Effect of diazoxide on brain capillary insulin receptor binding and food intake in hyperphagic obese Zucker rats

Toward the Decipherment of Molecular Interactions in the Diabetic Brain

Diabetes mellitus (DM) has been associated with cognitive complications in the brain resulting from acute and chronic metabolic disturbances happening peripherally and centrally. Numerous studies have reported on the morphological, electrophysiological, biochemical, and cognitive changes in the brains of diabetic individuals. The detailed pathophysiological mechanisms implicated in the development of the diabetic cognitive phenotype remain unclear due to intricate molecular changes evolving over time and space. This review provides an insight into recent advances in understanding molecular events in the diabetic brain, focusing on cerebral glucose and insulin uptake, insulin action in the brain, and the role of the brain in the regulation of glucose homeostasis. Fully competent mitochondria are essential for energy metabolism and proper brain function; hence, the potential contribution of mitochondria to the DM-induced impairment of the brain is also discussed.

The Potential Role of Activating the ATP-Sensitive Potassium Channel in the Treatment of Hyperphagic Obesity

To evaluate the potential role of ATP-sensitive potassium (K_ATP) channel activation in the treatment of hyperphagic obesity, a PubMed search was conducted focused on the expression of genes encoding the K_ATP channel, the response to activating the K_ATP channel in tissues regulating appetite and the establishment and maintenance of obesity, the evaluation of K_ATP activators in obese hyperphagic animal models, and clinical studies on syndromic obesity. K_ATP channel activation is mechanistically involved in the regulation of appetite in the arcuate nucleus; the regulation of hyperinsulinemia, glycemic control, appetite and satiety in the dorsal motor nucleus of vagus; insulin secretion by β-cells; and the synthesis and β-oxidation of fatty acids in adipocytes. K_ATP channel activators have been evaluated in hyperphagic obese animal models and were shown to reduce hyperphagia, induce fat loss and weight loss in older animals, reduce the accumulation of excess body fat in growing animals, reduce circulating and hepatic lipids, and improve glycemic control. Recent experience with a K_ATP channel activator in Prader-Willi syndrome is consistent with the therapeutic responses observed in animal models. K_ATP channel activation, given the breadth of impact and animal model and clinical results, is a viable target in hyperphagic obesity.

High-Dose, Diazoxide-Mediated Insulin Suppression Boosts Weight Loss Induced by Lifestyle Intervention

CONTEXT

Obesity-related hyperinsulinism may impede lifestyle-initiated weight loss.

OBJECTIVE

Proof-of-concept study to investigate the amplifying effects of diazoxide (DZX)-mediated insulin suppression on lifestyle-induced weight loss in nondiabetic, hyperinsulinemic, obese men.

DESIGN

Twelve-month study comprising an initial 6-month, double-blind trial, followed by a partially de-blinded 6-month extension in men with obesity with a body mass index of 30 to 37.5 kg/m2 and a fasting serum C-peptide level >1.00 nM. Patients were randomized into three treatment groups: DZX + placebo (DZX + PL), DZX + metformin (DZX + MTF), and double PL (PL + PL).

RESULTS

At 6 months, DZX treatment was associated with a 6.1-kg PL-subtracted decline in fat mass (FM), and at 12 months, FM had decreased by a total of 15.7 ± 2.5 kg. Twelve months of DZX treatment was also associated with a significant decline in systolic (-6.6%) and diastolic (-8.6%) blood pressure and low-density lipoprotein-cholesterol (-18%) and triglycerides (-43%) and a 39% rise in high-density lipoprotein-cholesterol. These effects were achieved at the cost of a small rise in fasting glucose (95% CI: 0.2 to 1.0 mM) and hemoglobin A1c (95% CI: -0.08% to 0.44%). There were no differences between DZX monotherapy and the combination of DZX + MTF.

CONCLUSION

High-dose DZX treatment of 1 year resulted in a substantial decrease in FM, blood pressure, and lipid levels at the cost of a small rise in blood glucose levels.

Some cannabinoid receptor ligands and their distomers are direct-acting openers of SUR1 K(ATP) channels

