Mg2+ sensitizes KATP channels to inhibition by DIDS: dependence on the sulphonylurea receptor subunit

The Role of Potassium Channels in the Vasodilatation Induced by Resveratrol and Naringenin in Isolated Human Umbilical Vein

Preclinical Research Potassium (K⁺ ) channels have a key role in the maintenance of smooth muscle tone; a variety of agonists can modify the tone by altering K⁺ -channel activity. The aim of this study was assess the effects of the phenols, resveratrol, and naringenin on K⁺ -channels of the vascular smooth muscle. Segments of human umbilical vein (HUV) without endothelium were precontracted using serotonin (100 μM) or 100 mM K⁺ to derive cumulative concentration-response curves using increasing concentrations of resveratrol or naringenin. K⁺ -channel inhibitors were added in the bath before resveratrol (1-100 μM) or naringenin (0.01-1 mM) in assess the role of K⁺ -channels in their effects on HUV precontracted by serotonin. 4-Aminopiridine (4-AP; 1 mM), a nonselective blocker of voltage-dependent, tetraethylammonium (TEA; 1 mM) and barium chloride (1 mM), a nonselective blocker of Ca²⁺ -dependent and inward rectifier K⁺ -channels (respectively) induced significant shifts to the right (P < 0.05) of resveratrol. concentration-response curves. The effect of naringenin was antagonized by 4-AP (1 mM). 4-AP-, TEA-, and barium chloride-sensitive K⁺ -channels are probably involved in the resveratrol vasodilatatory effect, while naringenin seems to affect 4-AP-sensitive K⁺ -channels. However, other mechanisms of vasodilation induced by polyphenols could not be excluded. Drug Dev Res, 2015. © 2015 Wiley Periodicals, Inc.

CFTR and Anoctamin 1 (ANO1) contribute to cAMP amplified exocytosis and insulin secretion in human and murine pancreatic beta-cells

BACKGROUND

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene lead to the disease cystic fibrosis (CF). Although patients with CF often have disturbances in glucose metabolism including impaired insulin release, no previous studies have tested the hypothesis that CFTR has a biological function in pancreatic beta-cells.

METHODS

Experiments were performed on islets and single beta-cells from human donors and NMRI-mice. Detection of CFTR was investigated using PCR and confocal microscopy. Effects on insulin secretion were measured with radioimmunoassay (RIA). The patch-clamp technique was used to measure ion channel currents and calcium-dependent exocytosis (as changes in membrane capacitance) on single cells with high temporal resolution. Analysis of ultrastructure was done on transmission electron microscopy (TEM) images.

RESULTS

We detected the presence of CFTR and measured a small CFTR conductance in both human and mouse beta-cells. The augmentation of insulin secretion at 16.7 mM glucose by activation of CFTR by cAMP (forskolin (FSK) or GLP-1) was significantly inhibited when CFTR antagonists (GlyH-101 and/or CFTRinh-172) were added. Likewise, capacitance measurements demonstrated reduced cAMP-dependent exocytosis upon CFTR-inhibition, concomitant with a decreased number of docked insulin granules. Finally, our studies demonstrate that CFTR act upstream of the chloride channel Anoctamin 1 (ANO1; TMEM16A) in the regulation of cAMP- and glucose-stimulated insulin secretion.

CONCLUSION

Our work demonstrates a novel function for CFTR as a regulator of pancreatic beta-cell insulin secretion and exocytosis, and put forward a role for CFTR as regulator of ANO1 and downstream priming of insulin granules prior to fusion and release of insulin. The pronounced regulatory effect of CFTR on insulin secretion is consistent with impaired insulin secretion in patients with CF.

Effect of type 1 diabetes on the production and vasoactivity of hydrogen sulfide in rat middle cerebral arteries

Hydrogen sulfide (H₂S) is produced endogenously in vascular tissue and has both vasoregulation and antioxidant effects. This study examines the effect of diabetes-induced oxidative stress on H₂S production and function in rat middle cerebral arteries. Diabetes was induced in rats with streptozotocin (50 mg/kg, i.v.). Middle cerebral artery function was examined using a small vessel myograph and superoxide anion generation measured using nicotinamide adenine dinucleotide phosphate (NADPH)-dependent lucigenin-enhanced chemiluminescence. Cystathionine-γ-lyase (CSE) mRNA expression was measured via RT-PCR. Diabetic rats had elevated blood glucose and significantly reduced cerebral artery endothelial function. Maximum vasorelaxation to the H₂S donor NaHS was unaffected in diabetic cerebral arteries and was elicited via a combination of K⁺, Cl⁻, and Ca²⁺ channel modulation, although the contribution of Cl⁻ channels was significantly less in the diabetic cerebral arteries. Vasorelaxation to the H₂S precursor l-cysteine and CSE mRNA were significantly increased in diabetic cerebral arteries. Cerebral artery superoxide production was significantly increased in diabetes, but this increase was attenuated ex vivo by incubation with the H₂S donor NaHS. These data confirm that cerebral artery endothelial dysfunction and oxidative stress occurs in diabetes. Endogenous H₂S production and activity is upregulated in cerebral arteries in this model of diabetes. Vasorelaxation responses to exogenous H₂S are preserved and exogenous H₂S attenuates the enhanced cerebral artery generated superoxide observed in the diabetic group. These data suggest that upregulation of endogenous H₂S in diabetes may play an antioxidant and vasoprotective role.

