovascular and dietary responses to leptin infusion. A cannula was placed in the lateral ventricle of male Sprague-Dawley rats for chronic intracerebroventricular (ICV) infusion via osmotic minipump, and arterial and venous catheters were implanted for measurement of mean arterial pressure (MAP) and heart rate (HR) 24 h/d and IV infusions. After a 5-day control period, rats received (1) 0.9% saline vehicle ICV for 12 days plus leptin (1 μg/kg per minute IV, n=5) during the final 7 days; (2) MC3/4-R antagonist SHU-9119 (1 nmol/h ICV) for 12 days plus leptin (1 μg/kg per minute IV, n=6) during the final 7 days; and (3) SHU-9119 (1 nmol/h ICV, n=8) for 12 days. Leptin infusion in vehicle-treated rats caused a small increase in MAP (5±1 mm Hg) despite reduced food intake (23±1 to 10±1 g/d) and decreased body weight (−6%±1%). SHU-9119 infusion completely prevented the cardiovascular and dietary actions of leptin, leading to increased food intake (23±1 to 49±4 g/d) and body weight (+30%±2%), markedly decreased HR (−77±9 bpm), and caused a decrease in MAP (−6±1 mm Hg). Similar results were observed when SHU-9119 was infused alone in vehicle-treated rats. Leptin decreased plasma insulin to 30% of control values, an effect that was also abolished by SHU-9119 treatment, which caused a 5-fold increase in plasma insulin concentration. Thus, MC3/4-R antagonism completely blocked the chronic cardiovascular, satiety, and metabolic effects of leptin, suggesting that the hypothalamic melanocortin system plays an important role in mediating these actions of leptin.

Control of Blood Pressure, Appetite, and Glucose by Leptin in Mice Lacking Leptin Receptors in Proopiomelanocortin Neurons

Although the central nervous system melanocortin system is an important regulator of energy balance, the role of proopiomelanocortin (POMC) neurons in mediating the chronic effects of leptin on appetite, blood pressure, and glucose regulation is unknown. Using Cre/loxP technology we tested whether leptin receptor deletion in POMC neurons (LepRflox/flox/POMC-Cre mice) attenuates the chronic effects of leptin to increase mean arterial pressure (MAP), enhance glucose use and oxygen consumption, and reduce appetite. LepRflox/flox/POMC-Cre, wild-type, LepRflox/flox, and POMC-Cre mice were instrumented for MAP and heart rate measurement by telemetry and venous catheters for infusions. LepRflox/flox/POMC-Cre mice were heavier, hyperglycemic, hyperinsulinemic, and hyperleptinemic compared with wild-type, LepRflox/flox, and POMC-Cre mice. Despite exhibiting features of metabolic syndrome, LepRflox/flox/POMC-Cre mice had normal MAP and heart rate compared with LepRflox/flox but lower MAP and heart rate compared with wild-type mice. After a 5-day control period, leptin was infused (2 &mgr;g/kg per minute, IV) for 7 days. In control mice, leptin increased MAP by ≈5 mm Hg despite decreasing food intake by ≈35%. In contrast, leptin infusion in LepRflox/flox/POMC-Cre mice reduced MAP by ≈3 mm Hg and food intake by ≈28%. Leptin significantly decreased insulin and glucose levels in control mice but not in LepRflox/flox/POMC-Cre mice. Leptin increased oxygen consumption in LepRflox/flox/POMC-Cre and wild-type mice. Activation of POMC neurons is necessary for the chronic effects of leptin to raise MAP and reduce insulin and glucose levels, whereas leptin receptors in other areas of the brain other than POMC neurons appear to play a key role in mediating the chronic effects of leptin on appetite and oxygen consumption.

Endogenous Melanocortin System Activity Contributes to the Elevated Arterial Pressure in Spontaneously Hypertensive Rats

Previous studies suggest that activation of the CNS melanocortin system reduces appetite while increasing sympathetic activity and arterial pressure. The present study tested whether endogenous activity of the CNS melanocortin 3/4 receptors (MC3/4-R) contributes to elevated arterial pressure in the spontaneously hypertensive rat (SHR), a model of hypertension with increased sympathetic activity. A cannula was placed in the lateral ventricle of male SHR and Wistar (WKY) rats for chronic intracerebroventricular (ICV) infusions (0.5 &mgr;L/h). Mean arterial pressure (MAP) and heart rate (HR) were recorded 24 hour/d using telemetry. After 5-day control period, rats were infused with MC3/4-R antagonist (SHU-9119, 1 nmol/h-ICV) for 12 days, followed by 5-day posttreatment period. MC3/4-R antagonism increased food intake in SHR by 90% and in WKY by 125%, resulting in marked weight gain, insulin resistance, and hyperleptinemia in SHR and WKY. Despite weight gain, MC3/4-R antagonism reduced HR in SHR and WKY (≈40 bpm), while lowering MAP to a greater extent in SHR (−22±4 mm Hg) than WKY (−4±3 mm Hg). SHU9119 treatment failed to cause further reductions in MAP during chronic adrenergic blockade with propranolol and terazosin. These results suggest that endogenous activity of the CNS melanocortin system contributes to the maintenance of adrenergic tone and elevated arterial pressure in SHR even though mRNA levels for POMC and MC4R in the mediobasal hypothalamus were not increased compared to WKY. These results also support the hypothesis that weight gain does not raise arterial pressure in the absence of a functional MC3/4-R.

