The role of I(1)-imidazoline receptors and alpha(2)-adrenergic receptors in the modulation of glucose and lipid metabolism in the SHROB model of metabolic syndrome X

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

Development of insulin resistance in Nischarin mutant female mice

BACKGROUND/OBJECTIVES

NISCH-STAB1 is a newly identified locus correlated to human waist-hip ratio (WHR), which is a risk indicator of developing obesity-associated diabetes. Our previous studies have shown that Nisch mutant male mice increased glucose tolerance in chow-fed conditions. Thus we hypothesized that Nisch mutant mice will have changes in insulin resistance, adipocytes, hepatic steatosis when mice are fed with high-fat diet (HFD).

METHODS

Insulin resistance was assessed in Nisch mutant mice and WT mice fed with high-fat diet (60% by kCal) or chow diet. Whole-body energy metabolism was examined using an indirect calorimeter. Adipose depots including inguinal white adipose tissue (WAT), perigonadal WAT, retroperitoneal WAT, and mesenteric WAT were extracted. Area and eqdiameter of each adipocyte were determined, and insulin signaling was examined as well. Paired samples of subcutaneous and omental visceral adipose tissue were obtained from 400 individuals (267 women, 133 men), and examined the expression of Nischarin.

RESULTS

We found that insulin signaling was impaired in major insulin-sensitive tissues of Nisch mutant female mice. When mice were fed with HFD for 15 weeks, the Nisch mutant female mice not only developed severe insulin resistance and decreased glucose tolerance compared with wild-type control mice, but also accumulated more white fat, had larger adipocytes and developed severe hepatic steatosis than wild-type control mice. To link our animal studies to human diseases, we further analyzed Nischarin expression in the paired human samples of visceral and subcutaneous adipose tissue from Caucasians. In humans, we found that Nischarin expression is attenuated in adipose tissue with obesity. More importantly, we found that Nischarin mRNA inversely correlated with parameters of obesity, fat distribution, lipid and glucose metabolism.

CONCLUSIONS

Taken together, our data revealed sexual dimorphism of Nischarin in body fat distribution, insulin resistance, and glucose tolerance in mice.

Effects of sympatholytic therapy with moxonidine on serum adiponectin levels in hypertensive women

We examined whether moxonidine influences lipid profile, insulin resistance, adiponectin levels, renal function and microalbuminuria in women with essential hypertension in a study of 55 non-diabetic hypertensive patients and 53 normotensive women. Hypertensive patients received moxonidine for 12 weeks. At baseline the hypertensive group had significantly higher mean blood pressure, low-density lipoprotein cholesterol, triglycerides, total cholesterol, fasting glucose, urinary albumin excretion and homeostasis model assessment of insulin resistance (HOMA-IR), together with significantly lower mean high-density lipoprotein cholesterol, creatinine clearance and serum adiponectin than the normotensive group. Moxonidine significantly decreased blood pressure, fasting glucose, triglycerides, total cholesterol, HOMA-IR and albumin excretion, but significantly increased serum adiponectin. The change in adiponectin level was negatively correlated with the change in HOMA-IR. Moxonidine treatment may improve unfavourable metabolic status related to insulin resistance by increasing adiponectin levels in patients with essential hypertension. Since it can improve adiponectin levels, it may be used in the antihypertensive treatment of patients at high risk of diabetes and cardiovascular disease.

Metabolic actions of angiotensin receptor antagonists: PPAR-gamma agonist actions or a class effect?

Accumulating basic and clinical data support the hypothesis that angiotensin receptor blockers have beneficial effects on glucose and lipid metabolism that are not shared by other classes of antihypertensive agents. These metabolic actions might only partially be shared by angiotensin-converting enzyme inhibitors. Specific benefits beyond those of other angiotensin receptor blockers have been claimed for telemesartan and, to a lesser extent, irbesartan based on a partial agonist action on PPAR-gamma receptors. Although the evidence is strong in vitro, specific actions not shared by other angiotensin receptor blockers have not yet been convincingly demonstrated in vivo or in clinical trials. In many cases, a full range of doses has not been compared, and the apparent superiority of telmesartan could be an artifact of its higher receptor binding affinity, greater tissue penetration owing to lipophilicity, and longer half life.

