Chronic antidiabetic and cardiovascular actions of leptin: role of CNS and increased adrenergic activity

This study examined the importance of direct central nervous system (CNS) actions and increased adrenergic activity in mediating the chronic antidiabetic and cardiovascular actions of leptin. Insulin-deficient rats (streptozotocin, 50 mg/kg) were used to examine the effects of leptin on glucose homeostasis independent of changes in insulin. Male Sprague-Dawley rats were instrumented with arterial and venous catheters and intracerebroventricular cannula for 24-h/day blood pressure (BP) and heart rate (HR) monitoring and intravenous and intracerebroventricular infusions. Insulin-deficient diabetes was associated with marked hyperglycemia, hyperphagia, decreased BP, and pronounced fall in HR. Leptin treatment, intravenous or intracerebroventricular, completely restored to control values plasma glucose levels (384+/-58 to 102+/-28 and 307+/-38 to 65+/-7 mg/dl, respectively), food intake, BP, and HR (304+/-8 to 364+/-7 and 317+/-13 to 423+/-9 bpm, respectively). Combined blockade of alpha1-, beta1-, and beta2-adrenergic receptors attenuated the rise in HR by 30 to 50% but had no effect on the antidiabetic and dietary actions of leptin. Blockade of beta3-adrenergic receptors did not attenuate the chronic cardiovascular or metabolic effects of leptin. These data demonstrate that leptin, via its direct actions in the CNS, has powerful antidiabetic actions in insulin-deficient rats independent of increased peripheral alpha1, beta1, beta2, and beta3-adrenergic activity. Leptin also exerts important long-term cardiovascular actions that are partially mediated via alpha1- and beta1/beta2-adrenergic activation. These findings provide new insights into novel pathways for long-term control of glucose homeostasis and cardiovascular regulation.     

Paracrine actions of the stomach-derived leptin

Leptin, a 16 kilodalton protein-encoded by the ob gene, is involved in the regulation of food intake, body composition, and energy expenditure through a central feedback mechanism. Initially thought to be adipocyte-specific, the ob gene, as well as the leptin receptor, has been found in a variety of other tissues. Relevant to this review, the leptin gene and its receptor have been identified in the stomach, intestine, liver, and pancreas. Recent data also suggest that gut leptin may act locally within the gastrointestinal tract to influence intestinal functions such as nutrient absorption and may have a physiopathological implication. This review emphasises the concept that leptin may be a new gastrointestinal hormone.     

Modulation of the cardiovascular system by leptin

It is well established that individuals with the metabolic syndrome have a significantly increased risk of cardiovascular disease and much effort has been expended to elicit the underlying mechanisms. Various studies have proposed that excessive or deficient physiological effects mediated by leptin make an important contribution, yet many paradoxical observations often preclude a clear definition of the role of leptin. This review article will briefly discuss principal and most recent evidence on direct and indirect regulation of the cardiovascular system by leptin, focusing on cardiac structural and functional as well as vascular effects.     

Circulating leptin and stress-induced cardiovascular activity in humans

Obesity is associated with an elevated risk of hypertension and cardiovascular disease. The adipocyte hormone leptin, which stimulates energy expenditure in animals by activating the sympathetic nervous system (SNS), is believed to play a role in this association. However, evidence in humans remains sparse. We investigated the relationship between circulating leptin and cardiovascular and inflammatory responses to acute psychological stress in humans. Participants were 32 men and 62 women aged 18-25 years. Cardiovascular activity was assessed using impedance cardiography at baseline, during acute laboratory stress, and during a 45-min recovery period. Plasma cytokines were measured in blood drawn at baseline and 45-min poststress. In women only, baseline plasma leptin was significantly associated with stress-induced changes in heart rate (beta = 0.53, P = 0.006), heart rate variability (HRV) (beta = -0.44, P = 0.015), and cardiac preejection period (PEP) (beta = -0.51, P = 0.004), independent of age, adiposity, and smoking. Women's plasma leptin levels also correlated with stress-induced elevations in the proinflammatory cytokine interleukin-6 (IL-6) (beta = 0.35, P = 0.042). Circulating leptin is an independent predictor of sympathetic cardiovascular activity, parasympathetic withdrawal, and inflammatory responses to stress in women. Because cardiovascular and inflammatory stress responses are predictive of future cardiovascular disease, leptin may be a mechanism mediating the adverse effects of stress and obesity on women's cardiovascular health.     

