Regulation of KATP channel subunit gene expression by hyperglycemia in the mediobasal hypothalamus of female rats

Could α-Synuclein Modulation of Insulin and Dopamine Identify a Novel Link Between Parkinson's Disease and Diabetes as Well as Potential Therapies?

Characterizing the normal function(s) of the protein α-Synuclein (aSyn) has the potential to illuminate links between Parkinson’s disease (PD) and diabetes and also point the way toward new therapies for these disorders. Here we provide a perspective for consideration based on our discovery that aSyn normally acts to inhibit insulin secretion from pancreatic β-cells by interacting with the Kir6.2 subunit of the ATP-sensitive potassium channel (K-ATP). It is also known that K-ATP channels act to inhibit brain dopamine secretion, and we have also shown that aSyn is a normal inhibitor of dopamine synthesis. The finding, that aSyn modulates Kir6.2 and other proteins involved in dopamine and insulin secretion, suggests that aSyn interacting proteins may be negatively impacted when aSyn aggregates inside cells, whether in brain or pancreas. Furthermore, identifying therapies for PD that can counteract dysfunction found in diabetes, would be highly beneficial. One such compound may be the multiple sclerosis drug, FTY720, which like aSyn can stimulate the activity of the catalytic subunit of protein phosphatase 2A (PP2Ac) as well as insulin secretion. In aging aSyn transgenic mice given long term oral FTY720, the mice had reduced aSyn pathology and increased levels of the protective molecule, brain derived neurotrophic factor (BDNF) (Vidal-Martinez et al., 2016). In collaboration with medicinal chemists, we made two non-immunosuppressive FTY720s that also enhance PP2Ac activity, and BDNF expression (Vargas-Medrano et al., 2014; Enoru et al., 2016; Segura-Ulate et al., 2017a). FTY720 and our novel FTY720-based-derivatives, may thus have therapeutic potential for both diabetes and PD.

The role of KATP channels in cerebral ischemic stroke and diabetes

ATP-sensitive potassium (KATP) channels are ubiquitously expressed on the plasma membrane of cells in multiple organs, including the heart, pancreas and brain. KATP channels play important roles in controlling and regulating cellular functions in response to metabolic state, which are inhibited by ATP and activated by Mg-ADP, allowing the cell to couple cellular metabolic state (ATP/ADP ratio) to electrical activity of the cell membrane. KATP channels mediate insulin secretion in pancreatic islet beta cells, and controlling vascular tone. Under pathophysiological conditions, KATP channels play cytoprotective role in cardiac myocytes and neurons during ischemia and/or hypoxia. KATP channel is a hetero-octameric complex, consisting of four pore-forming Kir6.x and four regulatory sulfonylurea receptor SURx subunits. These subunits are differentially expressed in various cell types, thus determining the sensitivity of the cells to specific channel modifiers. Sulfonylurea class of antidiabetic drugs blocks KATP channels, which are neuroprotective in stroke, can be one of the high stoke risk factors for diabetic patients. In this review, we discussed the potential effects of KATP channel blockers when used under pathological conditions related to diabetics and cerebral ischemic stroke.

ABCC8 R1420H Loss-of-Function Variant in a Southwest American Indian Community: Association With Increased Birth Weight and Doubled Risk of Type 2 Diabetes

Missense variants in KCNJ11 and ABCC8, which encode the KIR6.2 and SUR1 subunits of the β-cell KATP channel, have previously been implicated in type 2 diabetes, neonatal diabetes, and hyperinsulinemic hypoglycemia of infancy (HHI). To determine whether variation in these genes affects risk for type 2 diabetes or increased birth weight as a consequence of fetal hyperinsulinemia in Pima Indians, missense and common noncoding variants were analyzed in individuals living in the Gila River Indian Community. A R1420H variant in SUR1 (ABCC8) was identified in 3.3% of the population (N = 7,710). R1420H carriers had higher mean birth weights and a twofold increased risk for type 2 diabetes with a 7-year earlier onset age despite being leaner than noncarriers. One individual homozygous for R1420H was identified; retrospective review of his medical records was consistent with HHI and a diagnosis of diabetes at age 3.5 years. In vitro studies showed that the R1420H substitution decreases KATP channel activity. Identification of this loss-of-function variant in ABCC8 with a carrier frequency of 3.3% affects clinical care as homozygous inheritance and potential HHI will occur in 1/3,600 births in this American Indian population.

