Paracrine role for periadventitial adipose tissue in the regulation of arterial tone

Omentin, a novel adipokine, induces vasodilation in rat isolated blood vessels

Omentin is a recently identified adipose tissue-derived cytokine and is implicated in obesity-related cardiovascular disorders. In the present study, we tested the hypothesis that omentin could directly affect vascular reactivity of isolated blood vessels. In endothelium-intact rat isolated aorta, pretreatment with omentin (300 ng/ml, 30 min) inhibited noradrenaline (NA; 1 nM-1 microM)-induced concentration-dependent contraction. In NA (100 nM)-pre-contracted aorta, omentin (1-300 ng/ml) directly induced an endothelium-dependent relaxation. While a nitric oxide (NO) synthase (NOS) inhibitor, N(G)-nitro-l-arginine methyl ester (100 microM, 30 min) inhibited the relaxation, a PI3K/Akt inhibitor, LY294002 (10 microM, 30 min) or a tyrosine kinase inhibitor, genistein (30 microM, 30 min) was ineffective. Omentin (300 ng/ml, 5 min) induced a phosphorylation of endothelial NOS at serine 1177 but not a phosphorylation of Akt at serine 473. Omentin (1-300 ng/ml) also relaxed NA pre-contracted mesenteric artery. Present study for the first time demonstrated that omentin has a vasodilating effect on isolated blood vessels, which is mediated through endothelium-derived NO.

Influence of exercise and perivascular adipose tissue on coronary artery vasomotor function in a familial hypercholesterolemic porcine atherosclerosis model

Our lab has shown that left circumflex coronary artery (LCX) perivascular adipose tissue (PAT) blunts endothelin-1 (ET-1)-induced maximal contractions in normal pigs on low- and high-fat diets. Other studies report that PAT exerts anticontractile effects on agonist-induced arterial contraction via release of a relaxing factor that acts on the underlying vasculature. The purpose of this study was to test the hypotheses that PAT blunts LCX contraction in familial hypercholesterolemic pigs and that exercise training (Ex) augments this anticontractile effect. Male familial hypercholesterolemic pigs were divided into Ex (n = 13) and sedentary (Sed) (n = 15) groups. LCX reactivity to angiotensin II (ANG II), bradykinin (BK), ET-1, and sodium nitroprusside (SNP) was evaluated in vitro with intact or removed PAT in Sed and Ex familial hypercholesterolemic pigs. LCX relaxation induced by BK and SNP was not altered by Ex or PAT removal. LCX contractions stimulated by ANG II and ET-1 were not significantly altered by Ex or PAT removal across doses; however, Ex did act to significantly reduce ET-1 maximal contractions in familial hypercholesterolemic pig LCX compared with Sed familial hypercholesterolemic pig LCX, independent of PAT (P < 0.05). We conclude that LCX PAT in Sed and Ex familial hypercholesterolemic pigs exerts no substantial anticontractile influence over LCX vasomotor responses to endogenous constrictors such as ANG II and ET-1. Our results suggest that exercise training significantly reduces familial hypercholesterolemic pig LCX maximal contractile responses to the endogenous constrictor ET-1, independent of PAT.

