The problem of obesity : is there a role for antagonists of the renin-angiotensin system?

Obesity is a major health problem worldwide; it is associated with more than 30 medical conditions and is a leading cause of unnecessary deaths. Adipose tissue not only acts as an energy store, but also behaves like an endocrine organ, synthesising and secreting numerous hormones and cytokines. Angiotensin II (ANG II) is the biologically active component of the renin-angiotensin system (RAS). The RAS is present in adipose tissue and evidence suggests that ANG II is intimately linked to obesity. Indeed, ANG II increases fat cell growth and differentiation, increases synthesis, uptake and storage of fatty acids and triglycerides and possibly inhibits lipolysis. Evidence obtained using genetically modified animals has shown that the amount of body fat is directly related to the amount of ANG II, i.e., animals with low levels of ANG II have reduced fat stores while animals with excessive ANG II have increased fat stores. In humans, epidemiological evidence has shown that body fat is correlated with angiotensinogen, a precursor of ANG II, or other components of the RAS. Furthermore, blocking the production and/or actions of ANG II with drugs or natural substances decreases body fat. The decrease in body fat caused by such treatments predominantly occurs in abdominal fat depots and appears to be independent of energy intake and digestibility. Clearly, ANG II has an important role in the accumulation of body fat and the possibility exists that treatment of obesity will be enhanced by the use of natural or synthetic substances that interfere with ANG II.

Splanchnic blood flow and hepatic glucose production in exercising humans : role of renin-angiotensin system

The study examined the implication of the renin-angiotensin system (RAS) in regulation of splanchnic blood flow and glucose production in exercising humans. Subjects cycled for 40 min at 50% maximal O2 consumption (VO2 max) followed by 30 min at 70% VO2 max either with [angiotensin-converting enzyme (ACE) blockade] or without (control) administration of the ACE inhibitor enalapril (10 mg iv). Splanchnic blood flow was estimated by indocyanine green, and splanchnic substrate exchange was determined by the arteriohepatic venous difference. Exercise led to an ~20-fold increase (P < 0.001) in ANG II levels in the control group (5.4 ± 1.0 to 102.0 ± 25.1 pg/ml), whereas this response was blunted during ACE blockade (8.1 ± 1.2 to 13.2 ± 2.4 pg/ml) and in response to an orthostatic challenge performed postexercise. Apart from lactate and cortisol, which were higher in the ACE-blockade group vs. the control group, hormones, metabolites, VO2, and RER followed the same pattern of changes in ACE-blockade and control groups during exercise. Splanchnic blood flow (at rest: 1.67 ± 0.12, ACE blockade; 1.59 ± 0.18 l/min, control) decreased during moderate exercise (0.78 ± 0.07, ACE blockade; 0.74 ± 0.14 l/min, control), whereas splanchnic glucose production (at rest: 0.50 ± 0.06, ACE blockade; 0.68 ± 0.10 mmol/min, control) increased during moderate exercise (1.97 ± 0.29, ACE blockade; 1.91 ± 0.41 mmol/min, control). Refuting a major role of the RAS for these responses, no differences in the pattern of change of splanchnic blood flow and splanchnic glucose production were observed during ACE blockade compared with controls. This study demonstrates that the normal increase in ANG II levels observed during prolonged exercise in humans does not play a major role in the regulation of splanchnic blood flow and glucose production.

