Gluconeogenesis activation after intravenous angiotensin II in freely moving rats

Angiotensin II Inhibits Insulin Receptor Signaling in Adipose Cells

Angiotensin II (Ang II) is a critical regulator of insulin signaling in the cardiovascular system and metabolic tissues. However, in adipose cells, the regulatory role of Ang II on insulin actions remains to be elucidated. The effect of Ang II on insulin-induced insulin receptor (IR) phosphorylation, Akt activation, and glucose uptake was examined in 3T3-L1 adipocytes. In these cells, Ang II specifically inhibited insulin-stimulated IR and insulin receptor substrate-1 (IRS-1) tyrosine-phosphorylation, Akt activation, and glucose uptake in a time-dependent manner. These inhibitory actions were associated with increased phosphorylation of the IR at serine residues. Interestingly, Ang II-induced serine-phosphorylation of IRS was not detected, suggesting that Ang II-induced desensitization begins from IR regulation itself. PKC inhibition by BIM I restored the inhibitory effect of Ang II on insulin actions. We also found that Ang II promoted activation of several PKC isoforms, including PKCα/βI/βII/δ, and its association with the IR, particularly PKCβII, showed the highest interaction. Finally, we also found a similar regulatory effect of Ang II in isolated adipocytes, where insulin-induced Akt phosphorylation was inhibited by Ang II, an effect that was prevented by PKC inhibitors. These results suggest that Ang II may lead to insulin resistance through PKC activation in adipocytes.

Chronic AT1 blockade improves hyperglycemia by decreasing adipocyte inflammation and decreasing hepatic PCK1 and G6PC1 expression in obese rats

Inappropriate activation of the renin-angiotensin system decreases glucose uptake in peripheral tissues. Chronic angiotensin receptor type 1 (AT1) blockade (ARB) increases glucose uptake in skeletal muscle and decreases the abundance of large adipocytes and macrophage infiltration in adipose. However, the contributions of each tissue to the improvement in hyperglycemia in response to AT1 blockade are not known. Therefore, we determined the static and dynamic responses of soleus muscle, liver, and adipose to an acute glucose challenge following the chronic blockade of AT1. We measured adipocyte morphology along with TNF-α expression, F4/80- and CD11c-positive cells in adipose and measured insulin receptor (IR) phosphorylation and AKT phosphorylation in soleus muscle, liver, and retroperitoneal fat before (T0), 60 (T60) and 120 (T120) min after an acute glucose challenge in the following groups of male rats: 1) Long-Evans Tokushima Otsuka (LETO; lean control; n = 5/time point), 2) obese Otsuka Long Evans Tokushima Fatty (OLETF; n = 7 or 8/time point), and 3) OLETF + ARB (ARB; 10 mg olmesartan/kg/day; n = 7 or 8/time point). AT1 blockade decreased adipocyte TNF-α expression and F4/80- and CD11c-positive cells. In retroperitoneal fat at T60, IR phosphorylation was 155% greater in ARB than in OLETF. Furthermore, in retroperitoneal fat AT1 blockade increased glucose transporter-4 (GLUT4) protein expression in ARB compared with OLETF. IR phosphorylation and AKT phosphorylation were not altered in the liver of OLETF, but AT1 blockade decreased hepatic Pck1 and G6pc1 mRNA expressions. Collectively, these results suggest that chronic AT1 blockade improves obesity-associated hyperglycemia in OLETF rats by improving adipocyte function and by decreasing hepatic glucose production via gluconeogenesis.NEW & NOTEWORTHY Inappropriate activation of the renin-angiotensin system increases adipocyte inflammation contributing to the impairment in adipocyte function and increases hepatic Pck1 and G6pc1 mRNA expression in response to a glucose challenge. Ultimately, these effects may contribute to the development of glucose intolerance.

