John H. Dubinion

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.

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.

Chronic HO-1 induction with cobalt protoporphyrin (CoPP) treatment increases oxygen consumption, activity, heat production and lowers body weight in obese melanocortin-4 receptor-deficient mice

Objective:Heme oxygenase-1 induction (HO-1) elicits chronic weight loss in several rodent models of obesity. Despite these findings, the mechanism by which HO-1 induction reduces body weight is unclear. Chronic HO-1 induction does not alter food intake, suggesting other mechanisms such as increases in metabolism and activity may be responsible for the observed reduction of body weight. In this study, we investigated the mechanism of weight loss elicited by chronic HO-1 induction in a model of genetic obesity due to melanocortin-4 receptor (MC4R) deficiency.Design:Experiments were performed on loxTB MC4R-deficient mice as well as lean controls. Mice were administered cobalt protoporphyrin (CoPP, 5 mg kg−1), an inducer of HO-1, once weekly, from 4 to 23 weeks of age. Body weights were measured weekly and fasted blood glucose and insulin, as well as food intake were determined at 18 weeks of age. Oxygen consumption (VO2), CO2 production (VCO2), activity and body heat production were measured at 20 weeks of age.Results:Chronic CoPP treatment resulted in a significant decrease in body weight from 5 weeks on in loxTB mice. Chronic CoPP treatment resulted in a significant decrease in fasted blood glucose levels, plasma insulin and a significant increase in plasma adiponectin levels in MC4R-deficient mice. Chronic CoPP treatment increased VO2 (47±4 vs 38±3 ml kg−1 per min, P<0.05) and VCO2 (44±7 vs 34±4 ml kg−1 per min, P<0.05) in treated vs non-treated, MC4R-deficient mice (n=4). Heat production (10%) and activity (18%) were also significantly (P<0.05) increased in CoPP-treated MC4R-deficient mice.Conclusion:Our results suggest that chronic HO-1 induction with CoPP induction elicits weight loss by increasing metabolism and activity by an MC4R-independent pathway.

Enhanced blood pressure and appetite responses to chronic central melanocortin-3/4 receptor blockade in dietary-induced obesity

Method We examined the role of central nervous system (CNS) endogenous melanocortin 3/4 receptors (MC3/4R) activity in controlling cardiovascular and metabolic functions in Sprague Dawley rats fed a high-fat diet (n = 6) for 10 months compared with rats fed a standard chow (normal fat, n = 8) starting at 3 weeks of age. Results At 7 months of age, high-fat rats were heavier (473 ± 3 vs. 424 ± 7 g), consumed more calories with larger, less frequent meals and had reduced respiratory quotient (RQ) compared with normal-fat rats. After 10 months on the diets, arterial and venous catheters were implanted for measurement of mean arterial pressure (MAP) and heart rate (HR) 24-h/day and i.v. (intravenous) infusions, and a 21G steel cannula was placed in the lateral ventricle for intracerebroventricular (ICV) infusions. High-fat rats were heavier (528 ± 14 vs. 477 ± 11 g) with increased visceral adiposity and significantly higher MAP (108 ± 3 vs. 102 ± 1 mmHg). After a 5-day control period, the rats were infused with a MC3/4R antagonist (SHU-9119, 1 nmol/h, ICV) for 10 days followed by a 5-day recovery period. SHU-9119 infusion for 10 days increased caloric intake significantly more in high-fat rats (159 ± 19 vs. 64 ± 8 kcal). Despite increasing caloric intake and rapid weight gain, MC3/4R antagonism reduced MAP more in high-fat diet compared with normal-fat rats (−7.9 ± 0.3 vs. −4.7 ± 1.3 mmHg, average reduction of last 4 days of blockade). Conclusion These observations suggest that a high-fat diet increases endogenous activity of the CNS MC3/4R and that an intact MC3/4 appears to play an important role in linking increased blood pressure with diet-induced obesity.