Sexual dimorphism in offspring glucose-sensitive hypothalamic gene expression and physiological responses to maternal high-fat diet feeding

A wealth of animal and human studies demonstrate that early life environment significantly influences adult metabolic balance, however the etiology for offspring metabolic misprogramming remains incompletely understood. Here, we determine the effect of maternal diet per se on offspring sex-specific outcomes in metabolic health and hypothalamic transcriptome regulation in mice. Furthermore, to define developmental periods of maternal diet misprogramming aspects of offspring metabolic balance, we investigated offspring physiological and transcriptomic consequences of maternal high-fat/high-sugar diet feeding during pregnancy and/or lactation. We demonstrate that female offspring of high-fat/high-sugar diet-fed dams are particularly vulnerable to metabolic perturbation with body weight increases due to postnatal processes, whereas in utero effects of the diet ultimately lead to glucose homeostasis dysregulation. Furthermore, glucose- and maternal-diet sensitive gene expression modulation in the paraventricular hypothalamus is strikingly sexually dimorphic. In summary, we uncover female-specific, maternal diet-mediated in utero misprogramming of offspring glucose homeostasis and a striking sexual dimorphism in glucose- and maternal diet-sensitive paraventricular hypothalamus gene expression adjustment. Notably, female offspring metabolic vulnerability to maternal high-fat/high-sugar diet propagates a vicious cycle of obesity and type 2 diabetes in subsequent generations.

Expression and function of K(ATP) channels in normal and osteoarthritic human chondrocytes: possible role in glucose sensing

ATP-sensitive potassium [K(ATP)] channels sense intracellular ATP/ADP levels, being essential components of a glucose-sensing apparatus in various cells that couples glucose metabolism, intracellular ATP/ADP levels and membrane potential. These channels are present in human chondrocytes, but their subunit composition and functions are unknown. This study aimed at elucidating the subunit composition of K(ATP) channels expressed in human chondrocytes and determining whether they play a role in regulating the abundance of major glucose transporters, GLUT-1 and GLUT-3, and glucose transport capacity. The results obtained show that human chondrocytes express the pore forming subunits, Kir6.1 and Kir6.2, at the mRNA and protein levels and the regulatory sulfonylurea receptor (SUR) subunits, SUR2A and SUR2B, but not SUR1. The expression of these subunits was no affected by culture under hyperglycemia-like conditions. Functional impairment of the channel activity, using a SUR blocker (glibenclamide 10 or 20 nM), reduced the protein levels of GLUT-1 and GLUT-3 by approximately 30% in normal chondrocytes, while in cells from cartilage with increasing osteoarthritic (OA) grade no changes were observed. Glucose transport capacity, however, was not affected in normal or OA chondrocytes. These results show that K(ATP) channel activity regulates the abundance of GLUT-1 and GLUT-3, although other mechanisms are involved in regulating the overall glucose transport capacity of human chondrocytes. Therefore, K(ATP) channels are potential components of a broad glucose sensing apparatus that modulates glucose transporters and allows human chondrocytes to adjust to varying extracellular glucose concentrations. This function of K(ATP) channels seems to be impaired in OA chondrocytes.

Decreased brain K(ATP) channel contributes to exacerbating ischemic brain injury and the failure of neuroprotection by sevoflurane post-conditioning in diabetic rats

