William G. Haynes

Receptor-mediated regional sympathetic nerve activation by leptin.

Leptin is a peptide hormone produced by adipose tissue which acts centrally to decrease appetite and increase energy expenditure. Although leptin increases norepinephrine turnover in thermogenic tissues, the effects of leptin on directly measured sympathetic nerve activity to thermogenic and other tissues are not known. We examined the effects of intravenous leptin and vehicle on sympathetic nerve activity to brown adipose tissue, kidney, hindlimb, and adrenal gland in anesthetized Sprague-Dawley rats. Intravenous infusion of mouse leptin over 3 h (total dose 10-1,000 microg/kg) increased plasma concentrations of immunoreactive murine leptin up to 50-fold. Leptin slowly increased sympathetic nerve activity to brown adipose tissue (+286+/-64% at 1,000 microg/kg; P = 0.002). Surprisingly, leptin infusion also produced gradual increases in renal sympathetic nerve activity (+228+/-63% at 1,000 microg/kg; P = 0.0008). The effect of leptin on sympathetic nerve activity was dose dependent, with a threshold dose of 100 microg/kg. Leptin also increased sympathetic nerve activity to the hindlimb (+287+/-60%) and adrenal gland (388+/-171%). Despite the increase in overall sympathetic nerve activity, leptin did not increase arterial pressure or heart rate. Leptin did not change plasma glucose and insulin concentrations. Infusion of vehicle did not alter sympathetic nerve activity. Obese Zucker rats, known to possess a mutation in the gene for the leptin receptor, were resistant to the sympathoexcitatory effects of leptin, despite higher achieved plasma leptin concentrations. These data demonstrate that leptin increases thermogenic sympathetic nerve activity and reveal an unexpected stimulatory effect of leptin on overall sympathetic nerve traffic.

Impairment of Endothelium-Dependent Vasodilation of Resistance Vessels in Patients With Obstructive Sleep Apnea

Background—Patients with obstructive sleep apnea (OSA) experience repetitive episodic hypoxemia with consequent sympathetic activation and marked blood pressure surges, each of which may impair endothelial function. We tested the hypothesis that patients with OSA have impaired endothelium-dependent vasodilation, even in the absence of overt cardiovascular disease. Methods and Results—We studied 8 patients with OSA (age 44±4 years) and 9 obese control subjects (age 48±3 years). Patients with OSA were newly diagnosed, never treated for OSA, on no medications, and free of any other known diseases. All obese control subjects underwent complete overnight polysomnographic studies to exclude occult OSA. Resistance-vessel function was tested by use of forearm blood flow responses to intra-arterial infusions of acetylcholine (a vasodilator that stimulates endothelial release of nitric oxide), sodium nitroprusside (an exogenous nitric oxide donor), and verapamil (a calcium channel blocker). Conduit-vessel function was also evaluated by ultrasonography. Brachial artery diameter was measured under baseline conditions, during reactive hyperemia (with flow increase causing endothelium-dependent dilatation), and after sublingual administration of nitroglycerin (an endothelium-independent vasodilator). Patients with OSA had a blunted vasodilation in response to acetylcholine (P <0.007), but responses to sodium nitroprusside and verapamil were not significantly different from those of control subjects. No significant difference in conduit-vessel dilation was evident between OSA patients and obese control subjects. Conclusions—Patients with OSA have an impairment of resistance-vessel endothelium-dependent vasodilation. This may be implicated in the pathogenesis of hypertension and heart failure in this condition.

