Lakshmi S. Tallam

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.

Obesity Exacerbates Sepsis-Induced Inflammation and Microvascular Dysfunction in Mouse Brain

Objective: Obese patients with sepsis have higher morbidity and mortality than lean counterparts, but the mechanisms involved are unknown. The authors examined the inflammatory and thrombogenic responses of the cerebral microvasculature to sepsis induced by cecal ligation and perforation in obese and lean wild‐type mice.

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.

A Functional Melanocortin System May Be Required for Chronic CNS-Mediated Antidiabetic and Cardiovascular Actions of Leptin

OBJECTIVE We recently showed that leptin has powerful central nervous system (CNS)-mediated antidiabetic and cardiovascular actions. This study tested whether the CNS melanocortin system mediates these actions of leptin in diabetic rats. RESEARCH DESIGN AND METHODS A cannula was placed in the lateral ventricle of Sprague-Dawley rats for intracerebroventricular infusions, and arterial and venous catheters were implanted to measure mean arterial pressure (MAP) and heart rate 24 h/day and for intravenous infusions. After recovery from surgery for 8 days, rats were injected with streptozotocin (STZ), and 5 days later, either saline or the melanocortin 3 and 4 receptor (MC3/4R) antagonist SHU-9119 (1 nmol/h) was infused intracerebroventricularly for 17 days. Seven days after starting the antagonist, leptin (0.62 μg/h) was added to the intracerebroventricular infusion for 10 days. Another group of diabetic rats was infused with the MC3/4R agonist MTII (10 ng/h i.c.v.) for 12 days, followed by 7 days at 50 ng/h. RESULTS Induction of diabetes caused hyperphagia, hyperglycemia, and decreases in heart rate (−76 bpm) and MAP (−7 mmHg). Leptin restored appetite, blood glucose, heart rate, and MAP back to pre-diabetic values in vehicle-treated rats, whereas it had no effect in SHU-9119–treated rats. MTII infusions transiently reduced blood glucose and raised heart rate and MAP, which returned to diabetic values 5–7 days after starting the infusion. CONCLUSIONS Although a functional melanocortin system is necessary for the CNS-mediated antidiabetic and cardiovascular actions of leptin, chronic MC3/4R activation is apparently not sufficient to mimic these actions of leptin that may involve interactions of multiple pathways.

Role of Endothelin-1 in Blood Pressure Regulation in a Rat Model of Visceral Obesity and Hypertension

Abstract—Endothelial dysfunction has been suggested to play an important role in the development of obesity-induced hypertension. Because endothelin release increases in response to endothelial damage, we examined whether endothelin-1 contributes to increased arterial pressure in a model of visceral obesity produced by feeding Sprague-Dawley rats a high-fat (HF) diet (40% fat w/w, n=6) for 12 months. Arterial and venous catheters were implanted for measurement of mean arterial pressure (MAP) and heart rate (HR) 24 hours per day and intravenous infusions. After a 5-day control period, rats were infused with the selective endothelin-1 type A receptor (ET-A) blocker ABT-627 (2.5 mg/kg per day, IV) for 9 days, followed by a recovery period. Rats fed a standard chow (normal fat, or NF, group: n=6) for 12 months were also infused with ET-A blocker and were used as controls. Compared with NF rats, HF rats had higher MAP (113±4 versus 98±2 mm Hg), increased visceral fat (18.7±2.0 versus 10.8±1.4 g), and 3.2-fold increase in plasma leptin despite similar total body weight gain. Long-term ET-A blockade markedly reduced MAP in HF (−14±3 mm Hg) and NF (−14±2 mm Hg), but it had no effect on HR, GFR, or PRA. These results indicate that a long-term HF diet may cause visceral obesity and increased MAP, even in the absence of major changes in total body weight. Endothelin-1 appears to play an important role in the maintenance of arterial pressure in rats fed HF and NF diets, but it does not appear to contribute to increased MAP in this model of diet-induced hypertension.

Does Obesity Induce Resistance to the Long-Term Cardiovascular and Metabolic Actions of Melanocortin 3/4 Receptor Activation?

Previous studies suggest that blockade of melanocortin 3 and 4 receptors (MC3/4-R) markedly attenuates the chronic hypertensive effects of leptin. Although obesity has been reported to be associated with leptin “resistance,” it is unclear whether obesity alters the cardiovascular and metabolic effects of chronic MC3/4-R activation. Therefore, we tested whether the cardiovascular and metabolic actions of MC3/4-R activation are attenuated in Sprague-Dawley rats fed a high-fat diet (HF, n=6) compared with rats fed a standard chow (NF, n=6) for 12 months. A 21G steel cannula was placed in the lateral ventricle for ICV infusion, and arterial and venous catheters were implanted for measurement of mean arterial pressure (MAP) 24 hours/day and IV infusions. After a 5-day control period, rats were infused with MC3/4-R agonist melanotan II (10 ng/h, ICV), for 10 days followed by a 5-day recovery period. HF rats were heavier (558±21 versus 485±13 g) with 140% more visceral fat than NF rats, hyperleptinemic (8.9±0.5 versus 2.7±0.5 ng/mL), and insulin resistant. HF rats also had higher MAP (109±3 versus 100±1 mm Hg). Chronic melanotan II infusion significantly increased MAP in HF and NF (7±2 and 6±1 mm Hg), decreased caloric intake (−32±2 and −25 ±2 kcal/day), and reduced insulin levels in both groups by ≈50%. Thus, the metabolic and cardiovascular actions of chronic MC3/4-R activation are preserved in diet-induced obesity, supporting a potential role for the hypothalamic melanocortin system in obesity hypertension.