Role of Adrenergic Activity in Pressor Responses to Chronic Melanocortin Receptor Activation

Abstract—Acute studies have shown that MC3/4-R stimulation increases sympathetic activity, but the role of adrenergic activation in mediating the cardiovascular and renal responses to chronic melanocortin 3- and 4-receptor (MC3/4-R) activation is unknown. The present study tested whether chronic MC3/4-R activation raises blood pressure and whether these changes are attenuated by &agr;1+&bgr;-adrenergic blockade. Rats were instrumented with an intracerebroventricular (ICV) cannula and arterial and venous catheters for measurements of mean arterial pressure (MAP) and heart rate (HR) 24 hours per day, and intravenous infusions. After control measurements, rats were intravenously infused with either saline vehicle (n=7) or &;1+ &bgr;-adrenergic antagonists (n=6, terazosin+propranolol, 10 mg/kg per day each) for 21 days. Five days after starting the vehicle or adrenergic blockade, the MC3/4-R agonist, MTII (10 ng/h), was infused ICV for 11 days followed by a 5-day recovery period. Another group of rats was infused with the adrenergic antagonists for 21 days but received the saline vehicle ICV for 11 days (n=7). MC3/4-R activation decreased food intake from 21±1 to 8±2 g/d by day 3 of MC3/4-R activation, and increased MAP and HR by an average of 8±2 mm Hg and 9±5 bpm, respectively. Adrenergic blockade did not alter the MC3/4-R-mediated decrease in food intake but abolished the increases in MAP and HR (1±2 mm Hg and −12±5 bpm, respectively, compared with control). ICV vehicle infusion during adrenergic blockade did not alter food intake or MAP. Glomerular filtration rate was unchanged in both the vehicle-infused and adrenergic blocked rats during MC3/4-R activation. These results indicate that the chronic actions of MC3/4-R activation on MAP and HR are mediated by adrenergic activation.

Impact of obesity on renal structure and function in the presence and absence of hypertension: evidence from melanocortin-4 receptor-deficient mice

The purpose of this study was to determine the long-term impact of obesity and related metabolic abnormalities in the absence and presence of hypertension on renal injury and salt-sensitivity of blood pressure. Markers of renal injury and blood pressure salt sensitivity were assessed in 52- to 55-wk-old normotensive melanocortin-4 receptor-deficient (MC4R-/-) mice and lean C57BL/6J wild-type (WT) mice and in 22-wk-old MC4R-/- and WT mice made hypertensive by N(G)-nitro-L-arginine methyl ester (L-NAME) in the drinking water for 8 wk. Old MC4R-/- mice were 60% heavier, hyperinsulinemic, and hyperleptinemic but had similar mean arterial pressure (MAP) as WT mice (115 +/- 2 and 117 +/- 2 mmHg) on normal salt diet (0.4% NaCl). A high-salt diet (4.0% NaCl) for 12 days did not raise MAP in obese or lean mice [DeltaMAP: MC4R (-/-) 4 +/- 2 mmHg; WT, 2 +/- 1 mmHg]. Obese MC4R-/- mice had 23% greater glomerular tuft area and moderately increased GFR compared with WT mice. Bowmans space, total glomerular area, mesangial matrix, urinary albumin excretion (UAE), renal TGF-beta and collagen expression were not significantly different between old MC4R-/- and WT mice. Renal lipid content was greater but renal macrophage count was markedly lower in MC4R-/- than WT mice. Mild increases in MAP during L-NAME treatment (approximately 16 mmHg) caused small, but greater, elevations in UAE, renal TGF-beta content, and macrophage infiltration in MC4R-/- compared with WT mice without significant changes in glomerular structure. Thus despite long-term obesity and multiple metabolic abnormalities, MC4R-/- mice have no evidence of renal injury or salt-sensitivity of blood pressure. These observations suggest that elevations in blood pressure may be necessary for obesity and related metabolic abnormalities to cause major renal injury or that MC4R-/- mice are protected from renal injury by mechanisms that are still unclear.