Effects of obstructive sleep apnea on endothelin-1 and blood pressure

OBJECTIVE To evaluate blood pressure and humoral vasoconstrictor responses to recurrent episodes of obstructive sleep apnea and the effects of therapy by means of continuous positive airway pressure. PATIENTS AND METHODS We prospectively evaluated overnight changes in hemodynamics, oxygen saturation, the apnea-hypopnea index, circulating endothelin-1, norepinephrine and plasma renin activity in 22 patients with severe obstructive sleep apnea before and after successful therapy using continuous positive airway pressure therapy (three measurements). Measurements of endothelin-1 and blood pressure were also obtained on three occasions, at similar times, in 12 healthy control subjects without sleep disturbances. RESULTS Mean arterial pressure and endothelin-1 concentrations increased significantly after 4 h of untreated obstructive sleep apnea, and decreased after 5 h of continuous positive airway pressure. Changes in endothelin-1 levels were correlated with changes in mean arterial pressure (r = 0.44, P < 0.02) and with changes in oxygen saturation (r = 0.37, P < 0.05). Norepinephrine levels and plasma renin activity did not change significantly in patients with obstructive sleep apnea, and were not correlated with changes in blood pressure or oxygen saturation. In controls, blood pressure measurements at similar times during the night showed changes directionally opposite to that seen in obstructive sleep apnea, while endothelin-1 levels remained unchanged. CONCLUSIONS Sleep apnea elicits increases in blood pressure and endothelin-1, with reductions in both after treatment. Vasoconstrictor and mitogenic effects of endothelin-1 may be implicated in increased cardiovascular risk in patients with obstructive sleep apnea.

Role of Oxidant Stress in Endothelial Dysfunction Produced by Experimental Hyperhomocyst(e)inemia in Humans

BACKGROUND Moderate elevations in plasma homocyst(e)ine concentrations are associated with atherosclerosis and hypertension. We tested the hypothesis that experimental perturbation of homocysteine levels produces resistance and conduit vessel endothelial dysfunction and that this occurs through increased oxidant stress. METHODS AND RESULTS Oral administration of L-methionine (100 mg/kg) was used to induce moderate hyperhomocyst(e)inemia ( approximately 25 micromol/L) in healthy human subjects. Endothelial function of forearm resistance vessels was assessed by use of forearm vasodilatation to brachial artery administration of the endothelium-dependent dilator acetylcholine. Conduit vessel endothelial function was assessed with flow-mediated dilatation of the brachial artery. Forearm resistance vessel dilatation to acetylcholine was significantly impaired 7 hours after methionine (methionine, 477+/-82%; placebo, 673+/-110%; P=0.016). Methionine did not alter vasodilatation to nitroprusside and verapamil. Flow-mediated dilatation was significantly impaired 8 hours after methionine loading (0.3+/-2.7%) compared with placebo (8. 2+/-1.6%, P=0.01). Oral administration of the antioxidant ascorbic acid (2 g) prevented methionine-induced endothelial dysfunction in both conduit and resistance vessels (P=0.03). CONCLUSIONS Experimentally increasing plasma homocyst(e)ine concentrations by methionine loading rapidly impairs both conduit and resistance vessel endothelial function in healthy humans. Endothelial dysfunction in conduit and resistance vessels may underlie the reported associations between homocysteine and atherosclerosis and hypertension. Increased oxidant stress appears to play a pathophysiological role in the deleterious endothelial effects of homocysteine.

Interactions Between the Melanocortin System and Leptin in Control of Sympathetic Nerve Traffic

Leptin plays an important role in regulation of body weight through regulation of food intake and sympathetically mediated thermogenesis. The hypothalamic melanocortin system, via activation of the melanocortin-4 receptor (MC4-R), decreases appetite and weight, but its effects on sympathetic nerve activity (SNA) are unknown. In addition, it is not known whether sympathoactivation to leptin is mediated by the melanocortin system. We tested the interactions between these systems in regulation of brown adipose tissue (BAT) and renal and lumbar SNA in anesthetized Sprague-Dawley rats. Intracerebroventricular administration of the MC4-R agonist MT-II (200 to 600 pmol) produced a dose-dependent sympathoexcitation affecting BAT and renal and lumbar beds. This response was completely blocked by the MC4-R antagonist SHU9119 (30 pmol ICV). Administration of leptin (1000 microg/kg IV) slowly increased BAT SNA (baseline, 41+/-6 spikes/s; 6 hours, 196+/-28 spikes/s; P=0.001) and renal SNA (baseline, 116+/-16 spikes/s; 6 hours, 169+/-26 spikes/s; P=0.014). Intracerebroventricular administration of SHU9119 did not inhibit leptin-induced BAT sympathoexcitation (baseline, 35+/-7 spikes/s; 6 hours, 158+/-34 spikes/s; P=0.71 versus leptin alone). However, renal sympathoexcitation to leptin was completely blocked by SHU9119 (baseline, 142+/-17 spikes/s; 6 hours, 146+/-25 spikes/s; P=0.007 versus leptin alone). This study demonstrates that the hypothalamic melanocortin system can act to increase sympathetic nerve traffic to thermogenic BAT and other tissues. Our data also suggest that leptin increases renal SNA through activation of hypothalamic melanocortin receptors. In contrast, sympathoactivation to thermogenic BAT by leptin appears to be independent of the melanocortin system.