Loss of alpha2B-adrenoceptors increases magnitude of hypertension following nitric oxide synthase inhibition

Vascular alpha(2B)-adrenoceptors (alpha(2B)-AR) may mediate vasoconstriction and contribute to the development of hypertension. Therefore, we hypothesized that blood pressure would not increase as much in mice with mutated alpha(2B)-AR as in wild-type (WT) mice following nitric oxide (NO) synthase (NOS) inhibition with N(omega)-nitro-l-arginine (l-NNA, 250 mg/l in drinking water). Mean arterial pressure (MAP) was recorded in heterozygous (HET) alpha(2B)-AR knockout mice and WT littermates using telemetry devices for 7 control and 14 l-NNA treatment days. MAP in HET mice was increased significantly on treatment days 1 and 4 to 14, whereas MAP did not change in WT mice (days 0 and 14 = 113 +/- 3 and 114 +/- 4 mmHg in WT, 108 +/- 0.3 and 135 +/- 13 mmHg in HET, P < 0.05). MAP was significantly higher in HET than in WT mice days 10 through 14 (P < 0.05). Thus blood pressure increased more rather than less in mice with decreased alpha(2B)-AR expression. We therefore examined constrictor responses to phenylephrine (PE, 10(-9) to 10(-4) M) with and without NOS inhibition to determine basal NO contributions to arterial tone. In small pressurized mesenteric arteries (inner diameter = 177 +/- 5 microm), PE constriction was decreased in untreated HET arteries compared with WT (P < 0.05). l-NNA (100 microM) augmented PE constriction more in HET arteries than in WT arteries, and responses were not different between groups in the presence of l-NNA. Acetylcholine dilated preconstricted arteries from HET mice more than arteries from WT mice. Endothelial NOS expression was increased in HET compared with WT mesenteric arteries by Western analysis. Griess assay showed increased NO(x) concentrations in HET plasma compared with those in WT plasma. These data demonstrate that diminished alpha(2B)-AR expression increases the dependence of arterial pressure and vascular tone on NO production and that vascular alpha(2B)-AR either directly or indirectly regulates vascular endothelial NOS function.

Metabolic effects of antihypertensive agents: role of sympathoadrenal and renin-angiotensin systems

Reports of beneficial, neutral and adverse impacts of antihypertensive drug classes on glucose and lipid metabolism can be found in human data. Furthermore, mechanisms for these diverse effects are often speculative and controversial. Clinical trial data on the metabolic effects of antihypertensive agents are highly contradictory. Comparisons of clinical trials involving different agents are complicated by differences in the spectrum of metabolic disturbances that accompany hypertension in different groups of patients. Two physiological systems are predominant at the interface between metabolic and cardiovascular regulation: the sympathetic nervous system (SNS) and the renin-angiotensin system (RAS). These two systems are major targets of antihypertensive drug actions, and also mediate many of the beneficial and adverse effects of antihypertensive agents on glucose and lipid metabolism. Thiazides and beta-adrenergic antagonists can adversely affect glucose and lipid metabolism, which are frequently compromised in human essential hypertension, and increase the incidence of new cases of diabetes. Laboratory studies confirm these effects, and suggest that compensatory activation of the SNS and RAS may be one mechanism. Other antihypertensives directly targeting the SNS and RAS may have beneficial effects on glucose and lipid metabolism, and may prevent diabetes. Resolution of the controversies surrounding the metabolic effects of antihypertensive agents can only be resolved by further laboratory studies, in addition to controlled clinical trials.

Efficacy of moxonidine in the treatment of hypertension in obese, noncontrolled hypertensive patients

BACKGROUND

Obesity has become an epidemic problem, contributing to metabolic syndrome, type 2 diabetes, hypertension, and cardiovascular disease. An adequate blood pressure control in this population of obese individuals is extremely difficult to achieve, and in most cases, therapeutic combinations are required. Pharmacologic treatment with moxonidine, a central I(1) imidazole receptor agonist, is a very interesting option because it acts upon the mechanisms implicated in the development of arterial hypertension in these patients. In addition, the drug improves the peripheral insulin resistance often found in obese patents, which contributes to maintain high blood pressure.

METHODS

An interventional study has been designed, adding moxonidine to noncontrolled hypertensive, obese subjects in whom a hypocaloric diet was previously recommended. A total of 25 primary care centers participated in the study, with a total of 135 patients recruited.

RESULTS

One hundred twelve patients were included in the study; 25 of them had type 2 diabetes. The mean reduction in systolic and diastolic blood pressure after 6 months treatment with moxonidine was 23.0 and 12.9 mm Hg, respectively. The mean systolic and diastolic pressures were 158.5 +/- 10.6 and 95.1 +/- 9 mm Hg, respectively, at baseline, versus 135.5 +/- 11.6 and 82.2 +/- 5.8 mm Hg at the end of the study. Creatinine clearance was significantly decreased in hyperfiltrating obese patients (143.6 +/- 31 vs. 128.2 +/- 27.9, P < 0.0001), without any significant change in patients with normal or slightly decreased renal function (81.9 +/- 18.9 vs. 80.9 +/- 17.5). Only 8 mild adverse reactions in 7 patients were recorded during the study.

CONCLUSION

Moxonidine is useful and safe for controlling arterial hypertension in obese patients.