Adipocyte-derived factor reduces vasodilatory capability in ob-/ob- mice

Obesity is associated with impaired functional hyperemic response. We have shown that ATP-sensitive potassium (K(ATP)) channels are important in mediating functional vasodilation. Adipocyte-derived factors (ADFs) can alter vascular tone via opening K(ATP) channels. We hypothesize that, in an animal model of obesity, ADFs will decrease basal arteriolar tone by opening K(ATP) channels, resulting in an attenuated functional vasodilation. We used wild-type (WT) mice and ob(-)/ob(-) mice (ob) to test this hypothesis. The spinotrapezius muscle was prepared for the microcirculatory observation of arcade arterioles, and we measured the vasodilatory responses to muscle stimulation. The basal arteriolar diameter was larger in ob mice compared with WT mice. The K(ATP) channel inhibitor glibenclamide (10 microM) decreased arteriolar diameter in ob mice with no effect in WT mice. The increase in arteriolar diameter induced by muscle stimulation was attenuated in ob mice compared with WT mice. To determine the mechanisms for the opening of K(ATP) channels, fat was collected from the ob mice, subcutaneous fat from around the spinotrapezius muscle (OBSF) or visceral fat (OBVF) and was incubated in physiological saline solution (PSS). The vasodilatory responses to the fat-conditioned PSS were determined in WT mice. Treatment with OBSF- or OBVF -conditioned PSS increased the arteriolar diameters in WT mice, a dilation that was inhibited by glibenclamide. The absolute diameters induced by muscle stimulation were not altered by the fat-conditioned PSS. These results suggest that, in ob mice, local ADFs reduce the functional vasodilatory capability via opening K(ATP) channels.

Visfatin causes endothelium-dependent relaxation in isolated blood vessels

Visfatin is a novel adipocyte-derived cytokine. We hypothesized that visfatin could directly affect vascular reactivity. To test the hypothesis, effects of visfatin on contraction of isolated blood vessels were examined. In endothelium-intact rat aorta, pretreatment with visfatin (100 ng/ml, 30 min) inhibited noradrenaline (NA; 1 nM-1 microM)-induced contraction. In NA (100 nM)-pre-contracted aorta, visfatin (1-100 ng/ml) directly induced a relaxation. Although an N(G)-Nitro-L-arginine methyl ester (300 microM, 15 min) inhibited the relaxation, an insulin receptor inhibitor, AGL2263 (10 microM, 20 min) was ineffective. Visfatin (100 ng/ml, 20 min) induced a phosphorylation of eNOS at serine 1177 and a de-phosphorylation of eNOS at threonine 495. Visfatin also induced a phosphorylation of Akt at serine 473 and a substrate of cGMP-dependent protein kinase, vasodilator stimulated phosphoprotein at serine 239. Present study revealed for the first time that visfatin has a vasodilating effect on isolated blood vessels, which is mediated via endothelium-derived NO.

Local inflammation and hypoxia abolish the protective anticontractile properties of perivascular fat in obese patients

BACKGROUND

Inflammation in adipose tissue has been implicated in vascular dysfunction, but the local mechanisms by which this occurs are unknown.

METHODS AND RESULTS

Small arteries with and without perivascular adipose tissue were taken from subcutaneous gluteal fat biopsy samples and studied with wire myography and immunohistochemistry. We established that healthy adipose tissue around human small arteries secretes factors that influence vasodilation by increasing nitric oxide bioavailability. However, in perivascular fat from obese subjects with metabolic syndrome (waist circumference 111+/-2.8 versus 91.1+/-3.5 cm in control subjects, P<0.001; insulin sensitivity 41+/-5.9% versus 121+/-18.6% in control subjects, P<0.001), the loss of this dilator effect was accompanied by an increase in adipocyte area (1786+/-346 versus 673+/-60 mum(2), P<0.01) and immunohistochemical evidence of inflammation (tumor necrosis factor receptor 1 12.4+/-1.1% versus 6.7+/-1%, P<0.001). Application of the cytokines tumor necrosis factor receptor-alpha and interleukin-6 to perivascular fat around healthy blood vessels reduced dilator activity, resulting in the obese phenotype. These effects could be reversed with free radical scavengers or cytokine antagonists. Similarly, induction of hypoxia stimulated inflammation and resulted in loss of anticontractile capacity, which could be rescued by catalase and superoxide dismutase or cytokine antagonists. Incubation with a soluble fragment of adiponectin type 1 receptor or inhibition of nitric oxide synthase blocked the vasodilator effect of healthy perivascular adipose tissue.

CONCLUSIONS

We conclude that adipocytes secrete adiponectin and provide the first functional evidence that it is a physiological modulator of local vascular tone by increasing nitric oxide bioavailability. This capacity is lost in obesity by the development of adipocyte hypertrophy, leading to hypoxia, inflammation, and oxidative stress.

