Tissue specificity of sulfonylureas: studies on cloned cardiac and beta-cell K(ATP) channels

ATP-sensitive K(+) channels in rat colonic epithelium

ATP-sensitive K(+) (KATP) channels couple the metabolic state of a cell to its electrical activity. They consist of a hetero-octameric complex with pore-forming Kir6.x (Kir6.1, Kir6.2) and regulatory sulfonylurea receptor (SUR) subunits. Functional data indicate that KATP channels contribute to epithelial K(+) currents at colonic epithelia. However, their molecular identity and their properties are largely unknown. Therefore, changes in short-circuit current (I sc) induced by the KATP channel opener pinacidil (5 10(-4) mol l(-1)) were measured in Ussing chambers under control conditions and in the presence of different blockers of KATP channels. The channel subunits expressed by the colonic epithelium were identified by immunohistochemistry and by RT-PCR. The K(+) channel opener, when administered at the serosal side, induced an increase in I sc consistent with the induction of transepithelial Cl(-) secretion after activation of basolateral K(+) channels, whereas mucosal administration of pinacidil resulted in a negative I sc. The increase in I sc evoked by serosal pinacidil was inhibited by serosal administration of glibenclamide (5 10(-4) mol l(-1)) and gliclazide (10(-6) mol l(-1)), but was resistant even against a high concentration (10(-2) mol l(-1)) of tolbutamide. In contrast, none of these inhibitors (administered at the mucosal side) reduced significantly the negative I sc induced by mucosal pinacidil. Instead, pinacidil inhibited Cl(-) currents across apical Cl(-) channels in basolaterally depolarized epithelia indicating that the negative I sc induced by mucosal pinacidil is due to a transient inhibition of Cl(-) secretion. In mRNA prepared from isolated colonic crypts, messenger RNA for both pore-forming subunits, Kir6.1 and Kir6.2, and two regulatory subunits (SUR1 and SUR2B) was found. Expression within the colonic epithelium was confirmed for these subunits by immunohistochemistry. In consequence, KATP channels are present in the basolateral membrane of the colonic epithelium; their exact subunit composition, however, has still to be revealed.

Localization of the ATP-sensitive K(+) channel regulatory subunits SUR2A and SUR2B in the rat brain

ATP-sensitive K(+) (K(ATP)) channel subunits SUR2A and SUR2B in the rat brain were investigated by RT-PCR assay, western blot analysis, in situ hybridization histochemistry, and immunohistochemical staining. The results show that the mRNA and protein of SUR2A and SUR2B are expressed in whole rat brain extracts and selected regions. SUR2 mRNA is widely expressed in many neurons and glial cells as revealed by in situ hybridization histochemistry. Immunohistochemical staining shows SUR2A to be widely expressed in neurons of the brain, especially in the large pyramidal neurons and their main dendrites in the neocortex and in the Purkinje cells of the cerebellar cortex. In contrast to SUR2A, SUR2B is potently expressed in small cells in the corpus callosum and cerebellar white matter, but is also weakly expressed in some neurons. Double immunostaining shows SUR2B to be localized in astrocytes and oligodendrocytes, while SUR2A is only localized in oligodendrocytes. These results suggest that SUR2A might be mainly a regulatory subunit of the K(ATP) channel in most neurons and part of oligodendrocytes, while SUR2B might be mainly a regulatory subunit of the K(ATP) channel in astrocytes, oligodendrocytes, and some neurons.

Pharmacogenomic analysis of ATP-sensitive potassium channels coexpressing the common type 2 diabetes risk variants E23K and S1369A

OBJECTIVES

The common ATP-sensitive potassium (KATP) channel variants E23K and S1369A, found in the KCNJ11 and ABCC8 genes, respectively, form a haplotype that is associated with an increased risk for type 2 diabetes. Our previous studies showed that KATP channel inhibition by the A-site sulfonylurea gliclazide was increased in the K23/A1369 haplotype. Therefore, we studied the pharmacogenomics of seven clinically used sulfonylureas and glinides to determine their structure-activity relationships in KATP channels containing either the E23/S1369 nonrisk or K23/A1369 risk haplotypes.

RESEARCH DESIGN AND METHODS

The patch-clamp technique was used to determine sulfonylurea and glinide inhibition of recombinant human KATP channels containing either the E23/S1369 or the K23/A1369 haplotype.