Diabetes leads to exacerbating brain injury after ischemic stroke, but the underlying mechanisms and whether therapeutic intervention with anesthetic post-conditioning can induce neuroprotection in this population are not known. We tested the hypothesis that alteration of brain mitochondrial (mito) K_ATP channels might cause exacerbating brain injury after ischemic stroke and attenuate anesthetic post-conditioning induced neuroprotection in diabetes. We also examined whether hyperglycemic correction with insulin would restore anesthetic post-conditioning in diabetes. Non-diabetic rats and diabetic rats treated with or without insulin were subjected to focal cerebral ischemia for 2 h followed by 24 h of reperfusion. Post-conditioning was performed by exposure to sevoflurane for 15 min, immediately at the onset of reperfusion. The role of the mitoK_ATP channel was assessed by administration of a selective blocker 5-hydroxydecanoate (5-HD) before sevoflurane post-conditioning or by diazoxide (DZX), a mitoK_ATP channel opener, given in place of sevoflurane. Compared with non-diabetic rats, diabetic rats had larger infarct volume and worse neurological outcome at 24 h after ischemia. Sevoflurane or DZX reduced the infarct volume and improved neurological outcome in non-diabetic rats but not in diabetic rats, and the protective effects of sevoflurane in non-diabetic rats were inhibited by pretreatment with 5-HD. Molecular studies revealed that expression of Kir6.2, an important mitoK_ATP channel component, was decreased in the brain of diabetic rats as compared to non-diabetic rats. In contrast, hyperglycemic correction with insulin in diabetic rats normalized expression of brain Kir6.2, reduced ischemic brain damage and restored neuroprotective effects of sevoflurane post-conditioning. Our findings suggest that decreased brain mitoK_ATP channel contributes to exacerbating ischemic brain injury and the failure of neuroprotection by anesthetic post-conditioning in diabetes. Insulin glycemic control in diabetes may restore the neuroprotective effects of anesthetic post-conditioning by modulation of brain mitoK_ATP channel.

Intrahypothalamic estradiol regulates glucose metabolism via the sympathetic nervous system in female rats

Long-term reduced hypothalamic estrogen signaling leads to increased food intake and decreased locomotor activity and energy expenditure, and ultimately results in obesity and insulin resistance. In the current study, we aimed to determine the acute obesity-independent effects of hypothalamic estrogen signaling on glucose metabolism. We studied endogenous glucose production (EGP) and insulin sensitivity during selective modulation of systemic or intrahypothalamic estradiol (E2) signaling in rats 1 week after ovariectomy (OVX). OVX caused a 17% decrease in plasma glucose, which was completely restored by systemic E2. Likewise, the administration of E2 by microdialysis, either in the hypothalamic paraventricular nucleus (PVN) or in the ventromedial nucleus (VMH), restored plasma glucose. The infusion of an E2 antagonist via reverse microdialysis into the PVN or VMH attenuated the effect of systemic E2 on plasma glucose. Furthermore, E2 administration in the VMH, but not in the PVN, increased EGP and induced hepatic insulin resistance. E2 administration in both the PVN and the VMH resulted in peripheral insulin resistance. Finally, sympathetic, but not parasympathetic, hepatic denervation blunted the effect of E2 in the VMH on both EGP and hepatic insulin sensitivity. In conclusion, intrahypothalamic estrogen regulates peripheral and hepatic insulin sensitivity via sympathetic signaling to the liver.

Central insulin and leptin-mediated autonomic control of glucose homeostasis

Largely as a result of rising obesity rates, the incidence of type 2 diabetes is escalating rapidly. Type 2 diabetes results from multi-organ dysfunctional glucose metabolism. Recent publications have highlighted hypothalamic insulin- and adipokine-sensing as a major determinant of peripheral glucose and insulin responsiveness. The preponderance of evidence indicates that the brain is the master regulator of glucose homeostasis, and that hypothalamic insulin and leptin signaling in particular play a crucial role in the development of insulin resistance. This review discusses the neuronal crosstalk between the hypothalamus, autonomic nervous system, and tissues associated with the pathogenesis of type 2 diabetes, and how hypothalamic insulin and leptin signaling are integral to maintaining normal glucose homeostasis.

Gender discrimination in the influence of hyperglycemia and hyperosmolarity on rat aortic tissue responses to insulin