Central benzodiazepine involvement in clonidine cardiovascular actions

It is well known that the GABAergic and noradrenergic systems play an important role in blood pressure and heart rate regulation. Benzodiazepines and beta-carbolines, respectively, increase or decrease the probability of chloride-channel opening induced by GABA. The aim of this study was to determine, in conscious rats, the interaction existing between the central alpha2-adrenoceptor stimulation induced by clonidine and the facilitation or impairment of benzodiazepine receptor activity through the administration of either diazepam, a benzodiazepine receptor agonist, or methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), an inverse benzodiazepine agonist. Clonidine (5-10 microg, intracerebroventricularly) reduced heart rate and increased mean blood pressure by activation of central alpha2-adrenoceptors. Diazepam (2 mg/kg, intravenously (i.v.)) induced an increase in heart rate, while DMCM (0.3 mg/kg, i.v.) elicited a bradycardic effect. The bradycardic effects induced by both clonidine and DMCM were antagonized by the prior administration of methylatropine (1.5 mg/kg, i.v.). DMCM (0.3 mg/kg, i.v.) prevented the clonidine effects on heart rate and mean blood pressure, while diazepam (2 mg/kg, i.v.) failed to modify these effects. Our results suggest that the bradycardic effects of clonidine are mediated by a vagal stimulation and are related to the activation of a GABAergic pathway.     

Central cardiovascular actions of vasopressin in the rat

Arginine vasopressin (AVP) containing neurones and pathways have been localized in various cardiovascular control centers of the central nervous system in rats. AVP influences cardiovascular regulation when injected into various areas of the central nervous system. The blood pressure increases in response to central AVP injections were shown to be initiated by stimulation of central V1-AVP receptors and mediated by stimulation of sympathetic outflow to the periphery. On the other hand, AVP has also been shown to attenuate the pressor responses to electrical stimulation of the mesencephalic reticular formation when injected into the brain ventricular system. In addition, AVP can participate in cardiovascular regulation by modulating baroreceptor reflex sensitivity. We have shown that in rats peripheral (hormonal) AVP can sensitize the heart rate component of the baroreceptor reflex by acting on V2-AVP receptors accessible from the blood, while at the same time central (neuronal) AVP can attenuate the baroreceptor reflex through brain V1-AVP receptors that cannot be reached from the blood. Binding and functional studies favour the existence of V1-AVP receptors in the central nervous system, whereas evidence for central V2-AVP receptors is still scarce. The role of AVP in hypertension remains controversial, but recent evidence suggests that a discordance between the various central and peripheral cardiovascular actions of AVP, rather than its hormonal vasopressor effects, may contribute to the pathogenesis of hypertension.     

Potent cardiovascular actions of homologous adrenomedullins in eels

Adrenomedullin (AM), known as a multifunctional hormone in mammals, forms a unique family of five paralogous peptides in teleost fish. To examine their cardiovascular effects using homologous AMs in eels, we isolated cDNAs encoding four eel AMs, and named AM1 (ortholog of mammalian AM), AM2, AM3 (paralog of AM2 generated only in teleost lineage), and AM5 according to the known teleost AM sequences. Unlike pufferfish, not only AM1 but AM2/3 and AM5 were expressed ubiquitously in various eel tissues. Synthetic mature AM1, AM2, and AM5 exhibited vasodepressor effects after intra-arterial injections, and the effects were more potent at dorsal aorta than at ventral aorta. This indicates that AMs preferentially act on peripheral resistance vessels rather than on branchial arterioles. The potency was in the order of AM2 = AM5 > AM1 in both freshwater (FW) and seawater (SW) eels, which is different from the result of mammals in which AM1 is as potent as, or more potent than, AM2 when injected peripherally. The minimum effective dose of AM2 and AM5 in eels was 1/10 that of AM1 in mammals. The hypotension reached 50% at 1.0 nmol/kg of AM2 and AM5, which is much greater than atrial natriuretic peptide (20%), another potent vasodepressor hormone. Even with such hypotension, AMs did not change heart rate in eels. In addition, AM1 increased blood pressure at ventral aorta and dorsal aorta immediately after an initial hypotension at 5.0 nmol/kg, but not with AM2 and AM5. These data strongly suggest that specific receptors for AM2 and AM5 exist in eels, which differ from the AM1 receptors identified in mammals.     