RESULTS

KATP channels containing the K23/A1369 risk haplotype were significantly less sensitive to inhibition by tolbutamide, chlorpropamide, and glimepiride (IC50 values for K23/A1369 vs. E23/S1369=1.15 vs. 0.71 μmol/l; 4.19 vs. 3.04 μmol/l; 4.38 vs. 2.41 nmol/l, respectively). In contrast, KATP channels containing the K23/A1369 haplotype were significantly more sensitive to inhibition by mitiglinide (IC50=9.73 vs. 28.19 nmol/l for K23/A1369 vs. E23/S1369) and gliclazide. Nateglinide, glipizide, and glibenclamide showed similar inhibitory profiles in KATP channels containing either haplotype.

CONCLUSION

Our results demonstrate that the ring-fused pyrrole moiety in several A-site drugs likely underlies the observed inhibitory potency of these drugs on KATP channels containing the K23/A1369 risk haplotype. It may therefore be possible to tailor existing therapy or design novel drugs that display an increased efficacy in type 2 diabetes patients homozygous for these common KATP channel haplotypes.

Gene networks modified by sulphonylureas in beta cells: a pathway-based analysis of insulin secretion and cell death

Sulphonylureas (SUs) used in the treatment for type 2 diabetes have been shown to result in different clinical outcome. This study hypothesized that three widely used SUs, glibenclamide, glimepiride and gliclazide, may affect function and survival of insulin-producing cells differently. To evaluate differences between SUs, insulin secretion and cell death were measured, and genome-wide gene expression patterns were compared using a bioinformatics approach focusing on functional relationships between molecules. Insulin-producing INS-1E cells exposed to SUs for 6 and 24 hr were assayed using GeneChip. Cluster and pathway analyses were used to identify differentially expressed genes and patterns of potential biological functions associated with SU treatment. Cell death was measured using acridine orange/Hoechst 33342 staining. Short-term treatment (6 hr) yielded up-regulation of insulin secretion and genes associated with insulin secretion for all three SUs applied. While long-term treatment (24-72 hr) with gliclazide did not change gene expression or cell survival, treatment with glibenclamide or glimepiride up-regulated genes associated with oxidative stress and hypoxia, but did not induce cell death. Short-term treatment with SUs initiates gene regulation that can be attributed to insulin secretion with few differences between individual SUs. This regulation was temporal and returned to baseline after 24 hr. Individual differences observed after 24-72 hr indicate that glibenclamide and glimepiride induce potentially harmful cell signalling insufficient for triggering beta cell death.

Variations in tissue selectivity amongst insulin secretagogues: a systematic review

AIM

Insulin secretagogues promote insulin release by binding to sulfonylurea receptors on pancreatic β-cells (SUR1). However, these drugs also bind to receptor isoforms on cardiac myocytes (SUR2A) and vascular smooth muscle (SUR2B). Binding to SUR2A/SUR2B may inhibit ischaemic preconditioning, an endogenous protective mechanism enabling cardiac tissue to survive periods of ischaemia. This study was designed to identify insulin secretagogues that selectively bind to SUR1 when given at therapeutic doses.

METHODS

Using accepted systematic review methods, three electronic databases were searched from inception to 13 June 2011. Original studies measuring the half-maximal inhibitory concentration (IC(50)) for an insulin secretagogue on K(ATP) channels using standard electrophysiological techniques were included. Steady-state concentrations (C(SS)) were estimated from the usual oral dose and clearance values for each drug.

RESULTS

Data were extracted from 27 studies meeting all inclusion criteria. IC(50) values for SUR1 were below those for SUR2A/SUR2B for all insulin secretagogues and addition of C(SS) values identified three distinct patterns. The C(SS) for gliclazide, glipizide, mitiglinide and nateglinide lie between IC(50) values for SUR1 and SUR2A/SUR2B, suggesting that these drugs bind selectively to pancreatic receptors. The C(SS) for glimepiride and glyburide (glibenclamide) was above IC(50) values for all three isoforms, suggesting these drugs are non-selective. Tolbutamide and repaglinide may have partial pancreatic receptor selectivity because IC(50) values for SUR1 and SUR2A/SUR2B overlapped somewhat, with the C(SS) in the midst of these values.

CONCLUSIONS

Insulin secretagogues display different tissue selectivity characteristics at therapeutic doses. This may translate into different levels of cardiovascular risk.