Hyperglycaemia initiates endothelial dysfunction causing diabetic macro- and micro-vasculopathy, the main causes of morbidity and mortality in diabetes mellitus. The vasculopathy exhibits gender peculiarities. We therefore explored gender differences in comparing the effects of hyperglycaemia (50 mM) per se with its hyperosmolar (50 mM) effects on vascular tissue responses to insulin. Endothelium-intact or denuded thoracic aortic rings from age-matched male and female Sprague-Dawley rats were incubated for 10 min or 6 h (acute versus chronic exposure) in normal, hyperglycaemic or hyperosmolar Krebs solution. Relaxant responses to insulin (6.9x10(-7)-6.9x10(-5) M) of the phenylephrine-contracted tissues were recorded. Endothelium denudation in both genders inhibited relaxation to insulin in all conditions, more significantly in female than in male tissues, suggesting the female response to insulin is more endothelium-dependent than the male. Acutely and chronically exposed normoglycemic endothelium-intact or -denuded tissues responded similarly to insulin. Chronic hyperglycemic or hyperosmolar exposure did not alter the endothelium-denuded tissue responses to insulin, whereas the responses of the endothelium-intact male and female hyperosmolar, and male hyperglycemic tissues were enhanced. The results show that insulin exerts an endothelium-dependent and independent relaxation with the female tissue responses more endothelium-dependent than the male. The data also suggest that hyperosmolarity per se enhances aortic tissue relaxant responses to insulin whereas hyperglycemia per se inhibits the same and more so in female than male tissues. These effects are endothelium-dependent.

Localization of ATP-sensitive K+ channel subunits in rat submandibular gland

ATP-sensitive K(+) (K(ATP)) channel subunits were investigated in rat submandibular gland (SMG). RT-PCR detected the presence of mRNA transcripts of the Kir6.1, Kir6.2, SUR2A, and SUR2B in the SMG, whereas SUR1 mRNA was barely detected. Western blot analysis provided the evidence that these four K(ATP) channel subunits are expressed in rat SMG. Immunostaining detected that these four K(ATP) channel subunits are widely distributed, with different intensities, in myoepithelial cells, epithelial cells of intercalated ducts, granular convoluted tubules, striated ducts, and excretory ducts. Immunofluorescence double staining showed that Kir6.1 and Kir6.2 colocalized with SUR2A in the myoepithelial cells, granular convoluted tubules, striated ducts, and excretory ducts. Kir6.1 and Kir6.2 also colocalized with SUR2B, mainly in the duct system, e.g., the granular convoluted tubules, striated ducts, and excretory ducts. Taken together, these results indicate that the K(ATP) channels in SMG may consist of Kir6.1, Kir6.2, SUR2A, and SUR2B, with various combinations of colocalization with each other, and may play important roles in rat SMG during salivary secretion.

Hormone and glucose signalling in POMC and AgRP neurons

In the wake of the obesity pandemic, increased research efforts are under way to define how peripheral hormones and metabolites regulate energy homeostasis. The melanocortin system, comprising anorexigenic proopiomelanocortin (POMC) expressing neurons and orexigenic agouti-related protein (AgRP)/neuropeptide Y (NPY) coexpressing neurons in the arcuate nucleus of the hypothalamus are crucial for normal energy homeostasis both in rodents and humans. They are regulated by peripheral hormones such as leptin and insulin, as well as nutrients such as glucose, amino acids and fatty acids. Although much progress has been made, recent reports continue to underline how restricted our understanding of POMC and AgRP/NPY neuron regulation by these signals is. Importantly, ATP-dependent potassium (K(ATP)) channels are regulated both by ATP (from glucose metabolism) and by leptin and insulin, and directly control electrical excitability of both POMC and AgRP neurons. Thus, this review attempts to offer an integrative overview about how peripheral signals, particularly leptin, insulin and glucose, converge on a molecular level in POMC and AgRP neurons of the arcuate nucleus of the hypothalamus to control energy homeostasis.

A role for ATP-sensitive potassium channels in male sexual behavior

ATP-sensitive potassium (K(+)(ATP)) channels regulate cell excitability and are expressed in steroid-responsive brain regions involved in sexual behavior, such as the preoptic area (POA) and medial basal hypothalamus (MBH). We hypothesized that K(+)(ATP) channels serve as a mechanism by which testosterone can control the electrical activity of neurons and consequently elicit male sexual responsiveness. RT-PCR analysis indicated that castration induces, while testosterone inhibits, mRNA expression of the K(+)(ATP) channel subunit Kir6.2 in both the POA and MBH of adult male rats. Intracerebral infusion of the pharmacological K(+)(ATP) channel inhibitor tolbutamide increased the proportion of long-term castrates displaying sexual behavior and restored mount frequency, intromission frequency, and copulatory efficacy to values observed in testes-intact animals. Infusions of tolbutamide, but not vehicle, also decreased latencies to mount and intromit in castrated males. Unilateral tolbutamide infusion directly into the POA significantly reduced mount latency of castrates; however, it did not affect other copulatory measures, suggesting that blockade of K(+)(ATP) channels in additional brain regions may be necessary to recover the full range of sexual behavior. These data indicate that blockade of K(+)(ATP) channels is sufficient to elicit the male sexual response in the absence of testosterone. Our observations are consistent with the hypothesis that testosterone modulates male sexual behavior by regulating K(+)(ATP) channels in the brain. Decreased channel expression or channel blockade may increase the excitability of androgen-target neurons, rendering them more sensitive to the hormonal, chemical, and somatosensory inputs they receive, and potentially increase secretion of neurotransmitters that facilitate sexual behavior.