Endothelial dysfunction and diabetes: roles of hyperglycemia, impaired insulin signaling and obesity

Endothelial dysfunction comprises a number of functional alterations in the vascular endothelium that are associated with diabetes and cardiovascular disease, including changes in vasoregulation, enhanced generation of reactive oxygen intermediates, inflammatory activation, and altered barrier function. Hyperglycemia is a characteristic feature of type 1 and type 2 diabetes and plays a pivotal role in diabetes-associated microvascular complications. Although hyperglycemia also contributes to the occurrence and progression of macrovascular disease (the major cause of death in type 2 diabetes), other factors such as dyslipidemia, hyperinsulinemia, and adipose-tissue-derived factors play a more dominant role. A mutual interaction between these factors and endothelial dysfunction occurs during the progression of the disease. We pay special attention to the possible involvement of endoplasmic reticulum stress (ER stress) and the role of obesity and adipose-derived adipokines as contributors to endothelial dysfunction in type 2 diabetes. The close interaction of adipocytes of perivascular adipose tissue with arteries and arterioles facilitates the exposure of their endothelial cells to adipokines, particularly if inflammation activates the adipose tissue and thus affects vasoregulation and capillary recruitment in skeletal muscle. Hence, an initial dysfunction of endothelial cells underlies metabolic and vascular alterations that contribute to the development of type 2 diabetes.

Obesity and vascular dysfunction

One of the most profound challenges facing public health and public health policy in Western society is the increased incidence and prevalence of both overweight and obesity. While this condition can have significant consequences for patient mortality and quality of life, it can be further exacerbated as overweight/obesity can be a powerful stimulus for the development of additional risk factors for a negative cardiovascular outcome, including increased insulin resistance, dyslipidemia and hypertension. This manuscript will present the effects of systemic obesity on broad issues of vascular function in both afflicted human populations and in the most relevant animal models. Among the topics that will be covered are alterations to vascular reactivity (both dilator and constrictor responses), adaptations in microvascular network and vessel wall structure, and alterations to the patterns of tissue/organ perfusion as a result of the progression of the obese condition. Additionally, special attention will be paid to the contribution of chronic inflammation as a contributor to alterations in vascular function, as well as the role of perivascular adipose tissue in terms of impacting vessel behavior. When taken together, it is clearly apparent that the development of the obese condition can have profound, and frequently difficult to predict, impacts on integrated vascular function. Much of this complexity appears to have its basis in the extent to which other co-morbidities associated with obesity (e.g., insulin resistance) are present and exert contributing effects.

Effects of fetal and neonatal exposure to nicotine on blood pressure and perivascular adipose tissue function in adult life

In Wistar rats, maternal exposure to nicotine was shown to impair the inhibitory function of perivascular adipose tissue on vascular contractility in the aorta of the offspring. It is not known whether an impairment of perivascular adipose tissue function occurs in smaller arteries, and whether the control of blood pressure is affected. Here we studied the blood pressure effects and the alteration of perivascular adipose tissue function in mesenteric arteries of the offspring born to Wistar-Kyoto rat (WKY) dams exposed to nicotine. Nulliparous female WKY rats were given either nicotine bitartrate (1 mg/kg/day) or saline (vehicle) by subcutaneous injection 2 weeks prior to mating, during pregnancy and until weaning. Blood pressure of the offspring and functional studies with mesenteric arteries were conducted. Tissue samples (thoracic aorta, mesenteric arteries, and kidneys) were collected for morphological and immunohistochemical examinations. Blood pressure increased from 14 weeks of age onwards in the offspring born to nicotine-exposed dams. Nicotine-exposed offspring showed a significant increase in the number of brown adipocytes in aortic perivascular adipose tissue relative to control offspring. In mesenteric arteries from control offspring, contractile responses induced by phenylephrine, serotonin, and 9,11-dideoxy-11alpha, 9alpha-epoxymethanoprostaglandin F(2)alpha (U44619) were significantly attenuated in the presence of perivascular adipose tissue, an effect not observed in the nicotine-exposed tissues. Endothelium-dependent relaxation responses to carbachol, kidney weight, the total number of nephrons and glomerulus' size were comparable in nicotine and saline groups. We conclude that fetal and neonatal exposure to nicotine caused blood pressure elevation. Alterations in perivascular adipose tissue composition and modulatory function are some of the mechanisms associated with this blood pressure increase.