The rat placental renin-angiotensin system - a gestational gene expression study

BACKGROUND: The placenta is an essential organ that provides nutrients and oxygen to the developing fetus and removes toxic waste products from the fetal circulation. Maintaining placental blood osmotic pressure and blood flow is crucial for viable offspring. The renin-angiotensin system (RAS) in the placenta is a key player in the regulation of maternal-fetal blood flow during pregnancy. Therefore, the aim of this study was to determine if RAS genes are differentially expressed in mid to late gestation in rat placenta. METHODS: Whole placental tissue samples from pregnant Sprague Dawley rats at embryonic (E) days 14.25, 15.25, 17.25 and 20 (n = 6 for each gestational age) were used for genome-wide gene expression by microarray. RAS genes with expression differences of >2 fold were further analyzed. Quantitative Real-Time PCR (qPCR) was performed on independent samples to confirm and validate microarray data. Immunohistochemisty and Western blotting were performed on a differentially expressed novel RAS pathway gene (ANPEP). RESULTS: Six out of 17 genes of the RAS pathway were differentially expressed at different gestational ages. Gene expression of four genes (Angiotensin converting enzyme (Ace), angiotensin converting enzyme 2 (Ace2), membrane metalloendopeptidase (Mme) and angiotensin II receptor 1A (Agtr1a)) were significantly upregulated at E20 whereas two others (Thimet oligopeptidase 1 (Thop1) and Alanyl aminopeptidase (Anpep)) were downregulated at E20 prior to the onset of labour. These changes were confirmed by qPCR. Western blots revealed no overall differences in ANPEP protein expression in the placentae. Immunohistochemical studies, however, indicated that the localization of ANPEP differed at E17.25 and E20 as ANPEP localization in the giant trophoblast cell of the junctional zone was no longer detectable at E20. CONCLUSIONS: The current study investigated the expression of members of the RAS pathway in rat placentae and observed significantly altered expression of 6 RAS genes at 4 gestational ages. These findings present the need for further comprehensive investigation of RAS genes in normal and complicated pregnancies.

Mice lacking angiotensin-converting enzyme have increased energy expenditure, with reduced fat mass and improved glucose clearance

In addition to its role in the storage of fat, adipose tissue acts as an endocrine organ, and it contains a functional renin-angiotensin system (RAS). Angiotensin-converting enzyme (ACE) plays a key role in the RAS by converting angiotensin I to the bioactive peptide angiotensin II (Ang II). In the present study, the effect of targeting the RAS in body energy homeostasis and glucose tolerance was determined in homozygous mice in which the gene for ACE had been deleted (ACE^-/-) and compared with wild-type littermates. Compared with wild-type littermates, ACE^-/- mice had lower body weight and a lower proportion of body fat, especially in the abdomen. ACE^-/- mice had greater fed-state total energy expenditure (TEE) and resting energy expenditure (REE) than wild-type littermates. There were pronounced increases in gene expression of enzymes related to lipolysis and fatty acid oxidation (lipoprotein lipase, carnitine palmitoyl transferase, long-chain acetyl CoA dehydrogenase) in the liver of ACE^-/- mice and also lower plasma leptin. In contrast, no differences were detected in daily food intake, activity, fed-state plasma lipids, or proportion of fat excrete in fecal matter. In conclusion, the reduction in ACE activity is associated with a decreased accumulation of body fat, especially in abdominal fat depots. The decreased body fat in ACE^-/- mice is independent of food intake and appears to be due to a high energy expenditure related to increased metabolism of fatty acids in the liver, with the additional effect of increased glucose tolerance.

Influence of dietary modifications on the blood pressure response to antihypertensive medication

Identifying dietary modifications that potentiate the blood pressure (BP)-lowering effects of antihypertensive medications and that are practical for free-living people may assist in achieving BP reduction goals. We assessed whether two dietary patterns were effective in lowering BP in persons on antihypertensive therapy and in those not on therapy. Ninety-four participants (38/56 females/males), aged 55.6 (sd 9.9) years, consumed two 4-week dietary regimens in random order (Dietary Approaches to Stop Hypertension (DASH)-type diet and low-Na high-K (LNAHK) diet) with a control diet before each phase. Seated home BP was measured daily for the last 2 weeks in each phase. Participants were grouped based on antihypertensive drug therapy. The LNAHK diet produced a greater fall in systolic BP (SBP) in those on antihypertensive therapy ( - 6.2 (sd 6.0) mmHg) than in those not on antihypertensive therapy ( - 2.8 (sd 4.0) mmHg) (P = 0.036), and this was greatest for those on renin-angiotensin system (RAS) blocker therapy ( - 9.5 (sd 6.4) mmHg) (interaction P = 0.007). The fall in SBP on the DASH-type diet, in those on therapy (overall - 1.1 (sd 6.2) mmHg; renin-angiotensin blocker therapy - 4.2 (sd 4.7) mmHg), was not as marked as that observed on the LNAHK diet. Dietary modifications are an important part of all hypertension management regimens, and a low-Na and high-K diet enhances the BP-lowering effect of antihypertensive medications, particularly those targeting the RAS.