Expression of an activating mutation in the gene encoding the KATP channel subunit Kir6.2 in mouse pancreatic beta cells recapitulates neonatal diabetes

Neonatal diabetes is a rare monogenic form of diabetes that usually presents within the first six months of life. It is commonly caused by gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of the plasmalemmal ATP-sensitive K+ (KATP) channel. To better understand this disease, we generated a mouse expressing a Kir6.2 mutation (V59M) that causes neonatal diabetes in humans and we used Cre-lox technology to express the mutation specifically in pancreatic beta cells. These beta-V59M mice developed severe diabetes soon after birth, and by 5 weeks of age, blood glucose levels were markedly increased and insulin was undetectable. Islets isolated from beta-V59M mice secreted substantially less insulin and showed a smaller increase in intracellular calcium in response to glucose. This was due to a reduced sensitivity of KATP channels in pancreatic beta cells to inhibition by ATP or glucose. In contrast, the sulfonylurea tolbutamide, a specific blocker of KATP channels, closed KATP channels, elevated intracellular calcium levels, and stimulated insulin release in beta-V59M beta cells, indicating that events downstream of KATP channel closure remained intact. Expression of the V59M Kir6.2 mutation in pancreatic beta cells alone is thus sufficient to recapitulate the neonatal diabetes observed in humans. beta-V59M islets also displayed a reduced percentage of beta cells, abnormal morphology, lower insulin content, and decreased expression of Kir6.2, SUR1, and insulin mRNA. All these changes are expected to contribute to the diabetes of beta-V59M mice. Their cause requires further investigation.

Downregulation of Kir6.1/SUR2B channels in the obese rat aorta

OBJECTIVE

This study was designed to evaluate the contribution of adenosine triphosphate-dependent potassium channels to the increase in blood pressure observed in obese rats.

METHODS

The experiment was performed in male Sprague-Dawley rats. Glibenclamide-sensitive currents were measured in vascular smooth muscle cells by patch-clamp. Expressions of Kir6.1 and SUR2B were examined by reverse transcription polymerase chain reaction and western blot techniques, respectively.

RESULTS

In the aortic vascular smooth muscle cells, pinacidil induced glibenclamide-sensitive currents. The current from obese rats was significantly lower (-10.55 +/- 1.63 pA/pF) compared with that from the control rats (-20.18 +/- 2.79 pA/pF). Expressions of Kir6.1 and SUR2B were downregulated in vascular smooth muscle cells of aortas from the obese rats.

CONCLUSION

These findings suggest that the adenosine triphosphate-dependent potassium channel is downregulated in smooth muscle cells from the aortas of obese rats, which may contribute to the increase in blood pressure in these rats.

Altered ATP-sensitive potassium channels may underscore obesity-triggered increase in blood pressure

AIM

To determine whether ATP-sensitive potassium channels are altered in VSMC from arotas and mesenteric arteries of obese rat, and their association with obesity-triggered increase in blood pressure.

METHODS

Obesity was induced by 24 weeks of high-fat diet feeding in male Sprague-Dawley rats. Control rats were fed with standard laboratory rat chow. Blood pressure and body weight of these rats were measured every 4 weeks. At the end of 24 weeks, K(ATP) channelmediated relaxation responses in the aortas and mesenteric arteries, K(ATP) channel current, and gene expression were examined, respectively.

RESULTS

Blood pressure and body weight were increased in rats fed with high-fat diet. K(ATP) channelmediated relaxation responses, currents, and K(ATP) expression in VSMC of both aortas and mesenteric arteries were inhibited in these rats.