Perivascular fat alters reactivity of coronary artery: effects of diet and exercise

Perivascular adipose tissue (PAT) has been reported to blunt agonist-induced arterial tone via a relaxing factor acting in a paracrine manner. The purpose of this study was to test the hypothesis that PAT of porcine coronary artery blunts constriction similarly and that this anticontractile effect of PAT is altered by diet and/or exercise training.

METHODS

Fourteen adult male pigs were fed a normal-fat (NF) diet, and 10 adult male pigs were fed a high-fat/cholesterol (HF) diet. Four weeks after the initiation of diet, pigs were exercised (EX) or remained sedentary (SED) for 16 wk, yielding four groups: 1) NF-SED, 2) NF-EX, 3) HF-SED, and 4) HF-EX. Left circumflex coronary artery (LCX) rings were prepared with PAT left intact or removed. LCX reactivity to acetylcholine (ACh), endothelin (ET-1), bradykinin (BK), and sodium nitroprusside (SNP) was assessed in vitro using standard techniques.

RESULTS

The results demonstrate that both ACh and ET-1 elicited dose-dependent increases in tension from LCX rings from all groups. Removal of PAT had no significant effect on ACh-induced contractions in any group. In contrast, removal of PAT increased ET-1-induced tension in LCX from NF-SED, HF-SED, and HF-EX but not NF-EX. PAT had no significant effect on relaxation responses to BK except in HF-EX animals, where removal of PAT increased BK-induced relaxation. PAT removal decreased SNP-induced relaxation in HF-LCX, but not LCX from NF pigs, suggesting basal release of a relaxing factor LCX from HF pigs.

CONCLUSION

PAT blunts contractions induced by ET-1 in LCX from NF and HF pigs. Whereas EX abolished this effect of PAT in NF pigs, exercise did not alter the anticontractile effect in HF pigs.

Do cardiac and perivascular adipose tissue play a role in atherosclerosis?

Recent evidence suggests that epicardial and perivascular adipose tissue could mechanically and functionally affect the heart and vasculature, thereby possibly playing a role in adiposity-related atherosclerosis. Experimental and clinical observations suggest both favorable and unfavorable effects of epicardial and perivascular fat. The double role of epicardial and perivascular adipose tissue in the development of cardiovascular pathology and/or in protecting the heart and arteries warrants further studies.

Human epicardial adipose tissue: a review

We discuss the anatomy, physiology, and pathophysiology of epicardial adipose tissue and its relationship to coronary atherosclerosis. Epicardial fat stores triglyceride to supply free fatty acids for myocardial energy production and produces adipokines. It shares a common embryological origin with mesenteric and omental fat. Like visceral abdominal fat, epicardial fat thickness, measured by echocardiography, is increased in obesity. Epicardial fat could influence coronary atherogenesis and myocardial function because there is no fibrous fascial layer to impede diffusion of free fatty acids and adipokines between it and the underlying vessel wall as well as the myocardium. Segments of coronary arteries lacking epicardial fat or separated from it by a bridge of myocardial tissue are protected against the development of atherosclerosis in those segments. However, when epicardial fat is totally absent in congenital generalized lipodystrophy, coronary atherosclerosis can still occur. Macrophages are more numerous and densely packed in the periadventitial fat of human atherosclerotic coronary arteries with lipid cores than in that of fibrocalcific or nonatherosclerotic coronary arteries. In obese patients with multiple cardiovascular risk factors, epicardial fat around atheromatous coronaries secretes several proinflammatory cytokines and is infiltrated by macrophages, lymphocytes, and basophils. Epicardial adipokine expression in obesity without coronary atherosclerosis has not been determined. In nonobese patients, epicardial fat around atheromatous coronary arteries expresses proinflammatory cytokines but produces either less adiponectin, a vasoprotective adipokine, than fat around nonatheromatous coronaries or a similar amount compared with thoracic subcutaneous fat. Further studies should be done to test the hypothesis that adipokines produced by and released from human epicardial adipose tissue might contribute locally to the pathogenesis of coronary atherosclerosis.