Chronic central nervous system hyperinsulinemia and regulation of arterial pressure and food intake.

Background and objective Acute studies suggest that insulin has anorexic and hypertensive actions through direct effects on the central nervous system (CNS). However, the importance of these actions in the long-term regulation of food intake and cardiovascular function is still unclear. The goal of the present study was to determine whether chronic increases in CNS levels of insulin, in the absence of changes in plasma insulin concentration, decrease food intake and raise arterial pressure in rats. Methods An intracerebroventricular cannula was placed in the lateral ventricle of male Sprague–Dawley rats for chronic insulin or vehicle (artificial cerebrospinal fluid, 0.5 μl/h) infusion, via osmotic minipump, and arterial and venous catheters were implanted for measurement of mean arterial pressure (MAP) and heart rate (HR) 24 h/day and intravenous infusions. After 4 days of control measurements, insulin (6 mU/h, n = 7 or 12 mU/h, n = 6) or vehicle (n = 4) was infused intracerebroventricularly for 7 days. Results Insulin infusion at 6 or 12 mU/h caused transient decreases in food intake, but did not significantly alter plasma glucose or insulin concentrations. Insulin intracerebroventricular infusion also did not alter MAP or HR (106 ± 2 and 105 ± 5 versus 104 ± 2 mmHg in control; 411 ± 9 and 407 ± 9 versus 410 ± 7 bpm in control, respectively). Food intake, MAP and HR remained unchanged in the vehicle-treated group. Conclusions These findings provide no evidence that insulin plays a major role in the long-term regulation of appetite, arterial pressure or HR. However, the CNS actions of insulin may exert modest acute effects on appetite that could play a role in limiting meal size and influencing caloric balance.

Cardiovascular, Renal, and Metabolic Responses to Chronic Central Administration of Agouti-Related Peptide

Although excess hypothalamic agouti-related peptide (AGRP), an endogenous antagonist of the melanocortin 3/4 receptor, causes hyperphagia and obesity, its role in regulating cardiovascular function is unclear. This study examined control of mean arterial pressure (MAP), heart rate (HR), and metabolism during chronic central administration of AGRP in rats. A cannula was placed in the lateral ventricle for intracerebroventricular infusion, and arterial and venous catheters were implanted for monitoring MAP and HR 24 hours per day, as well as intravenous infusions. After a 5-day control period, rats received AGRP (n=6; 0.02 nmol per hour ICV) or artificial cerebrospinal fluid (aCSF; n=9; 0.02 nmol per hour ICV) for 12 days, followed by a 5-day recovery period. A third group was infused intracerebroventricularly with AGRP and pair-fed to match food intake of control rats (n=7). AGRP produced a peak decrease in MAP and HR of −7±2 mm Hg and −68±7 bpm, respectively, despite increased food intake (from 23±0.5 to 36±3 g per day) and weight gain (from 350±8 to 454±5 g). AGRP also increased glomerular filtration rate, plasma insulin, glucose, and leptin. AGRP infusion in pair-fed rats produced a peak decrease in HR of −70±8 bpm but did not alter MAP or other variables. The metabolic effects of AGRP may be secondary to hyperphagia because they were abolished in pair-fed rats. aCSF infusion did not change any of the variables studied. These results demonstrate that increased central nervous system AGRP levels produce chronic reductions in MAP and HR despite marked increases in food intake and weight gain that normally tend to raise arterial pressure.

Influence of plasma insulin levels on antinatriuretic and vasoconstrictor actions of angiotensin-II.

The objective of the present study is to investigate whether plasma insulin levels play a role in the antinatriuretic and vasoconstrictor actions of angiotensin‐II (Ang‐II). We evaluated antinatriuretic function of endogenous Ang‐II using an AT1 receptor antagonist, candesartan in anesthetized Sprague‐Dawley rats. In control rats, candesartan produced significant increases in natriuresis and diuresis and these effects were abolished in streptozocin (STZ, 55 mg/kg i.p.) treated rats. Replacement of insulin restored these renal effects of candesartan. In a separate group of rats pretreated with an autonomic ganglionic blocker, pressor responses to Ang‐II and norepinephrine (NE) before or after L‐NNA, a nitric oxide synthase inhibitor were not affected by STZ treatment. However, insulin replacement greatly augmented these responses. These data provide evidence in vivo showing that insulin can enhance both antinatriuretic and vasoconstrictor actions of Ang‐II. Hence exaggerated renal and vascular effects of Ang‐II in the obese Zucker rats observed in our previous studies may be related to hyperinsulimemia and this phenomena could contribute to salt‐sensitivity and development of sustained hypertension.