CONCLUSION

Altered ATP-sensitive potassium channels in obese rats may underscore obesity-triggered increase in blood pressure.

Altered dietary carbohydrates significantly affect gene expression of the major glucosensing components in Brockmann bodies and hypothalamus of rainbow trout

Carnivorous fish have a limited capacity to utilize dietary carbohydrates even though glucosensing components exist in the hypothalamus and Brockmann bodies. Therefore, we fed trout for 10 days with two experimental diets containing a high level of carbohydrates (20%) or a carbohydrate-free level (<0.3%) to test the capacity of dietary carbohydrates to regulate gene expression of glucosensing components. Fish were fed and killed 1, 6, and 24 h after the meal to analyze plasma glucose levels, glucosensing-related biochemical parameters, and gene expression of the major components of the glucosensing system in the hypothalamus and Brockmann bodies. Glucose facilitative transporter type 2 and glucokinase gene expression were confirmed by real-time PCR data and two new components of the glucosensing mechanism, Kir6.-like and sulfonylurea receptor-like, were detected for the first time in fish in both tissues. In addition, a clear adaptation to dietary carbohydrates was found in trout Brockmann bodies, based on increased gene expression of major components of the system as well as enhanced glucokinase activities and glycogen levels. In contrast, in the hypothalamus, only glucokinase gene expression and activity showed a response to dietary carbohydrates, supporting the key role of that enzyme in glucosensing mechanism. Finally, a differential postprandial profile was found between tissues regarding the glucosensing potential, since the hypothalamus seems to respond to hyperglycemia earlier than the Brockmann bodies, whose response took place later. Altogether, these data describe for the first time in fish a complete response of major glucosensing components to dietary carbohydrates in trout hypothalamus and Brockmann bodies, supporting an efficient adaptation of both tissues to those dietary components.

Metabolic environment in substantia nigra reticulata is critical for the expression and control of hypoglycemia-induced seizures

Seizures represent a common and serious complication of hypoglycemia. Here we studied mechanisms of control of hypoglycemic seizures induced by insulin injection in fasted and nonfasted rats. We demonstrate that fasting predisposes rats to more rapid and consistent development of hypoglycemic seizures. However, the fasting-induced decrease in baseline blood glucose concentration cannot account for the earlier onset of seizures in fasted versus nonfasted rats. Data obtained with c-Fos immunohistochemistry and [14C]2-deoxyglucose uptake implicate a prominent involvement of the substantia nigra reticulata (SNR) among other structures in the hypoglycemic seizure control. This is supported by data showing that fasting decreases the SNR expression of K(ATP) channels, which link metabolism with activity, and is further confirmed with microinfusions of K(ATP) channel agonist and antagonist. Data obtained with whole-cell and perforated patch recordings from SNR neurons in slices in vitro demonstrate that both presynaptic and postsynaptic K(ATP) channels participate in the failure of the SNR to control hypoglycemic seizures. The results suggest that fasting and insulin-induced hypoglycemia can lead to impairment in the function of the SNR, leading thus to hypoglycemic seizures.

Neurocircuits integrating hormone and nutrient signaling in control of glucose metabolism

As obesity, diabetes, and associated comorbidities are on a constant rise, large efforts have been put into better understanding the cellular and molecular mechanisms by which nutrients and metabolic signals influence central and peripheral energy regulation. For decades, peripheral organs as a source and a target of such cues have been the focus of study. Their ability to integrate metabolic signals is essential for balanced energy and glucose metabolism. Only recently has the pivotal role of the central nervous system in the control of fuel partitioning been recognized. The rapidly expanding knowledge on the elucidation of molecular mechanisms and neuronal circuits involved is the focus of this review.