Chronic Angiotensin-(1-7) Improves Insulin Sensitivity in High-Fat Fed Mice Independent of Blood Pressure

Angiotensin-(1-7) improves glycemic control in animal models of cardiometabolic syndrome. The tissue-specific sites of action and blood pressure dependence of these metabolic effects, however, remain unclear. We hypothesized that Ang-(1-7) improves insulin sensitivity by enhancing peripheral glucose delivery. Adult male C57BL/6J mice were placed on standard chow or 60% high-fat diet for 11 weeks. Ang-(1-7) (400 ng/kg per minute) or saline was infused subcutaneously during the last 3 weeks of diet, and hyperinsulinemic-euglycemic clamps were performed at the end of treatment. High-fat fed mice exhibited modest hypertension (systolic blood pressure: 137 ± 3 high fat versus 123 ± 5 mm Hg chow;P=0.001), which was not altered by Ang-(1-7) (141 ± 4 mm Hg;P=0.574). Ang-(1-7) did not alter body weight or fasting glucose and insulin in chow or high-fat fed mice. Ang-(1-7) increased the steady-state glucose infusion rate needed to maintain euglycemia in high-fat fed mice (31 ± 5 Ang-(1-7) versus 16 ± 1 mg/kg per minute vehicle;P=0.017) reflecting increased whole-body insulin sensitivity, with no effect in chow-fed mice. The improved insulin sensitivity in high-fat fed mice was because of an enhanced rate of glucose disappearance (34 ± 5 Ang-(1-7) versus 20 ± 2 mg/kg per minute vehicle;P=0.049). Ang-(1-7) enhanced glucose uptake specifically into skeletal muscle by increasing translocation of glucose transporter 4 to the sarcolemma. Our data suggest that Ang-(1-7) has direct insulin-sensitizing effects on skeletal muscle, independent of changes in blood pressure. These findings provide new insight into mechanisms by which Ang-(1-7) improves insulin action, and provide further support for targeting this peptide in cardiometabolic disease.

Decreased hepatic gluconeogenesis in transgenic rats with increased circulating angiotensin-(1-7)

The renin-angiotensin (Ang) system (RAS) plays an important role in the control of glucose metabolism and glycemia. Several studies demonstrated that the effects of angiotensin-(1-7) are mainly opposite to the actions of biological angiotensin II. Recent studies have demonstrated that rats with increased circulating angiotensin-(1-7), acting through the G protein coupled receptor Mas, have enhanced glucose tolerance and insulin sensitivity, presenting improved metabolic parameters. However, there is no data regarding the role of angiotensin-(1-7)-Mas axis in hepatic glycemic metabolism. In the present study, the gluconeogenesis and glycogenolysis was investigated in Sprague-Dawley (SD) and in TGR(A1-7)3292 (TGR) rats which present approximately twofold increase in plasma Ang-(1-7) levels compared to SD. The pyruvate administration in fasted rats showed a decreased synthesis of glucose in TGR compared to the SD rats, pointing to a downregulation of gluconeogenesis. Supporting this data, the mRNA evaluation of gluconeogenic enzymes showed a significant reduction in phosphoenolpyruvate carboxykinase reinforced by a significantly diminished expression of hepatocyte nuclear factor 4α (HNF-4α), responsible for the regulation of gluconeogenic enzymes. In conclusion our data show that the improved glucose metabolism induced by Ang-(1-7) could be due, at least in part, to a downregulation of hepatic gluconeogenesis.

Central angiotensin AT1 receptors are involved in metabolic adjustments in response to graded exercise in rats

To investigate the influence of central angiotensin AT1-receptors blockade on metabolic adjustments during graded exercise, Losartan (Los) was intracerebroventricularly injected in rats before running until fatigue. Oxygen consumption (VO2) was measured (n=6) and blood samples collected (n=7) to determine variations of glucose, lactate and free fatty acids (FFA). Los-rats exhibited a hyperglycemic response, already observed at 20% of maximal work, followed by a higher lactate levels and FFA mobilization from adipose tissue. Despite the reduced total time to fatigue and the higher VO2 associated with reduced mechanical efficiency, exercise led to the attainment of similar levels of effort in both groups. In summary, central AT1-receptor blockade during graded exercise induces hyperglycemia and higher FFA mobilization from adipose tissue at low exercise intensities in rats running at the same absolute exercise intensity. These data suggest that the central angiotensinergic system is involved in metabolic adjustments during exercise since central blockade of AT1-receptors shifts energy balance during graded exercise, similarly to situations of higher and premature sympathetic activation.