Hypertonic and isotonic potassium solutions have different effects on vessel contractility resulting in differences in optimal resting tension in rat aorta

AIM

To compare high K(+ )-induced contraction and optimal resting tension measured by two commonly used techniques of hypertonic and isotonic K(+ ) in aortas with and without adventitial fat from various age rats.

METHODS

Three age groups of rats (15, 25, and 62 weeks) were used to prepare thoracic aortic rings in which adventitial fat was either removed or left intact. High K(+ ) (30 mmol/L)-induced contractions were observed under increasing resting tensions of 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 g. Optimal resting tension was the resting tension at which the aorta showed a maximal contraction.

RESULTS

The contractions induced by 2 kinds of high K(+ ) were significantly different. Hypertonic and isotonic K(+ ) induced a different style of contraction, and the pattern varied with different ages. At the age of 15 weeks, isotonic K(+ )-induced contractions were greater than hypertonic K+-induced contractions. However, at the age of 62 weeks, isotonic K(+ )-induced contractions were smaller than hypertonic K(+ )-induced contractions. Optimal resting tensions measured by 2 kinds of high K(+ ) were inconsistent. Optimal resting tensions in different kinds of aortic preparations from various age rats were almost a constant of 2 g, determined by isotonic K(+ ), but a variable, determined by hypertonic K(+ ). The adventitial fat could delay the development of high K(+ )-induced contractions at different resting tensions, but had little effect on the maximal contractions.

CONCLUSION

Hypertonic and isotonic K(+ ) may produce different contractions resulting in differences in optimal resting tension in rat aorta.

The fatter the better? Perivascular adipose tissue attenuates vascular contraction through different mechanisms

Adipose tissue releases several hormones and autacoids and expansion of the adipose tissue and excessive obesity is a risk factor for hypertension. Perivascular adipose tissue, on the other hand, has been reported to lower the vascular tone through the release of a transferable, thermosensitive, non-lipid factor. In this issue of the British Journal of Pharmacology, Gao et al. (2007) report that a factor generated by the adipose tissue also stimulates the generation of NO by endothelium and that NO is the predominant mediator of adipose tissue-induced relaxation in endothelium-intact vessels.

Does periadventitial fat-derived nitric oxide play a role in improved saphenous vein graft patency in patients undergoing coronary artery bypass surgery?

BACKGROUND/AIMS

The saphenous vein is commonly used for coronary artery bypass surgery but its patency is poor. Vascular damage occurs during conventional surgery. However, patency improves when the graft is harvested with minimal surgical trauma, partly due to preservation of vascular endothelial nitric oxide synthase (eNOS) and tissue sources of nitric oxide (NO), a factor possessing both dilatory and anti-proliferative properties. Apart from these grafts exhibiting an intact luminal endothelium they are harvested complete with a surrounding cushion of tissue, much of which is fat.

METHODS

Immunostaining for eNOS was performed on vein graft sections and reverse-transcriptase polymerase chain reaction and Western blotting were used to identify eNOS mRNA and protein. NO synthase activity was measured using the citrulline assay.

RESULTS

Immunohistochemistry identified eNOS staining of vein graft segments, including dense staining of the cushion of perivascular fat and associated structures surrounding the vein. eNOS protein was confirmed in both the vein and surrounding fat by Western blot analysis. Using the citrulline assay, the perivascular fat and underlying vein possessed comparable NO synthase activity.

CONCLUSIONS

Our observations suggest that perivascular fat-derived NO plays a beneficial role in saphenous veins harvested atraumatically and used as grafts in patients undergoing coronary artery bypass surgery.