The involvement of adenosine triphosphate-sensitive potassium channels in the different effects of sevoflurane and propofol on glucose metabolism in fed rats

BACKGROUND

Recently, we reported marked differences in the effects of sevoflurane and propofol on glucose metabolism; glucose use is impaired by sevoflurane, but not by propofol. Opening of adenosine triphosphate-sensitive potassium channels (K(ATP) channels) in β islet cells attenuates insulin secretion, while inhibition of K(ATP) channels in β islet cells increases insulin secretion. It is reported that volatile anesthetics open K(ATP) channels, whereas propofol inhibits K(ATP) channels. In this study, we examined the effects of sevoflurane and propofol on glucose metabolism under normovolemic and hypovolemic conditions, focusing on insulin secretion.

METHODS

Anesthesia was induced with sevoflurane (3% in 1 L/min oxygen) in all rats. After surgical preparation, rats were assigned to 2 groups. Anesthesia was maintained with sevoflurane (2% in 1 L/min oxygen) in the 1st group, and with propofol (a bolus dose of 30 mg/kg followed by continuous infusion at a rate of 30 mg · kg(-1) · h(-1)) in the 2nd group. Each group was divided into 3 subgroups: rats without pretreatment, rats pretreated with glibenclamide, and rats pretreated with nicorandil. After a 30-minute stabilization period, we withdrew 15 mL/kg of blood to induce hypovolemia. We evaluated glucose metabolism under both normovolemic and hypovolemic conditions by measuring blood glucose levels and plasma insulin levels.

RESULTS

Under both normovolemia and hypovolemia, glucose levels in rats anesthetized with sevoflurane were significantly higher than those in rats anesthetized with propofol, and insulin levels in rats anesthetized with sevoflurane were significantly lower than those in rats anesthetized with propofol. Glibenclamide, a K(ATP) channel inhibitor, significantly decreased glucose levels and significantly increased insulin levels under sevoflurane anesthesia, suggesting that sevoflurane decreases insulin secretion by opening K(ATP) channels in β islet cells. Glibenclamide significantly decreased glucose levels and significantly increased insulin levels under propofol anesthesia as well; however, insulin levels in rats pretreated with glibenclamide under propofol anesthesia were much higher than those in rats pretreated with glibenclamide under sevoflurane anesthesia. Furthermore, insulin levels in rats without pretreatment under propofol anesthesia seemed to be equal to or higher than those in rats pretreated with glibenclamide under sevoflurane anesthesia. These results suggest that there are marked differences in the effects of sevoflurane and propofol on insulin secretion regulated by K(ATP) channels in β islet cells. Nicorandil, a K(ATP) channel opener, produced no significant effects on glucose metabolism under both sevoflurane and propofol anesthesia.

CONCLUSIONS

Insulin secretion regulated by K(ATP) channels in β islet cells is involved, at least in part, in the different effects of sevoflurane and propofol on glucose metabolism.

Subsets of ATP-sensitive potassium channel (KATP) inhibitors increase gap junctional intercellular communication in metastatic cancer cell lines independent of SUR expression

Gap junctional intercellular communication (GJIC) regulates cellular homeostasis by propagating signaling molecules, exchanging cellular metabolites, and coupling electrical signals. In cancer, cells exhibit altered rates of GJIC which may play a role in neoplastic progression. K(ATP) channels help maintain membrane polarity and linkages between K(ATP) channel activity and rates of GJIC have been established. The mechanistic relationship has not been fully elucidated. We report the effects of treatment with multiple K(ATP) antagonist compounds on GJIC in metastatic cell lines demonstrating an increase in communication rates following treatment with compounds possessing specificities towards the SUR2 subunit of K(ATP). These effects remained consistent using cell lines with different expression levels of SUR1 and SUR2, suggesting possible off target effects on GJIC by these compounds.

Comparable long-term mortality risk associated with individual sulfonylureas in diabetes patients with heart failure

AIMS

The aim was to investigate the outcomes of individual sulfonylureas in patients with heart failure (HF).

METHODS

All patients hospitalized with HF for the first time in 1997-2006, alive 30 days after discharge, and who received anti-diabetic monotherapy with glimepiride (n=1097), glibenclamide (glyburide) (n=1031), glipizide (n=557), gliclazide (n=251), or tolbutamide (n=541) were identified from nationwide registers. Risk of all-cause mortality was assessed by multivariable Cox regression models.