Sympathetic and Cardiorenal Actions of Leptin

Body weight is tightly regulated physiologically. The recent discovery of the peptide hormone leptin has permitted more detailed evaluation of the mechanisms responsible for control of body fat. Leptin is almost exclusively produced by adipose tissue and acts in the CNS through a specific receptor and multiple neuropeptide pathways to decrease appetite and increase energy expenditure. Leptin thus functions as the afferent component of a negative feedback mechanism to control adipose tissue mass. Increasing evidence suggests that leptin may have wider actions influencing autonomic, cardiovascular, and endocrine function. Intravenous leptin increases norepinephrine turnover and sympathetic nerve activity to thermogenic brown adipose tissue. Studies from our laboratory suggest that leptin also increases sympathetic nerve activity to kidney, hindlimb, and adrenal gland. However, systemic administration of leptin does not acutely increase arterial pressure or heart rate in anesthetized animals. Thus, longer-term exposure to hyperleptinemia may be necessary for full expression of the expected pressor effect of renal sympathoexcitation. Alternatively, leptin may have additional cardiovascular actions to oppose sympathetically mediated vasoconstriction. Leptin in high doses increases renal sodium and water excretion, apparently through a direct tubular action. In addition, leptin appears to increase systemic insulin sensitivity, even in the absence of weight loss. Although we are at an early stage of understanding, we speculate that abnormalities in the actions of leptin may have implications for the sympathetic, cardiovascular, and renal changes associated with obesity.

Selective leptin resistance: a new concept in leptin physiology with cardiovascular implications

Leptin, an adipocyte secreted hormone, acts in the hypothalamus to inhibit appetite and promote thermogenic metabolism, thereby reducing adiposity and body weight. Leptin has multiple autonomic and cardiovascular actions, including sympathetic activation, increases in endothelium derived nitric oxide (NO), and angiogenesis. The predominant cardiovascular effect of chronic hyperleptinemia is a pressor effect mediated by increased sympathetic activity. The sympathetic and cardiovascular actions of leptin are discussed and evidence derived from studies of obese mice for the novel concept of selective leptin resistance is reviewed. This concept holds that in some obese states, there is preservation of the sympathoexcitatory actions of leptin despite resistance to the satiety and weight-reducing actions of the hormone. Selective leptin resistance might explain how hyperleptinemia could contribute to increases in sympathetic activity and arterial pressure in obese states where there is resistance to the metabolic (satiety and weight-reducing) actions of leptin. It is speculated here, that this concept may have potential implications for human obesity, which is often associated with elevated plasma leptin and partial resistance to the satiety effects of leptin. If selective leptin resistance occurs in obese humans, then leptin could contribute to the sympathetic overactivity and hypertension despite resistance to its metabolic actions.

Contrasting blood pressure effects of obesity in leptin-deficient ob/ob mice and agouti yellow obese mice.