Here, we examined the chronic effects of two cannabinoid receptor-1 (CB1) inverse agonists, rimonabant and ibipinabant, in hyperinsulinemic Zucker rats to determine their chronic effects on insulinemia. Rimonabant and ibipinabant (10 mg·kg⁻¹·day⁻¹) elicited body weight-independent improvements in insulinemia and glycemia during 10 wk of chronic treatment. To elucidate the mechanism of insulin lowering, acute in vivo and in vitro studies were then performed. Surprisingly, chronic treatment was not required for insulin lowering. In acute in vivo and in vitro studies, the CB1 inverse agonists exhibited acute K channel opener (KCO; e.g., diazoxide and NN414)-like effects on glucose tolerance and glucose-stimulated insulin secretion (GSIS) with approximately fivefold better potency than diazoxide. Followup studies implied that these effects were inconsistent with a CB1-mediated mechanism. Thus effects of several CB1 agonists, inverse agonists, and distomers during GTTs or GSIS studies using perifused rat islets were unpredictable from their known CB1 activities. In vivo rimonabant and ibipinabant caused glucose intolerance in CB1 but not SUR1-KO mice. Electrophysiological studies indicated that, compared with diazoxide, 3 μM rimonabant and ibipinabant are partial agonists for K channel opening. Partial agonism was consistent with data from radioligand binding assays designed to detect SUR1 K(ATP) KCOs where rimonabant and ibipinabant allosterically regulated ³H-glibenclamide-specific binding in the presence of MgATP, as did diazoxide and NN414. Our findings indicate that some CB1 ligands may directly bind and allosterically regulate Kir6.2/SUR1 K(ATP) channels like other KCOs. This mechanism appears to be compatible with and may contribute to their acute and chronic effects on GSIS and insulinemia.

Pharmacological modulation of dopamine receptor D2-mediated transmission alters the metabolic phenotype of diet induced obese and diet resistant C57Bl6 mice

High fat feeding induces a variety of obese and lean phenotypes in inbred rodents. Compared to Diet Resistant (DR) rodents, Diet Induced Obese (DIO) rodents are insulin resistant and have a reduced dopamine receptor D2 (DRD2) mediated tone. We hypothesized that this differing dopaminergic tone contributes to the distinct metabolic profiles of these animals. C57Bl6 mice were classified as DIO or DR based on their weight gain during 10 weeks of high fat feeding. Subsequently DIO mice were treated with the DRD2 agonist bromocriptine and DR mice with the DRD2 antagonist haloperidol for 2 weeks. Compared to DR mice, the bodyweight of DIO mice was higher and their insulin sensitivity decreased. Haloperidol treatment reduced the voluntary activity and energy expenditure of DR mice and induced insulin resistance in these mice. Conversely, bromocriptine treatment tended to reduce bodyweight and voluntary activity, and reinforce insulin action in DIO mice. These results show that DRD2 activation partly redirects high fat diet induced metabolic anomalies in obesity-prone mice. Conversely, blocking DRD2 induces an adverse metabolic profile in mice that are inherently resistant to the deleterious effects of high fat food. This suggests that dopaminergic neurotransmission is involved in the control of metabolic phenotype.

Hyperinsulinemia precedes insulin resistance in mice lacking pancreatic beta-cell leptin signaling

The adipocyte hormone leptin acts centrally and peripherally to regulate body weight and glucose homeostasis. The pancreatic beta-cell has been shown to be a key peripheral target of leptin, with leptin suppressing insulin synthesis and secretion from beta-cells both in vitro and in vivo. Mice with disrupted leptin signaling in beta-cells (lepr(flox/flox) RIPcre tg+ mice) display hyperinsulinemia, insulin resistance, glucose intolerance, obesity, and reduced fasting blood glucose. We hypothesized that hyperinsulinemia precedes the development of insulin resistance and increased adiposity in these mice with a defective adipoinsular axis. To determine the primary defect after impaired beta-cell leptin signaling, we treated lepr(flox/flox) RIPcre tg+ mice with the insulin sensitizer metformin or the insulin-lowering agent diazoxide with the rationale that pharmacological improvement of the primary defect would alleviate the secondary symptoms. We show that improving insulin sensitivity with metformin does not normalize hyperinsulinemia, whereas lowering insulin levels with diazoxide improves insulin sensitivity. Taken together, these results suggest that hyperinsulinemia precedes insulin resistance in beta-cell leptin receptor-deficient mice, with insulin resistance developing as a secondary consequence of excessive insulin secretion. Therefore, pancreatic beta-cell leptin receptor-deficient mice may represent a model of obesity-associated insulin resistance that is initiated by hyperinsulinemia.

Six months of diazoxide treatment at bedtime in newly diagnosed subjects with type 1 diabetes does not influence parameters of {beta}-cell function and autoimmunity but improves glycemic control