The cardiovascular actions of centrally administered neuropeptide Y

The cardiovascular actions of intracerebroventricular (i.c.v.) administration of neuropeptide Y (NPY) were examined in conscious, unrestrained rats. A prolonged decrease in heart rate (HR) and a fall in mean arterial pressure (MAP) were obtained following i.c.v. administration of NPY (1 and 10 micrograms). Passive immunization with an antiserum directed against NPY confirmed that the slowing of HR following i.c.v. administration of NPY was mediated via a central nervous mechanism and not from leakage of NPY out of the brain. Administration of NPY into different brain parenchymal regions identified a putative site of action in the rostral region of the solitary tract. The mechanism of the decrease in HR caused by centrally administered NPY was investigated by i.c.v. administration of NPY to animals that were pretreated with agents that altered autonomic tone. Administration of NPY to atropine-treated animals produced a reversal of the atropine-induced tachycardia, suggesting that the NPY-induced decrease in HR was not due to augmented vagal tone. However, administration of NPY to animals pretreated with propranolol did not significantly lower HR below that obtained with propranolol alone. These data suggest that i.c.v. administration of NPY may cause a decrease in cardiac sympathetic outflow. The effects of centrally administered NPY on baroreflex function were studied. The changes in HR caused by NPY did not significantly alter baroreflex set-point or gain. These studies provide evidence that NPY acted within a brainstem region to decrease sympathetic nervous outflow, resulting in a decrease in HR and MAP.     

Chronic administration of leptin in Asian Blue Quail

It is well known that leptin has the capacity to reduce food intake, cause body weight loss, and increase energy expenditure in several vertebrate species. In this study, we investigated the effects of chronically elevated leptin levels on behavior and physiology of Asian Blue Quail (Coturnix chinensis). Fifteen male quail were treated with chicken leptin dissolved in phosphate-buffered saline (PBS) via subcutaneously inserted osmotic pumps that released approximately 1 microg/g body weight/day during a 14-day period. Another 15 males acted as controls and their pumps released PBS only. All males were housed together with two females. We observed a decrease in body weight and feeding behavior in leptin-treated birds, but not in control birds, after 2 days of treatment. Thereafter, all birds increased in weight. Males treated with leptin were more active and more likely to preen the day after the beginning of the treatment. Plasma cholesterol levels in leptin birds decreased during the first week of treatment and plasma triglycerides tended to remain lower compared to the controls during the whole 2-week period of treatment. Glucose levels appeared stable during the observation period. Leptin-treated males remained closer to accompanying females than did control males, and females together with leptin males took longer to lay their first egg compared to females together with control males. This is the first article showing the effect of leptin on cholesterol and triglyceride levels in birds. We also observed a change in the activity and male-female interaction pattern in tested quail.     

ENU mutagenesis identifies mice with morbid obesity and severe hyperinsulinemia caused by a novel mutation in leptin

Background

Obesity is a multifactorial disease that arises from complex interactions between genetic predisposition and environmental factors. Leptin is central to the regulation of energy metabolism and control of body weight in mammals.

Methodology/Principal Findings

To better recapitulate the complexity of human obesity syndrome, we applied N-ethyl-N-nitrosourea (ENU) mutagenesis in combination with a set of metabolic assays in screening mice for obesity. Mapping revealed linkage to the chromosome 6 within a region containing mouse Leptin gene. Sequencing on the candidate genes identified a novel T-to-A mutation in the third exon of Leptin gene, which translates to a V145E amino acid exchange in the leptin propeptide. Homozygous Leptin^145E/145E mutant mice exhibited morbid obesity, accompanied by adipose hypertrophy, energy imbalance, and liver steatosis. This was further associated with severe insulin resistance, hyperinsulinemia, dyslipidemia, and hyperleptinemia, characteristics of human obesity syndrome. Hypothalamic leptin actions in inhibition of orexigenic peptides NPY and AgRP and induction of SOCS1 and SOCS3 were attenuated in Leptin^145E/145E mice. Administration of exogenous wild-type leptin attenuated hyperphagia and body weight increase in Leptin^145E/145E mice. However, mutant V145E leptin coimmunoprecipitated with leptin receptor, suggesting that the V145E mutation does not affect the binding of leptin to its receptor. Molecular modeling predicted that the mutated residue would form hydrogen bond with the adjacent residues, potentially affecting the structure and formation of an active complex with leptin receptor within that region.

Conclusions/Significance

Thus, our evolutionary, structural, and in vivo metabolic information suggests the residue 145 as of special function significance. The mouse model harboring leptin V145E mutation will provide new information on the current understanding of leptin biology and novel mouse model for the study of human obesity syndrome.     