Resveratrol binds to the sulfonylurea receptor (SUR) and induces apoptosis in a SUR subtype-specific manner

Sulfonylurea receptors (SURs) constitute the regulatory subunits of ATP-sensitive K+ channels (K(ATP) channels). SUR binds nucleotides and synthetic K(ATP) channel modulators, e.g. the antidiabetic sulfonylurea glibenclamide, which acts as a channel blocker. However, knowledge about naturally occurring ligands of SUR is very limited. In this study, we show that the plant phenolic compound trans-resveratrol can bind to SUR and displace binding of glibenclamide. Electrophysiological measurements revealed that resveratrol is a blocker of pancreatic SUR1/K(IR)6.2 K(ATP) channels. We further demonstrate that, like glibenclamide, resveratrol induces enhanced apoptosis. This was shown by analyzing different apoptotic parameters (cell detachment, nuclear condensation and fragmentation, and activities of different caspase enzymes). The observed apoptotic effect was specific to cells expressing the SUR1 isoform and was not mediated by the electrical activity of K(ATP) channels, as it was observed in human embryonic kidney 293 cells expressing SUR1 alone. Enhanced susceptibility to resveratrol was not observed in pancreatic beta-cells from SUR1 knock-out mice or in cells expressing the isoform SUR2A or SUR2B or the mutant SUR1(M1289T). Resveratrol was much more potent than glibenclamide in inducing SUR1-specific apoptosis. Treatment with etoposide, a classical inducer of apoptosis, did not result in SUR isoform-specific apoptosis. In conclusion, resveratrol is a natural SUR ligand that can induce apoptosis in a SUR isoform-specific manner. Considering the tissue-specific expression patterns of SUR isoforms and the possible effects of SUR mutations on susceptibility to apoptosis, these observations could be important for diabetes and/or cancer research.

Human ether-a-go-go-related (HERG) gene and ATP-sensitive potassium channels as targets for adverse drug effects

Torsades de pointes (TdP) arrhythmia is a potentially fatal form of ventricular arrhythmia that occurs under conditions where cardiac repolarization is delayed (as indicated by prolonged QT intervals from electrocardiographic recordings). A likely mechanism for QT interval prolongation and TdP arrhythmias is blockade of the rapid component of the cardiac delayed rectifier K+ current (IKr), which is encoded by human ether-a-go-go-related gene (HERG). Over 100 non-cardiovascular drugs have the potential to induce QT interval prolongations in the electrocardiogram (ECG) or TdP arrhythmias. The binding site of most HERG channel blockers is located inside the central cavity of the channel. An evaluation of possible effects on HERG channels during the development of novel drugs is recommended by international guidelines. During cardiac ischaemia activation of ATP-sensitive K+ (KATP) channels contributes to action potential (AP) shortening which is either cardiotoxic by inducing re-entrant ventricular arrhythmias or cardioprotective by inducing energy-sparing effects or ischaemic preconditioning (IPC). KATP channels are formed by an inward-rectifier K+ channel (Kir6.0) and a sulfonylurea receptor (SUR) subunit: Kir6.2 and SUR2A in cardiac myocytes, Kir6.2 and SUR1 in pancreatic beta-cells. Sulfonylureas and glinides stimulate insulin secretion via blockade of the pancreatic beta-cell KATP channel. Clinical studies about cardiotoxic effects of sulfonylureas are contradictory. Sulfonylureas and glinides differ in their selectivity for pancreatic over cardiovascular KATP channels, being either selective (tolbutamide, glibenclamide) or non-selective (repaglinide). The possibility exists that non-selective KATP channel inhibitors might have cardiovascular side effects. Blockers of the pore-forming Kir6.2 subunit are insulin secretagogues and might have cardioprotective or cardiotoxic effects during cardiac ischaemia.

The Mechanism of Endothelium-Independent Relaxation Induced by the Wine Polyphenol Resveratrol in Human Internal Mammary Artery

Resveratrol, a stilbene polyphenol found in grapes and red wine, produces vasorelaxation in both endothelium-dependent and endothelium-independent manners. The mechanisms by which resveratrol causes vasodilatation are uncertain. The aim of this study was to investigate the mechanism(s) of endothelium-independent resveratrol-induced vasorelaxation in human internal mammary artery (HIMA) obtained from male patients undergoing coronary artery bypass surgery and to clarify the contribution of different K+ channel subtypes in resveratrol action in this blood vessel. HIMA rings without endothelium were precontracted with phenylephrine. Resveratrol induced a concentration-dependent relaxation of the HIMA. A highly selective blocker of ATP-sensitive K+ channels, glibenclamide, as well as nonselective blockers of Ca2+-sensitive K+ channels, tetraethylammonium and charybdotoxin, did not block resveratrol induced relaxation of HIMA rings. 4-Aminopyridine (4-AP), non selective blocker of voltage gated K+ (KV) channels, and margatoxin that inhibits KV1.2, KV1.3, and KV1.6 channels abolished relaxation of HIMA rings induced by resveratrol. In conclusion, we have shown that resveratrol potently relaxed HIMA rings with denuded endothelium. It seems that 4-AP- and margatoxin-sensitive K+ channels located in smooth muscle of HIMA mediated this relaxation.