The enhanced hyperglycemic response to hemorrhage hypotension in obese rats is related to an impaired baroreflex

The aim of the present study was to assess the metabolic adjustments in adult rats with autonomic imbalance induced by hypercaloric diet. Male Wistar rats (4 weeks of age) were fed a chow diet (CD, n = 12) or hypercaloric diet (HD, n = 13) for 19 weeks. Body weight and dietary intake were measured every week and the basal metabolic rate was assessed. After 19 weeks of diet, six animals from each group were anesthetized with a lethal dose of barbital sodium (100 mg/Kg body weight, intraperitoneal; i.p.). Lee index was evaluated and adipose pads weighted. The remaining animals had a silastic cannula placed into the jugular vein for drug administration, blood collecting, and hemorrhage (1.2 mL/100 g bw/2 min). A polyethylene catheter (PE50) was inserted into the abdominal artery through the femoral artery for cardiovascular monitoring. The assessment of autonomic balance was done by evaluation of baroreflex sensitivity (intravenous (IV) injection of phenylephrine and sodium nitroprusside) and hemorrhage (1.2 mL/100 g bw/2 min). As expected, the HD induced obesity; increased weight gain (28%), adipose pads weight, and baroreflex dysfunction. The plasma level of free fatty acids and triacylglycerols were increased in HD rats by about 124% and 424%, respectively, as well as the basal metabolic rate measured at 19th weeks of diet (p < 0.01). We observed that baroreflex sensitivity to phenylephrine was reduced by about 50%, and the hyperglycemic response to hemorrhage hypotension was increased by 128% in HD rats. We found also a negative correlation between the alteration in baroreflex sensitivity and the increase in hyperglycemic response to hemorrhage in the obese rats (r = 0.72, p < 0.01) and a strong positive correlation between the increased Lee index and the hemorrhagic hyperglycemia (r = 0.93, p < 0.01). Our data demonstrate that obesity induced by hypercaloric diet in Wistar rats promotes an autonomic imbalance, which interferes with metabolic responses dependent on baroreflex sensitivity. In addition, we showed the existence of close correlation between the loss of baroreflex sensitivity and the degree of obesity.

Genetically altered animals in the study of the metabolic functions of peptide hormone systems

PURPOSE OF REVIEW

Here we review the use of genetically altered animals to address the roles of peptide hormone systems in the modulation of energy homeostasis. Despite the disseminated use of transgenic techniques to establish the functional relevance of several peptide hormone systems, we focus on two multifunctional systems, the renin-angiotensin and the kallikrein-kinin systems. Initially, we explored the background information supporting the functional aspects of these systems, followed by novel knowledge obtained with the phenotypic characterization of genetically altered animals.

RECENT FINDINGS

A role for the renin-angiotensin system in the regulation of adiposity and glucose metabolism has been suggested. Studies using genetically altered animals not only confirmed the physiological relevance of angiotensin II in the control of energy homeostasis, but also revealed that the adipose tissue renin-angiotensin system participates in the endocrine modulation of cardiovascular and renal function. On the other hand, the involvement of the kallikrein-kinin system with metabolic processes was not so obvious. Recent reports using genetically altered animals, however, provided strong evidence to support an important role for kinins in the control of glucose homeostasis and energy balance.

SUMMARY

Here we present examples of how genetically altered animals contribute to a final postulation of the physiological roles of certain hormone systems, bringing new insights into the field.