Central ventilatory and cardiovascular actions of angiotensin peptides in trout

In the brains of teleosts, angiotensin II (ANG II), one of the main effector peptides of the renin-angiotensin system, is implicated in various physiological functions notably body fluid and electrolyte homeostasis and cardiovascular regulation, but nothing is known regarding the potential action of ANG II and other angiotensin derivatives on ventilation. Consequently, the goal of the present study was to determine possible ventilatory and cardiovascular effects of intracerebroventricular injection of picomole doses (5-100 pmol) of trout [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, ANG IV, and ANG 1-7 into the third ventricle of unanesthetized trout. The central actions of these peptides were also compared with their ventilatory and cardiovascular actions when injected peripherally. Finally, we examined the presence of [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, and ANG IV in the brain and plasma using radioimmunoassay coupled with high-performance liquid chromatography. After intracerebroventricular injection, [Asn(1)]-ANG II and [Asp(1)]-ANG II two ANG IIs, elevated the total ventilation through a selective stimulatory action on the ventilation amplitude. However, the hyperventilatory effect of [Asn(1)]-ANG II was threefold higher than the effect of [Asp(1)]-ANG II at the 50-pmol dose. ANG III, ANG IV, and ANG 1-7 were without effect. In addition, ANG IIs and ANG III increased dorsal aortic blood pressure (P(DA)) and heart rate (HR). After intra-arterial injections, none of the ANG II peptides affected the ventilation but [Asn(1)]-ANG II, [Asp(1)]-ANG II, and ANG III elevated P(DA) (50 pmol: +80%, +58% and +48%, respectively) without significant decrease in HR. In brain tissue, comparable amounts of [Asn(1)]-ANG II and [Asp(1)]-ANG II were detected (ca. 40 fmol/mg brain tissue), but ANG III was not detected, and the amount of ANG IV was about eightfold lower than the content of the ANG IIs. In plasma, ANG IIs were also the major angiotensins (ca. 110 fmol/ml plasma), while significant but lower amounts of ANG III and ANG IV were present in plasma. In conclusion, our study suggests that the two ANG II isoforms produced within the brain may act as a neurotransmitter and/or neuromodulator to regulate the cardioventilatory functions in trout. In the periphery, two ANG IIs and their COOH-terminal peptides may act as a circulating hormone preferentially involved in cardiovascular regulations.     

Central and peripheral cardiovascular actions of apelin in conscious rats

APJ was cloned as an orphan G protein-coupled receptor and shares a close identity with angiotensin II type 1 receptor (AT1R). Apelin is a peptide that has recently been identified as an endogenous ligand of the APJ. Apelin and APJ mRNA are expressed in peripheral tissue and the central nervous system. However, little is known about the effects of apelin in cardiovascular regulation. To examine the central and peripheral role of apelin, we injected the active fragment of apelin [(Pyr1)apelin-13] intracerebroventricularly (ICV, 5 and 20 nmol, n=6) or intravenously (IV, 20 and 50 nmol, n=4 or 5) in conscious rats. ICV injection of (Pyr1)apelin-13 dose-dependently increased mean arterial pressure (MAP) and heart rate (HR) (19+/-3 mm Hg and 162+/-26 bpm at 20 nmol). Pretreatment with ICV injection of the AT1R antagonist (CV-11974, 20 nmol) did not alter the apelin-induced increase in MAP and HR. IV injection of (Pyr1)apelin-13 also dose-dependently increased MAP and HR (13+/-2 mm Hg and 103+/-18 bpm at 50 nmol); however, the peripheral effects of apelin were relatively weak compared to its central effects. Expression of c-fos in the paraventricular nucleus (PVN) of hypothalamus was increased in the rat that received ICV injection of (Pyr1)apelin-13 but not in the rat that received IV injection of (Pyr1)apelin-13. These results suggest that apelin plays a role in both central and peripheral cardiovascular regulation in conscious rats, and that the cardiovascular effects of apelin are not mediated by the AT1R.     

Stretch-stimulated surfactant synthesis is coordinated by the paracrine actions of PTHrP and leptin

Intrauterine lung development, culminating in physiological pulmonary surfactant production by epithelial type II (TII) cells, is driven by fluid distension through unknown mechanisms. Differentiation of alveolar epithelial and mesenchymal cells is mediated by soluble factors like parathyroid hormone-related protein (PTHrP), a stretch-sensitive TII cell product. PTHrP stimulates pulmonary surfactant production by a paracrine feedback loop mediated by leptin, a soluble product of the mature lipofibroblast (LF). When LFs and TIIs are stretched in coculture, there is a fivefold increase in surfactant phospholipid synthesis that can be "neutralized" by inhibitors of PTHrP or leptin, implicating a paracrine feedback loop in this mechanism. Stretching LFs stimulates PTHrP binding (2.5-fold) and downstream stimulation of triglyceride uptake quantitatively (15-25%) due to upregulation of adipose differentiation-related protein expression. Stretching TII cells increases leptin stimulation of their surfactant phospholipid synthesis threefold, suggesting that retrograde signaling by leptin to TII cells is also stretch sensitive. We conclude that the effect of stretch on alveolar LF and TII differentiation is coordinated by PTHrP, leptin, and their receptors.     