Angiotensin converting enzyme inhibition lowers body weight and improves glucose tolerance in C57BL/6J mice maintained on a high fat diet

The renin–angiotensin system (RAS) is functional within adipose tissue and angiotensin II, the active component of RAS, has been implicated in adipose tissue hypertrophy and insulin resistance. In this study, captopril, an angiotensin converting enzyme (ACE) inhibitor that prevents angiotensin II formation, was used to study the development of diet-induced obesity and insulin resistance in obesity prone C57BL/6J mice. The mice were fed a high fat diet (w/w 21% fat) and allowed access to either water or water with captopril added (0.2 mg/ml). Body weight was recorded weekly and water and food intake daily. Glucose tolerance was determined after 11–12 weeks. On completion of the study (after 16 weeks of treatment), the mice were killed and kidney, liver, epididymal fat and extensor digitorum longus muscle (EDL) were weighed. Blood samples were collected and plasma analysed for metabolites and hormones. Captopril treatment decreased body weight in the first 2 weeks of treatment. Food intake of captopril-treated mice was similar to control mice prior to weight loss and was decreased after weight loss. Glucose tolerance was improved in captopril-treated mice. Captopril-treated mice had less epididymal fat than control mice. Relative to body weight, captopril-treated mice had increased EDL weight. Relative to control mice, mice administered captopril had a higher plasma concentration of adiponectin and lower concentrations of leptin and non-esterified fatty acids (NEFA). The results indicate that captopril both induced weight loss and improved insulin sensitivity. Thus, captopril may eventually be used for the treatment of obesity and Type 2 diabetes.

Alterations in renal and myocardial adrenoceptors associated with ethynyloestradiol- and levonorgestrel-induced hypertension in the rat

Hypertension is one of many side effects of oral contraceptive use in a small percentage of women. Although the underlying pathology has yet to be fully resolved, alterations in the renin-angiotensin-aldosterone axis, sympathetic nervous system/ renal and cardiac function have been implicated. In the thesis to be presented, the possible involvement of alterations in renal and myocardial adrenoceptor characteristics in the pathogenesis of steroid contraceptive-induced hypertension in rats was examined by radioligand binding techniques. In Chapter 2, a rat model of OC hypertension is described. Chronic low-dose administration of ethynyloestradiol (EE2), levonorgestrel (NG) or a combination of both steroids (EE2/NG) to female Sprague-Dawley rats was shown to significantly increase systolic blood pressure (SBP). Renal and cardiac hypertrophy developed in association with EE2-, EE2/NG- but not NG-induced hypertension. Moreover, whereas administration of NG alone attenuated body weight gain, combined EE2/NG administration increased body weight gain from the second week of treatment onwards. Based on the above observations, it is proposed that EE2 and NG induce hypertension in rats via different mechanisms. Although SBP was elevated to a similar maximum in all steroid-treated groups (+ 20 mmHg compared to controls), only with EE2 administration did SBP remain elevated for the duration of the 17 week treatment regimen. NG may therefore have a protective effect on blood pressure with long-term combined steroid contraceptive treatment. In Chapter 4, renal adrenoceptors were characterized using radioactively labelled adrenocephor antagonists. Under appropriate conditions, binding of [3H]-prazosin and [3H]-rauwolscine to membrane preparations of whole rat kidney displayed the kinetics, saturability and specificity of α1- and α2 -adrenoceptors respectively, which were present in a ratio 3:1. In contrast, [3H]-dihydroergocryptine ([3H]-DHE) apparently bound to both α1 and α2-adrenoceptors. Binding sites identified by [125I] –iodocyanopindolol (ICYP) had the recognition characteristics of β-adrenoceptors. In drug competition studies using the subtype-selective antagonists practolol (β1) and ICI 118,551 (β2)/ the ratio of β1- to β2 -adrenoceptors was found to be approximately 2:1. Subsequently, renal adrenoceptors were investigated at various stages during the development of hypertension with the different steroid contraceptive treatments (Chapters 5 and 6). Preliminary binding studies with [3H]-DHE and [3H]-prazosin suggested that the number of renal α2 - but not α1-adrenoceptors was reduced in rats with established EE2-induced hypertension (17 weeks treatment). This was subsequently confirmed using [3H]-rauwolscine, which in addition showed that the reduction in renal α2 -adrenoceptor number occurred during the developmental stage of EE2/NG~induced hypertension (6 weeks treatment) and established EE2-induced hypertension (12 weeks treatment). NG induced hypertension was unassociated with changes in renal α1- and α2-adrenoceptor characteristics. Renal β-adrenoceptor affinity was reduced in established EE2-, but not NG- or EE2/NG- induced hypertension. Moreover, the β-adrenoceptor agonist (-)-isoprenaline bound to renal β-adrenoceptors with reduced affinity following EE2 administration. Several endogenous and synthetic steroids were found to be ineffective inhibitors of [3H] –prazosin, [3H] –rauwolscine and ICYP binding excluding a direct interaction of these steroids with renal α1-, α2- and β -adrenoceptors. In Chapter 7, myocardial adrenoceptors were characterized and investigated in steroid-treated rats. In membrane preparations of whole myocardium, [3H]-prazosin binding was characteristically to α1- adrenoceptors, whereas there was a notable absence of [3H]-rauwolscine binding. Using ICYP, β-adrenoceptors were also detected, the ratio of β1- to β2~adrenoceptors being 3:1. Steroid contraceptive-induced hypertension was not associated with myocardial α1-adrenoceptor changes. Similarly, myocardial β-adrenoceptors were unchanged in established EE2-, NG- and EE2/NG-induced hypertension (12 weeks treatment). The affinity of (-)-isoprenaline for myocardial β-adrenoceptors was unaffected by EE2 aditiinistration. These studies suggest that established EE2- but not NG-induced hypertension in rats is associated with selective alterations in renal α2- and (β-adrenoceptors. These adrenoceptor changes may help to maintain elevated blood pressure by affecting the control of renal function by the sympathetic nervous system, catecholamines and several hormones which affect renin release and the transport of fluid and electrolytes in the nephron.