Control of Metabolic and Cardiovascular Function by the Leptin–Brain Melanocortin Pathway

Obesity is recognized as a major worldwide health problem. Excess weight gain is the most common cause of elevated blood pressure (BP) and markedly increases the risk of metabolic, cardiovascular and renal diseases. Although the mechanisms linking obesity with hypertension have not been fully elucidated, increased sympathetic nervous system (SNS) activity contributes to elevated BP in obese subjects. Recent evidence indicates that leptin and the central nervous system (CNS) melanocortin system, including melanocortin 4 receptors (MC4R), play a key role in linking obesity with increased SNS activity and hypertension. Leptin, a peptide‐hormone produced by adipose tissue, crosses the blood–brain barrier and activates brain centers that control multiple metabolic functions as well as SNS activity and BP via the CNS melanocortin system. The crosstalk between peripheral signals (e.g., leptin) and activation of CNS pathways (e.g., MC4R) that regulate energy balance, SNS activity and BP represents an important target for treating obesity and its metabolic and cardiovascular consequences.

Role of Proopiomelanocortin Neuron Stat3 in Regulating Arterial Pressure and Mediating the Chronic Effects of Leptin

Although signal transducer and activator of transcription 3 (Stat3) is a key second messenger by which leptin regulates appetite and body weight, its role in specific neuronal populations in metabolic regulation and in mediating the chronic effects of leptin on blood pressure is unknown. The current study tested the hypothesis that Stat3 signaling in proopiomelanocortin (POMC) neurons mediates the chronic effects of leptin on mean arterial pressure (MAP), as well as on glucose regulation, energy expenditure, and food intake. Stat3flox/flox mice were crossed with POMC-Cre mice to generate mice with Stat3 deletion specifically in POMC neurons (Stat3flox/flox/POMC-Cre). Oxygen consumption (VO2), carbon dioxide respiration (VCO2), motor activity, heat production, food intake, and MAP were measured 24 hours/d. After baseline measurements, leptin was infused (4 &mgr;g/kg per min, IP) for 7 days. Stat3flox/flox/POMC-Cre mice were hyperphagic, heavier, and had increased respiratory quotients compared with control Stat3flox/flox mice. Baseline MAP was not different between the groups, and chronic leptin infusion reduced food intake similarly in both groups (27 versus 29%). VO2, VCO2, and heat production responses to leptin were not significantly different in control and Stat3flox/flox/POMC-Cre mice. However, leptin-mediated increases in MAP were completely abolished, and blood pressure responses to acute air–jet stress were attenuated in male Stat3flox/flox/POMC-Cre mice. These results indicate that Stat3 signaling in POMC neurons is essential for leptin-mediated increases in MAP, but not for anorexic or thermogenic effects of leptin.

Chronic Central Nervous System MC3/4R Blockade Attenuates Hypertension Induced by Nitric Oxide Synthase Inhibition but Not by Angiotensin II Infusion

We examined whether central melanocortin 3 and 4 receptor (MC3/4R) blockade attenuates the blood pressure (BP) responses to chronic L-NAME or angiotensin II (Ang II) infusion in Sprague–Dawley rats implanted with telemetry transmitters, venous catheters, and intracerebroventricular cannula into the lateral ventricle. After 5 days of control measurements, L-NAME (10 &mgr;g/kg/min IV, groups 1 and 2) or Ang II (10 ng/kg/min IV, groups 3 and 4) were infused for 24 days, and starting on day 7 of L-NAME or Ang II infusion, the MC3/4R antagonist SHU-9119 (24 nmol/d, n=6/group; groups 1 and 3) or vehicle (saline 0.5 &mgr;L/h, n=6/group; groups 2 and 4) was infused intracerebroventricularly for 10 days. A control normotensive group also received SHU-9119 for 10 days (n=5). L-NAME and Ang II increased BP by 40±3 and 56±5 mm Hg, respectively, although heart rate was slightly reduced. MC3/4R blockade doubled food intake and reduced heart rate (≈40 to ≈50 bpm) in all groups. MC3/4R blockade caused only a small reduction in BP in normotensive group (4 mm Hg) and no change in rats receiving Ang II, although markedly reducing BP by 21±4 mm Hg in L-NAME–treated rats. After SHU-9119 infusion was stopped, food intake, heart rate, and BP gradually returned to values observed before SHU-9119 infusion was started. Ganglionic blockade at the end of L-NAME or Ang II infusion caused similar BP reduction in both groups. These results suggest that the brain MC3/4R contributes, at least in part, to the hypertension induced by chronic L-NAME infusion but not by Ang II.