OBJECTIVE Continuous beta-cell rest with diazoxide preserves residual endogenous insulin production in type 1 diabetes. However, side effects have hampered therapeutic usefulness. In a double-blind study, we tested whether lower, intermittent dosing of diazoxide had beneficial effects on insulin production, metabolic control, and autoimmunity markers in the absence of side effects. RESEARCH DESIGN AND METHODS Forty-one newly diagnosed type 1 diabetic patients were randomized to 6 months of treatment with placebo or 100 mg diazoxide at bedtime. A1C, C-peptide (fasting and glucagon stimulated), and FoxP3(+) regulatory T-cells (Tregs) were measured. Patients were followed for 6 months after intervention. RESULTS Of six dropouts, three were due to perceived side effects; one subject in the diazoxide group experienced rash, another dizziness, and one in the placebo group sleep disturbance. Adverse effects in others were absent. Diazoxide treatment reduced A1C from 8.6% at baseline to 6.0% at 6 months and 6.5% at 12 months. Corresponding A1C value in the placebo arm were 8.3, 7.3, and 7.5% (P < 0.05 for stronger reduction in the diazoxide group). Fasting and stimulated C-peptide decreased during 12 months similarly in both arms (mean -0.30 and -0.18 nmol/l in the diazoxide arm and -0.08 and -0.09 nmol/l in the placebo arm). The proportion of Tregs was similar in both arms and remained stable during intervention but was significantly lower compared with nondiabetic subjects. CONCLUSIONS Six months of low-dose diazoxide was without side effects and did not measurably affect insulin production but was associated with improved metabolic control.

Pancreatic islet overproduction of H2S and suppressed insulin release in Zucker diabetic rats

Hydrogen sulfide (H(2)S) has been traditionally known for its toxic effects on living organisms. The role of H(2)S in the homeostatic regulation of pancreatic insulin metabolism has been unclear. The present study is aimed at elucidating the effect of endogenously produced H(2)S on pancreatic insulin release and its role in diabetes development. Diabetes development in Zucker diabetic fatty (ZDF) rats was evaluated in comparison with Zucker fatty (ZF) and Zucker lean (ZL) rats. Pancreatic H(2)S production and insulin release were also assayed. It was found that H(2)S was generated in rat pancreas islets, catalyzed predominantly by cystathionine gamma-lyase (CSE). Pancreatic CSE expression and H(2)S production were greater in ZDF rats than in ZF or ZL rats. ZDF rats exhibited reduced serum insulin level, hyperglycemia, and insulin resistance. Inhibition of pancreatic H(2)S production in ZDF rats by intraperitoneal injection of DL-propargylglycine (PPG) for 4 weeks increased serum insulin level, lowered hyperglycemia, and reduced hemoglobin A1c level (P<0.05). Although in ZF rats it also reduced pancreatic H(2)S production and serum H(2)S level, PPG treatment did not alter serum insulin and glucose level. Finally, H(2)S significantly increased K(ATP) channel activity in freshly isolated rat pancreatic beta-cells. It appears that insulin release is impaired in ZDF because of abnormally high pancreatic production of H(2)S. New therapeutic approach for diabetes management can be devised based on our observation by inhibiting endogenous H(2)S production from pancreas.

Modulation of adipoinsular axis in prediabetic zucker diabetic fatty rats by diazoxide

Dysregulation of the adipoinsular axis in male obese Zucker diabetic fatty (ZDF; fa/fa) rats, a model of type 2 diabetes, results in chronic hyperinsulinemia and increased de novo lipogenesis in islets, leading to beta-cell failure and diabetes. Diazoxide (DZ; 150 mg/kg.d), an inhibitor of insulin secretion, was administered to prediabetic ZDF animals for 8 wk as a strategy for prevention of diabetes. DZ reduced food intake (P < 0.02) and rate of weight gain only in ZDF rats (P < 0.01). Plasma insulin response to glucose load was attenuated in DZ-Zucker lean rats (ZL; P < 0.01), whereas DZ-ZDF had higher insulin response to glucose than controls (P < 0.001). DZ improved hemoglobin A1c (P < 0.001) and glucose tolerance in ZDF (P < 0.001), but deteriorated hemoglobin A1c in ZL rats (P < 0.02) despite normal tolerance in the fasted state. DZ lowered plasma leptin (P < 0.001), free fatty acid, and triglyceride (P < 0.001) levels, but increased adiponectin levels (P < 0.02) only in ZDF rats. DZ enhanced beta3-adrenoreceptor mRNA (P < 0.005) and adenylate cyclase activity (P < 0.01) in adipose tissue from ZDF rats only, whereas it enhanced islet beta3- adrenergic receptor mRNA (P < 0.005) but paradoxically decreased islet adenylate cyclase activity (P < 0.005) in these animals. Islet fatty acid synthase mRNA (P < 0.03), acyl coenzyme A carboxylase mRNA (P < 0.01), uncoupling protein-2 mRNA (P < 0.01), and triglyceride content (P < 0.005) were only decreased in DZ-ZDF rats, whereas islet insulin mRNA and insulin content were increased in DZ-ZDF (P < 0.01) and DZ-ZL rats (P < 0.03). DZ-induced beta-cell rest improved the lipid profile, enhanced the metabolic efficiency of insulin, and prevented beta-cell dysfunction and diabetes in diabetes-prone animals. This therapeutic strategy may be beneficial in preventing beta-cell failure and progression to diabetes in humans.