Progression of cardiovascular and endocrine dysfunction in a rabbit model of obesity

In rabbits, mean arterial pressure (MAP) increases in response to fat feeding, but does not increase further with progressive weight gain. We documented the progression of adiposity and the alterations in endocrine/cardiovascular function in response to fat feeding in rabbits, to determine whether stabilization of MAP after 3 weeks could be explained by stabilization of neurohormonal factors. Rabbits were fed a control diet or high-fat diet for 9 weeks (n¼23). Fat feeding progressively increased body mass and adiposity. Heart rate (HR) was elevated by week 3 (15±3%) but changed little thereafter. The effects of fat feeding on MAP were dependent on baseline MAP and peaked at 3 weeks. From baseline, MAP p80mmHg, MAP had increased by 8.1±1.3, 4.7±1.7 and 5.6±1.2mmHg, respectively, 3, 6 and 9 weeks after commencing the high-fat diet, but by only 2.6±1.5, 3.0±1.7 and 3.9±1.4mmHg, respectively, in control rabbits. Fat feeding did not increase MAP from a baseline 480mmHg. Plasma concentrations of leptin and insulin increased during the first 3–6 weeks of fat feeding and then stabilized (increasing by 111±17% and 731±302% by week 9, respectively), coinciding with the pattern of changes in MAP and HR. Plasma total cholesterol, triglycerides, renin activity, aldosterone and atrial natriuretic peptide were not significantly altered by fat feeding. Given that the changes in plasma leptin and insulin mirrored the changes in MAP and HR, leptin and insulin may be important factors in the development of hypertensionand tachycardia in the rabbit model of obesity.

The impact of obesity on drug prescribing in primary care

Background

Healthcare costs attributable to obesity have previously involved estimations based on costs of diseases commonly considered as having obesity as an underlying factor.

Aim

To quantify the impact of obesity on total primary care drug prescribing.

Design of study

Review of computer generated and handwritten prescriptions to determine total prescribing volume for all drug classes.

Setting

Twenty-three general practice surgeries in the UK.

Method

Stratified random selection of 1150 patients who were obese (body mass index [BMI]>30 kg/m2) and 1150 age- and sex-matched controls of normal weight (BMI 18.5–<25 kg/m2). Retrospective review of medical records over an 18-month period.