OBJECTIVE Recent advances in understanding the neuroendocrine pathways regulating appetite, metabolism and body weight afford an opportunity to explore further the mechanisms by which obesity influences arterial pressure. ob/ob(Lep(ob)/Lep(ob)) mice have a mutation in the ob gene and are leptin-deficient. Leptin possesses pressor actions and has been shown to increase arterial pressure when infused chronically or over-expressed transgenically. In contrast, agouti yellow obese(Ay) mice have overexpression of an agouti peptide that blocks melanocortin receptors. Stimulation of melanocortin receptors by alpha-melanocyte-stimulating hormone decreases arterial pressure. DESIGN AND METHODS This study measured arterial pressure in leptin-deficient ob/ob mice, agouti yellow obese mice and their lean controls to test the hypothesis that the effects of obesity on arterial pressure are importantly influenced by the genetic and neuroendocrine mechanisms causing the obesity. We measured arterial pressure directly in conscious ob/ob mice (n = 14), agouti yellow obese mice (n = 6) and the same number of lean littermates. RESULTS Body weight was nearly twice as high in ob/ob mice as in their lean controls, but mean arterial pressure was significantly lower in ob/ob mice (92+/-3 mmHg) compared with their lean controls (106+/-2 mmHg; P = 0.00017). In contrast, mean arterial pressure was significantly higher in agouti yellow obese mice (124+/-3 mmHg) than in their lean controls (99+/-1 mmHg; P = 0.000002) despite the fact that the agouti mice had milder obesity. CONCLUSIONS This study prompts three conclusions: (1) leptin-deficient ob/ob mice and agouti yellow obese mice have contrasting blood pressure responses to obesity, (2) obesity does not invariably increase arterial pressure in mice, and (3) the arterial pressure response to obesity may depend critically on the underlying genetic and neuroendocrine mechanisms.

Elevation of asymmetrical dimethylarginine may mediate endothelial dysfunction during experimental hyperhomocyst(e)inaemia in humans.

Hyperhomocyst(e)inaemia is associated with endothelial dysfunction in animals and humans. Mechanisms responsible for endothelial dysfunction in hyperhomocyst(e)inaemia are poorly understood, but may involve impaired bioavailability of endothelium-derived nitric oxide (NO). We hypothesized that acute elevation of homocyst(e)ine by oral methionine loading may stimulate the formation of asymmetrical dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, due to a transmethylation reaction during the formation of homocyst(e)ine from methionine. We studied nine healthy human subjects (five males, four females) aged 29+/-2 years. Flow-mediated vasodilation (FMD) in the brachial artery (endothelium-dependent) and vasodilation induced by nitroglycerine (endothelium-independent) were measured with high-resolution ultrasound before and 8 h after oral methionine (100 mg/kg in cranberry juice) or placebo (cranberry juice), on separate days and in random order. Plasma homocyst(e)ine and ADMA concentrations were measured by specific HPLC methods. After a methionine bolus, elevation of homocyst(e)ine (28.4+/-3.5 micromol/l) was associated with an increased plasma concentration of ADMA (2.03+/-0.18 micromol/l) and reduced FMD (1.54+/-0.92%). Placebo had no effect on these parameters. There was a significant inverse linear relationship between ADMA concentration and FMD (r=-0.49; P<0.05), which was stronger than the relationship between the homocyst(e)ine concentration and FMD (r=-0.36; not significant). We conclude that acute elevation of the homocyst(e)ine concentration impairs vascular endothelial function by a mechanism in which an elevated concentration of ADMA may be involved. This finding may have importance for understanding the mechanism(s) leading to homocyst(e)ine-associated vascular disease, and its potential treatment.

Obesity-Induced Hypertension: New Concepts From the Emerging Biology of Obesity

Abstract —Obesity is associated with an increased risk of hypertension. In the past 5 years there have been dramatic advances into the genetic and neurobiological mechanisms of obesity with the discovery of leptin and novel neuropeptide pathways regulating appetite and metabolism. In this brief review, we argue that these mounting advances into the neurobiology of obesity have and will continue to provide new insights into the regulation of arterial pressure in obesity. We focus our comments on the sympathetic, vascular, and renal mechanisms of leptin and melanocortin receptor agonists and on the regulation of arterial pressure in rodent models of genetic obesity. We suggest 3 concepts. First, the effect of obesity on blood pressure may depend critically on the genetic-neurobiological mechanisms underlying the obesity. Second, obesity is not consistently associated with increased blood pressure, at least in rodent models. Third, the blood pressure response to obesity may be critically influenced by modifying alleles in the genetic background.