Attenuation of hyperinsulinemia by NN414, a SUR1/Kir6.2 selective K-adenosine triphosphate channel opener, improves glucose tolerance and lipid profile in obese Zucker rats

Chronic attenuation of hyperinsulinemia by diazoxide (DZ), a K-adenosine triphosphate (ATP) channel opener and an inhibitor of glucose-mediated insulin secretion, improved glucose tolerance and lipid profile and decreased the rate of weight gain in obese Zucker rats. To determine whether suppression of hyperinsulinemia alters daily food consumption, rate of weight gain, glucose tolerance, and lipid profile, we compared the effects of NN414, a potent and SUR1/Kir6.2 selective K(atp)() channel opener, with DZ in obese and lean Zucker rats. DZ (150 mg/kg/d), low-dose (LDNN414: 10 mg/kg/d), high-dose (HDNN414: 30 mg/kg/d), and vehicle (C) were administered to 7-week-old obese and lean female Zucker rats for a period of 6 weeks. Each animal underwent an intraperitoneal glucose tolerance test (IPGTT) at the end of study period. While NN414 treatment did not affect food intake and rate of weight gain in any of the strains, DZ treatment reduced food intake (P <.001) and rate of weight gain (P <.001) in obese rats. The fasting plasma insulin levels and area under the curve (AUC) insulin response to IPGTT were significantly attenuated in LDNN414 (P <.05), HDNN414 (P <.01), and DZ (P <.01) obese and lean rats compared with their controls. This was accompanied by a significant reduction in AUC glucose only in LDNN414 (P <.05), HDNN414 (P <.01), and DZ (P <.01) obese rats compared with controls. While hemoglobin A(1c) (HbA(1c)) was not affected in LDNN414 obese rats, it was higher in HDNN414 obese animals (P <.001), LD-, HDNN414 (P <.001), and DZ (P <.005) lean rats compared with their respective controls. DZ obese rats showed lower HbA(1c) levels than C obese rats (P <.02). The plasma free fatty acid (FFA) levels were only decreased in HDNN414 (P <.05) and DZ (P <.002) obese rats, whereas plasma triglyceride (TG) levels were decreased in LDNN414 (P <.05), HDNN414 (P <.001), and DZ (P <.001) obese rats compared with controls. Finally, plasma leptin level was only decreased in DZ obese rats compared with controls (P <.001). The new SUR1/Kir6.2 selective K(atp)() channel opener, NN414, reduced hyperinsulinemia in a dose-dependent manner without a significant effect on food consumption and rate of weight gain. NN414-induced beta-cell rest in obese rats was associated with a significant improvement in glucose responsiveness, suggesting an increase in insulin sensitivity after its withdrawal. There was an overall deterioration in glycemic control at the high dose as measured by HbA(1c). There was a dose-dependent improvement in lipid profiles of obese Zucker rats. These results suggest that pharmacologic attenuation of hyperinsulinemic state by low-dose NN414 may be therapeutically beneficial in insulin-resistant states without any deterioration in overall glycemic control.

Diazoxide down-regulates leptin and lipid metabolizing enzymes in adipose tissue of Zucker rats

We have previously reported that attenuation of hyperinsulinemia by diazoxide (DZ), an inhibitor of glucose-mediated insulin secretion, increased insulin sensitivity and reduced body weight in obese Zucker rats. These findings prompted us to investigate the effects of DZ on key insulin-sensitive enzymes regulating adipose tissue metabolism, fatty acid synthase (FAS), and lipoprotein lipase (LPL), as well as on circulating levels of leptin. We also determined the direct effects of diazoxide on FAS in 3T3-L1 adipocytes. Seven-week-old female obese and lean Zucker rats were treated with DZ (150 mg/kg/d) or vehicle (C, control) for a period of 6 wk. Changes in plasma parameters by DZ include significant decreases in triglycerides, free fatty acids, glucose, and insulin, consistent with our previous reports. DZ obese rats exhibited lower plasma leptin levels (P<0.03) compared to their C animals. DZ significantly reduced adipose tissue FAS activity in both lean (P<0.0001) and obese (P<0.01) animals. LPL mRNA content was also decreased significantly in DZ-treated obese animals (P<0.009) as compared to their respective controls without a significant effect on lean animals. The possibility that DZ exerted a direct effect on adipocytes was further tested in cultured 3T3-L1 adipocytes. Although diazoxide (5 microM) alone did not change FAS activity in cultured 3T3-L1 adipocytes, it significantly attenuated insulin's effect on FAS activity (P<0.001). We demonstrate that DZ regulates key insulin-sensitive enzymes involved in regulation of adipose tissue metabolism. These findings suggest that modification of insulin-sensitive pathways can be therapeutically beneficial in obesity management.