RESULTS

Over the median observational time of 744 (Inter Quartile Range 268-1451) days, 2242 patients (64%) died. The analysis demonstrated similar hazard ratio (HR) for mortality for treatment with glimepiride (1.10 [95% confidence interval 0.92-1.33]), glibenclamide (1.12 [0.93-1.34]), glipizide (1.14 [0.93-1.38]), tolbutamide (1.04 [0.85-1.26]), and gliclazide (reference). Grouped according to pancreatic specificity, i.e., with tolbutamide, glipizide, and gliclazide as specific, and glibenclamide, and glimepiride as non-specific agents, no differential prognosis was found between the two groups (HR 1.04 [0.96-1.14], for non-specific, compared to pancreas specific agents). The prognosis was not dependent on prior acute myocardial infarction or ischemic heart disease (p for interactions >0.3).

CONCLUSIONS

In current clinical practice, it is unlikely that there are considerable differences in risk of mortality associated with individual sulfonylureas in patients with heart failure.

Pharmacokinetic and pharmacodynamic drug interactions of carbamazepine and glibenclamide in healthy albino Wistar rats

AIMS

To find out the pharmacokinetic and pharmacodynamic drug interaction of carbamazepine, a protype drug used to treat painful diabetic neuropathy with glibenclamide in healthy albino Wistar rats following single and multiple dosage treatment.

MATERIALS AND METHODS

Therapeutic doses (TD) of glibenclamide and TD of carbamazepine were administered to the animals. The blood glucose levels were estimated by GOD/POD method and the plasma glibenclamide concentrations were estimated by a sensitive RP HPLC method to calculate pharmacokinetic parameters.

RESULTS

In single dose study the percentage reduction of blood glucose levels and glibenclamide concentrations of rats treated with both carbamazepine and glibenclamide were significantly increased when compared with glibenclamide alone treated rats and the mechanism behind this interaction may be due to inhibition of P-glycoprotein mediated transport of glibenclamide by carbamazepine, but in multiple dose study the percentage reduction of blood glucose levels and glibenclamide concentrations were reduced and it may be due to inhibition of P-glycoprotein mediated transport and induction of CYP2C9, the enzyme through which glibenclamide is metabolised.

CONCLUSIONS

In the present study there is a pharmacokinetic and pharmacodynamic interaction between carbamazepine and glibenclamide was observed. The possible interaction involves both P-gp and CYP enzymes. To investigate this type of interactions pre-clinically are helpful to avoid drug-drug interactions in clinical situation.

Drug-drug Interaction between Pravastatin and Gemfibrozil (Antihyperlipidemic) with Gliclazide (Antidiabetic) in Rats

Diabetes mellitus is a condition of increased blood glucose level in the body. Antihyperlipidemic drugs like statins and fibrates are widely used for prophylactic treatment in dyslipideamia and atherosclerosis. Diabetic dislipidemia exists with increased triglycerides, low HDL and high LDL levels. Hence, with oral hypoglycemic drugs, the addition of a lipid-lowering drug is necessary for controlling dislipidemia. In such a situation, there may be chances of drug–drug interactions between antidiabetic and antihyperlipidemic drugs. The present study is planned to evaluate the safety of gliclazide (antidiabetic) in the presence of pravastatin and gemfibrozil (antihyperlpidemic) in rats. Studies in normal and alloxan-induced diabetic rats were conducted with oral doses of gliclazide and their combination with pravastatin and gemfibrozil, with an adequate washout period in between the treatments. Blood samples were collected in rats by retroorbital puncture at 0, 1, 2, 3, 4, 6, 8, 10 and 12 h. All the blood samples were analyzed for glucose by GOD –POD. Gliclazide (½ TD) produced hypoglycemic activity in normal and diabetic rats, with peak activity at 2 and 8 h. Pravastatin (TD) + gemfibrozil (TD) combination treatment increased the hypoglycemic effect of gliclazide in normal rats or diabetic rats when administered together. The interaction observed due to inhibition of both the enzymes (CYP 450 2C9 and CYP 450 3A4) responsible for the metabolism of gliclazide showed increased half-life, which was seen in the present study. Because concomitant administration of gliclazide with provastatin and gemfibrozil in diabetes is associated with atherosclerosis, it should be contraindicated or used with caution.