Angiotensin II stimulates the reactivity of the pituitary-adrenal axis in leptin-resistant Zucker rats, thereby influencing the glucose utilization

The HPA axis is hyperactive under conditions of leptin and insulin resistance as well as after ANG II administration. We hypothesized that a hyperreactivity of the HPA axis to ANG contributes to an impaired glucose utilization in obesity, since leptin resistance and an overactive renin-angiotensin-aldosterone system are features of obesity. Zucker rats were treated with ANG via subcutaneous minipumps (0, 0.9, and 9.0 mug/h; 4 wk). PA axis reactivity and glucose homeostasis were characterized after CRH treatment and during an oral glucose tolerance test (OGTT). The elevated plasma profile of corticosterone after CRH stimulation in saline-treated OZR compared with LZR confirmed that the sensitization of the PA axis depended on leptin resistance. Irrespective of the rat strain, circulating ANG levels and blood pressure were selectively increased after administration of 9 mug/h ANG (high ANG). Only high ANG induced an elevation of the corticosterone and glucose response after CRH stimulation in OZR but did not affect the ACTH secretion. During OGTT, corticosterone and consequently glucose increased in OZR after high ANG, whereas the insulin secretion was decreased. In the adrenal glands of OZR, AT(1A) receptor mRNA levels increased after high ANG. We conclude that the impairment of glucose utilization after ANG stimulation is potentiated in leptin-resistant rats as a result of a hyperreactive PA axis, thereby confirming the functional importance of a dysregulation within the HPA axis in metabolic syndrome or obesity. The ACTH-independent stimulation of corticosterone release and the selective increase of AT(1A) receptor mRNA in the adrenals of OZR indicated a sensitization of adrenals toward ANG, causing a stimulation of the PA axis.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Impact of the renin-angiotensin system on lipid and carbohydrate metabolism

PURPOSE OF REVIEW

This review is intended to provide an update of the impact of the renin-angiotensin system on lipid and carbohydrate metabolism and of its relationship with adipose-tissue and skeletal muscle activities.

RECENT FINDINGS

The components of the renin-angiotensin system are fully represented in the adipose tissue and appear to be upregulated in obesity--a condition associated with enhanced circulating angiotensinogen levels. The local renin-angiotensin system plays a role in adipocyte differentiation and possibly in body-fat accumulation. In humans, angiotensin II produced by mature adipocytes appears to inhibit the differentiation of adipocyte precursors, thus decreasing the percentage of small insulin-sensitive adipocytes. In turn, the lipid-storage capacity of adipose tissue could become reduced and triglycerides might accumulate in liver and skeletal muscle, contributing to insulin resistance. Randomized controlled trials indicating that pharmacological renin-angiotensin system blockade improves insulin sensitivity and reduces the risk of type 2 diabetes are in keeping with this possibility. The local renin-angiotensin system in skeletal muscle may affect exercise performance and the individual response to different types of muscular performance. The concept that the local renin-angiotensin system plays a role in body-fat storage and in lipid and carbohydrate metabolism is further supported by genetic studies showing that susceptibility to weight gain and possibly insulin resistance is greater in individuals carrying certain renin-angiotensin system allelic variants associated with alterations in systemic and local angiotensinogen levels and angiotensin-converting enzyme activity.

SUMMARY

In summary, the aforementioned data imply that the renin-angiotensin system plays a substantial role in obesity, insulin resistance and the associated increase in blood pressure.

Possible involvement of the adipose tissue renin-angiotensin system in the pathophysiology of obesity and obesity-related disorders

Angiotensin II (Ang II), acting on the AT1 and AT2 receptors in mammalian cells, is the vasoactive component of the renin-angiotensin system (RAS). Several components of the RAS have been demonstrated in different tissues, including adipose tissue. Although the effects of Ang II on metabolism have not been studied widely, it is intriguing to assume that components of the RAS produced by adipocytes may play an autocrine, a paracrine and/or an endocrine role in the pathophysiology of obesity and provide a potential pathway through which obesity leads to hypertension and type 2 diabetes mellitus. In the first part of this review, we will describe the production of Ang II, the different receptors through which Ang II exerts its effects and summarize the concomitant intracellular signalling cascades. Thereafter, potential Ang II-induced mechanisms, which may be associated with obesity and obesity-related disorders, will be considered. Finally, we will focus on the different pharmaceutical agents that interfere with the RAS and highlight the possible implications of these drugs in the treatment of obesity-related disorders.

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 O(2) consumption (VO(2 max)) followed by 30 min at 70% VO(2 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 approximately 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, VO(2), 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.