Ovarian steroids stimulate adenosine triphosphate-sensitive potassium (KATP) channel subunit gene expression and confer responsiveness of the gonadotropin-releasing hormone pulse generator to KATP channel modulation

The ATP-sensitive potassium (K(ATP)) channels couple intracellular metabolism to membrane potential. They are composed of Kir6.x and sulfonylurea receptor (SUR) subunits and are expressed in hypothalamic neurons that project to GnRH neurons. However, their roles in regulating GnRH secretion have not been determined. The present study first tested whether K(ATP) channels regulate pulsatile GnRH secretion, as indirectly reflected by pulsatile LH secretion. Ovariectomized rats received sc capsules containing oil, 17beta-estradiol (E(2)), progesterone (P), or E(2)+P at 24 h before blood sampling. Infusion of the K(ATP) channel blocker tolbutamide into the third ventricle resulted in increased LH pulse frequency in animals treated with E(2)+P but was without effect in all other groups. Coinfusion of tulbutamide and the K(ATP) channel opener diazoxide blocked this effect, whereas diazoxide alone suppressed LH. Effects of steroids on Kir6.2 and SUR1 mRNA expression were then evaluated. After 24hr treatment, E(2)+P produced a modest but significant increase in Kir6.2 expression in the preoptic area (POA), which was reversed by P receptor antagonism with RU486. Neither SUR1 in the POA nor both subunits in the mediobasal hypothalamus were altered by any steroid treatment. After 8 d treatment, Kir6.2 mRNA levels were again enhanced by E(2)+P but to a greater extent in the POA. Our findings demonstrate that 1) blockade of preoptic/hypothalamic K(ATP) channels produces an acceleration of the GnRH pulse generator in a steroid-dependent manner and 2) E(2)+P stimulate Kir6.2 gene expression in the POA. These observations are consistent with the hypothesis that the negative feedback actions of ovarian steroids on the GnRH pulse generator are mediated, in part, by their ability to up-regulate K(ATP) channel subunit expression in the POA.

High-fat diet may impair K(ATP) channels in vascular smooth muscle cells

K(ATP) channel in vascular smooth muscle cell (VSMC) is closely linked to the etiology of hypertension. The aim of this study was to investigate effect of the high-fat diet-induced obesity on K(ATP) channel and blood pressure. Obesity was induced by a 24-week high-fat diet feeding in rats. Function and expression of K(ATP) channel in mesenteric arteries were examined using myography system, patch clamp and Western blotting. We show that high-fat diet increased blood pressure, decreased K(ATP) channel-mediated relaxation responses and currents, and down-regulated K(ATP) expression in VSMC. In conclusion, diet-induced obesity impairs K(ATP) channels in VSMC, which may underscore obesity-triggered increase in blood pressure.

Reduced expression of the KATP channel subunit, Kir6.2, is associated with decreased expression of neuropeptide Y and agouti-related protein in the hypothalami of Zucker diabetic fatty rats

The link between obesity and diabetes is not fully understood but there is evidence to suggest that hypothalamic signalling pathways may be involved. The hypothalamic neuropeptides, pro-opiomelanocortin (POMC), neuropeptide Y (NPY) and agouti-related protein (AGRP) are central to the regulation of food intake and have been implicated in glucose homeostasis. Therefore, the expression of these genes was quantified in hypothalami from diabetic Zucker fatty (ZDF) rats and nondiabetic Zucker fatty (ZF) rats at 6, 8, 10 and 14 weeks of age. Although both strains are obese, only ZDF rats develop pancreatic degeneration and diabetes over this time period. In both ZF and ZDF rats, POMC gene expression was decreased in obese versus lean rats at all ages. By contrast, although there was the expected increase in both NPY and AGRP expression in obese 14-week-old ZF rats, the expression of NPY and AGRP was decreased in 6-week-old obese ZDF rats with hyperinsulinaemia and in 14-week-old rats with the additional hyperglycaemia. Therefore, candidate genes involved in glucose, and insulin signalling pathways were examined in obese ZDF rats over this age range. We found that expression of the ATP-sensitive potassium (K(ATP)) channel component, Kir6.2, was decreased in obese ZDF rats and was lower compared to ZF rats in each age group tested. Furthermore, immunofluorescence analysis showed that Kir6.2 protein expression was reduced in the dorsomedial and ventromedial hypothalamic nuclei of 6-week-old prediabetic ZDF rats compared to ZF rats. The Kir6.2 immunofluorescence colocalised with NPY throughout the hypothalamus. The differences in Kir6.2 expression in ZF and ZDF rats mimic those of NPY and AGRP, which could infer that the changes occur in the same neurones. Overall, these data suggest that chronic changes in hypothalamic Kir6.2 expression may be associated with the development of hyperinsulinaemia and hyperglycaemia in ZDF rats.