Results

A higher percentage of patients who were obese, compared with those of normal weight, were prescribed at least one drug in the following disease categories: cardiovascular (36% versus 20%), central nervous system (46% versus 35%), endocrine (26% versus 18%), and musculoskeletal and joint disease (30% versus 22%). All of these categories had a P-value of <0.001. Other categories, such as gastrointestinal (24% versus 18%), infections (42% versus 35%), skin (24% versus 19%) had a P-value of <0.01, while respiratory diseases (18% versus 21%) had a P-value of <0.05. Total prescribing volume was significantly higher for the group with obesity and was increased in the region of two- to fourfold in a wide range of prescribing categories: ulcer healing drugs, lipid regulators, β-adrenoreceptor drugs, drugs affecting the rennin angiotensin system, calcium channel blockers, antibacterial drugs, sulphonylureas, biguanides, non-steroidal anti-inflammatories (NSAIDs) (P<0.001) and fibrates, angiotensin II antagonists, and thyroid drugs (P<0.05). The main impact on prescribing volumes is from numbers of patients treated, although in some areas there is an effect from greater dosage or longer treatment in those who are obese including calcium channel blockers, antihistamines, hypnotics, drugs used in the treatment of nausea and vertigo, biguanides, and NSAIDs (P<0.05) reflected in significantly increased defined daily dose prescribing.

Elevated intracardiac angiotensin II leads to cardiac hypertrophy and mechanical dysfunction in normotensive mice

Introduction
Angiotensin II (Ang II) is known to induce cardiac growth and modulate myocardial contractility. It has been reported that elevated levels of endogenous Ang II contribute to the development of cardiac hypertrophy in hypertensives. However, the long-term functional effects of cardiac exposure to Ang II in normotensives is unclear.

A recently developed transgenic mouse (TG1306/1R), in which cardiac-specific overproduction of Ang II produces primary hypertrophy, provides a new experimental model for investigation of this phenotype. The aim of the present study was to use this model to investigate whether there is a functional deficit in primary hypertrophy that may predispose to cardiac failure and sudden death. We hypothesised that primary cardiac hypertrophy is associated with mechanical dysfunction in the basal state.

Methods
Normotensive heterozygous TG1306/1R mice harbouring multiple copies of a cardiac-specific rat angiotensinogen gene were studied at age 30—40 weeks and compared with age-matched wild-type littermates. Left ventricular function was measured ex vivo in bicarbonate buffer-perfused, Langendorffmounted hearts ( at a perfusion pressure of 80 mmHg, 37°C) using a fluid-filled PVC balloon interfaced to a pressure transducer and digital data acquisition system.

Results
There was no difference in the mean (±SEM) intrinsic heart rate of TG1306/1R and wild-type control mice (357.4±11.8 vs. 367.5±20.9 bpm, n=9 & 7). Under standardised end-diastolic pressure conditions, TG1306/1R hearts exhibited a significant reduction in peak developed pressure (132.2±9.4 vs. 161.5±3.1 mmHg, n=9 & 7, p<0.05) and maximum rate of pressure development (3566.7±323.7 vs. 4486.3±109.4 mmHg, n=9 & 7, p<0.05). TG1306/1R mice show a significant correlation between incidence of arrhythmia and increasing heart size (Spearman's correlation coefficient 0.61).

Conclusion
These data demonstrate that chronic in vivo exposure to elevated levels of intra-cardiac Ang II is associated with significant contractile abnormalities evident in the ex vivo intact heart. Our findings suggest that endogenous overproduction of cardiac Ang II, independent of changes in blood pressure, is sufficient to induce ventricular remodelling that culminates in impaired cardiac function which may precede failure.

Role of angiotensin II in the hypertension induced by renal artery stenosis

Identical degrees of renal artery stenosis were induced in 5 dogs on two separate occasions; once during continuous inhibition of angiotensin I converting enzyme with enalapril, and once with the dogs untreated. Arterial pressure rose about 25 mm Hg during 3 days of stenosis in untreated dogs, due to increased total peripheral resistance. When the dogs were treated with enalapril, blood pressure had risen 14.5 ± 3.4 mm Hg 24 hours after stenosis due to a 35% increase in cardiac output while total peripheral resistance fell by 16%. By the third day, blood pressure had returned to pre-stenosis levels, cardiac output was close to normal and total peripheral resistance had increased. The stenosis on the renal artery increased the resistance to blood flow of the kidneys in both untreated and enalapril treated dogs. This increase in kidney resistance in the untreated dogs accounted for about 30% of the change in total peripheral resistance. In the enalapril treated dogs, the increased kidney resistance helped offset the vasodilatation in the rest of the vasculature. These results suggest that angiotensin II mediated vasoconstriction of nonrenal vascular beds was responsible for about ⅔ of the hypertension following renal artery stenosis, and the resistance of the stenosis responsible for about ⅓.