Direct cardiovascular actions of two metabolites of linoleic acid

Two newly discovered oxidation products of linoleic acid (i.e., 9,10-epoxy-12-octadecenoate termed Leukotoxin A, and 12,13-epoxy-9-octadecenoate termed Leukotoxin B) are produced by neutrophils in a variety of species. These substances appear to combat bacterial infection although they also have detrimental effects on normal organ function. Administration of Leukotoxin A or B to isolated cat papillary muscles decreased developed force, an index of myocardial contractility, in a concentration-dependent manner. Leukotoxin B was more active in decreasing the developed force than Leukotoxin A at high concentrations. Leukotoxin A or B, when added to isolated perfused cat carotid arteries, produced a significant vasoconstriction which in vivo would result in an increased vascular resistance. Thus, leukotoxins exert significant direct effects on the cardiovascular system in cats. Leukotoxins A and B are both cardiodepressant and vasoactive independent of release of other blood borne mediators.     

Regulation of free and bound leptin and soluble leptin receptors during inflammation in mice

Leptin, an appetite-regulating hormone/cytokine, circulates both free and bound to soluble leptin receptors (s-leptin R). An electrochemiluminescence (ECL) assay for the quantitative measurement of murine s-leptin R was developed. The absence of s-leptin R immunoreactivity in the serum of db(pas)/db(pas)mice demonstrated the specificity of the assay, which detected s-leptin R both in the free form and complexed with leptin. The distribution of free vs bound leptin and the regulation of s-leptin R were evaluated in mice following administration of the pro-inflammatory stimuli endotoxin (100 microg/mouse) and turpentine (100 microl/mouse). Both endotoxin and turpentine significantly increased serum leptin and s-leptin R levels compared to control mice. The distribution of free vs bound leptin was not altered by administration of endotoxin or turpentine. In fact, approximately 50% of total leptin was present in the free form in either control, endotoxin- or turpentine-injected mice. On the contrary, during the hyperleptinemia of pregnancy, only 10% of total leptin was present in the free form. We conclude that inflammation leads to the increase of both bound and free leptin. Therefore, the total amount of bioactive leptin is increased during acute inflammation, suggesting that leptin participates in the host response to inflammation.     

Sexually different actions of leptin in proopiomelanocortin neurons to regulate glucose homeostasis

Leptin regulates energy balance and glucose homeostasis, at least in part, via activation of receptors in the arcuate nucleus of the hypothalamus located in proopiomelanocortin (POMC) neurons. Females have greater sensitivity to central leptin than males, suggested by a greater anorectic effect of central leptin administration in females. We hypothesized that the regulation of energy balance and peripheral glucose homeostasis of female rodents would be affected to a greater extent than in males if the action of leptin in POMC neurons were disturbed. Male and female mice lacking leptin receptors only in POMC neurons gained significantly more body weight and accumulated more body fat. However, female mice gained disproportionately more visceral adiposity than males, and this appeared to be largely the result of differences in energy expenditure. When maintained on a high-fat diet (HFD), both male and female mutants had higher levels of insulin following exogenous glucose challenges. Chow- and HFD-fed males but not females had abnormal glucose disappearance curves following insulin administrations. Collectively, these data indicate that the action of leptin in POMC neurons is sexually different to influence the regulation of energy balance, fat distribution, and glucose homeostasis.     

Chronic administration of dehydroepiandrosterone reduces pancreatic beta-cell hyperplasia and hyperinsulinemia in genetically obese Zucker rats

The Zucker obese (fa/fa) rat is a model of hypertrophic/hyperplastic obesity. These rats develop marked hyperinsulinemia, insulin resistance, and pancreatic beta-cell hyperplasia. In the present study, chronic (22 weeks) administration of the 17-ketosteroid, dehydroepiandrosterone (DHEA), to obese Zucker rats significantly decreased body weight, and retroperitoneal and parametrial fat pad weights. In addition, beta-cell hyperplasia was reduced as well as pancreatic insulin content. DHEA treatment of lean Zucker rats also reduced body weight, fat depot weight, pancreatic islet diameter, and pancreatic insulin content. These data indicate that DHEA treatment appears to inhibit insulin synthesis and beta-cell proliferation. Whether this is due to a direct effect on the pancreas or due to improvement of peripheral insulin sensitivity remains to be elucidated.