Ontogeny of sulfonylurea-binding regulatory subunits of K(ATP) channels in the pregnant rat myometrium

ATP-sensitive potassium channels (K(ATP) channels) are composed of sulfonylurea receptors (SURs) and potassium inward rectifiers (Kir(6.x)) that assemble to form a large octameric channel. This study was designed to examine the expression and role of sulfonylurea-binding regulatory subunits 1 (SUR1 (ABCC8)) and 2 (SUR2 (ABCC9)) of the K(ATP) channels in the pregnant rat myometrium with particular regard to the contractility. RT-PCR and western blot analyses were performed to detect the presence of SUR1 and SUR2. The SUR1 levels were markedly increased in the early stages of pregnancy. The highest level was detected on day 6 of pregnancy, whereas in the late stages, the levels of SUR1 were significantly decreased. The SUR2 level remained unchanged throughout pregnancy. The SUR non-selective diazoxide and the SUR2-selective pinacidil inhibited oxytocin-induced contractions. Glibenclamide, a K(ATP) channel blocker, antagonized both pinacidil- and diazoxide-induced relaxations. It was established that SURs are responsible for pharmacological reactivity of K(ATP) channel openers. We conclude that both SURs are involved in the K(ATP) channel in the pregnant rat myometrium. It may further be concluded that 'pinacidil-like' K(ATP) channel openers may be of therapeutic relevance as tocolytic agents in the future.

Comparison of two sulfonylureas with high and low myocardial K(ATP) channel affinity on myocardial infarct size and metabolism in a rat model of type 2 diabetes

AIMS/HYPOTHESIS

Sulfonylureas (SUs) may impair outcome in patients with acute coronary syndrome. Most experimental studies of the myocardial effects of SU treatment are performed in non-diabetic models. We compared the effect of two widely used SUs, glibenclamide (gb) and gliclazide (gc), with high and low myocardial K(ATP) channel affinity, respectively, at therapeutic concentrations on infarct size, left ventricular (LV) function and myocardial glycogen, lactate and alanine content before and after ischaemia/reperfusion (I/R).

METHODS

Non-diabetic Wistar and diabetic Goto-Kakizaki rat hearts were investigated in a Langendorff preparation. Gb (0.1 μmol/l) and gc (1.0 μmol/l) were administrated throughout the study. Infarct size was evaluated after 120 min of reperfusion. Myocardial metabolite content was measured before and after ischaemia.

RESULTS

Infarct size was smaller in diabetic hearts than in non-diabetic hearts (0.33 ± 0.03 vs 0.51 ± 0.05, p < 0.05). Gb increased infarct size (0.54 ± 0.04 vs 0.33 ± 0.03, p < 0.05) and reduced post-ischaemic LV developed pressure (60 ± 3 vs 76 ± 3 mmHg, p < 0.05) and coronary flow (4.9 ± 0.5 vs 7.1 ± 0.4 ml min(-1) g(-1), p < 0.05) in gb-treated diabetic rats compared with untreated diabetic rats. On comparing gb-treated diabetic rats with untreated diabetic rats, glycogen content was reduced before (9.1 ± 0.6 vs 13.6 ± 1.0 nmol/mg wet weight, p < 0.01) and after ischaemia (0.9 ± 0.2 vs 1.8 ± 0.2 nmol/mg wet weight, p < 0.05), and lactate (4.8 ± 0.4 vs 3.2 ± 0.3 nmol/mg wet weight, p < 0.01) and alanine (1.38 ± 0.12 vs 0.96 ± 0.09 nmol/mg wet weight, p < 0.05) contents were increased during reperfusion. Gc-treatment of diabetic and non-diabetic rats did not affect any of the measured variables.

CONCLUSIONS/INTERPRETATIONS

Gb, but not gc, exacerbates I/R injury and deteriorates LV function in diabetic hearts. These effects of gb on diabetic hearts may be due to detrimental effects on myocardial carbohydrate metabolism.

HMR 1098 is not an SUR isotype specific inhibitor of heterologous or sarcolemmal K ATP channels

Murine ventricular and atrial ATP-sensitive potassium (K(ATP)) channels contain different sulfonylurea receptors (ventricular K(ATP) channels are Kir6.2/SUR2A complexes, while atrial K(ATP) channels are Kir6.2/SUR1 complexes). HMR 1098, the sodium salt of HMR 1883 {1-[[5-[2-(5-chloro-o-anisamido)ethyl]-2-methoxyphenyl]sulfonyl]-3-methylthiourea}, has been considered as a selective sarcolemmal (i.e. SUR2A-dependent) K(ATP) channel inhibitor. However, it is not clear whether HMR 1098 would preferentially inhibit ventricular K(ATP) channels over atrial K(ATP) channels. To test this, we used whole-cell patch clamp techniques on mouse atrial and ventricular myocytes as well as (86)Rb(+) efflux assays and excised inside-out patch clamp techniques on Kir6.2/SUR1 and Kir6.2/SUR2A channels heterologously expressed in COSm6 cells. In mouse atrial myocytes, both spontaneously activated and diazoxide-activated K(ATP) currents were effectively inhibited by 10 μM HMR 1098. By contrast, in ventricular myocytes, pinacidil-activated K(ATP) currents were inhibited by HMR 1098 at a high concentration (100 μM) but not at a low concentration (10 μM). Consistent with this finding, HMR 1098 inhibits (86)Rb(+) effluxes through Kir6.2/SUR1 more effectively than Kir6.2/SUR2A channels in COSm6 cells. In excised inside-out patches, HMR 1098 inhibited Kir6.2/SUR1 channels more effectively, particularly in the presence of MgADP and MgATP (mimicking physiological stimulation). Finally, dose-dependent enhancement of insulin secretion from pancreatic islets and decrease of blood glucose level confirm that HMR 1098 is an inhibitor of Kir6.2/SUR1-composed K(ATP) channels.

Hydrogen sulphide, a novel gasotransmitter involved in guard cell signalling

Hydrogen sulphide (H(2) S) has been proposed as the third gasotransmitter. In animal cells, H(2) S has been implicated in several physiological processes. H(2) S is endogenously synthesized in both animals and plants by enzymes with l-Cys desulphydrase activity in the conversion of l-Cys to H(2) S, pyruvate and ammonia. The participation of H(2) S in both stomatal movement regulation and abscisic acid (ABA)-dependent induction of stomatal closure was studied in epidermal strips of three plant species (Vicia faba, Arabidopsis thaliana and Impatiens walleriana). The effect of H(2) S on stomatal movement was contrasted with leaf relative water content (RWC) measurements of whole plants subjected to water stress. In this work we report that exogenous H(2) S induces stomatal closure and this effect is impaired by the ATP-binding cassette (ABC) transporter inhibitor glibenclamide; scavenging H(2) S or inhibition of the enzyme responsible for endogenous H(2) S synthesis partially blocks ABA-dependent stomatal closure; and H(2) S treatment increases RWC and protects plants against drought stress. Our results indicate that H(2) S induces stomatal closure and participates in ABA-dependent signalling, possibly through the regulation of ABC transporters in guard cells.

The molecular mechanisms and pharmacotherapy of ATP-sensitive potassium channel gene mutations underlying neonatal diabetes

Neonatal diabetes mellitus (NDM) is a monogenic disorder caused by mutations in genes involved in regulation of insulin secretion from pancreatic β-cells. Mutations in the KCNJ11 and ABCC8 genes, encoding the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel Kir6.2 and SUR1 subunits, respectively, are found in ∼50% of NDM patients. In the pancreatic β-cell, K(ATP) channel activity couples glucose metabolism to insulin secretion via cellular excitability and mutations in either KCNJ11 or ABCC8 genes alter K(ATP) channel activity, leading to faulty insulin secretion. Inactivation mutations decrease K(ATP) channel activity and stimulate excessive insulin secretion, leading to hyperinsulinism of infancy. In direct contrast, activation mutations increase K(ATP) channel activity, resulting in impaired insulin secretion, NDM, and in severe cases, developmental delay and epilepsy. Many NDM patients with KCNJ11 and ABCC8 mutations can be successfully treated with sulfonylureas (SUs) that inhibit the K(ATP) channel, thus replacing the need for daily insulin injections. There is also strong evidence indicating that SU therapy ameliorates some of the neurological defects observed in patients with more severe forms of NDM. This review focuses on the molecular and cellular mechanisms of mutations in the K(ATP) channel that underlie NDM. SU pharmacogenomics is also discussed with respect to evaluating whether patients with certain K(ATP) channel activation mutations can be successfully switched to SU therapy.

Recognition of sulfonylurea receptor (ABCC8/9) ligands by the multidrug resistance transporter P-glycoprotein (ABCB1): functional similarities based on common structural features between two multispecific ABC proteins

ATP-sensitive K(+) (K(ATP)) channels are the target of a number of pharmacological agents, blockers like hypoglycemic sulfonylureas and openers like the hypotensive cromakalim and diazoxide. These agents act on the channel regulatory subunit, the sulfonylurea receptor (SUR), which is an ABC protein with homologies to P-glycoprotein (P-gp). P-gp is a multidrug transporter expressed in tumor cells and in some healthy tissues. Because these two ABC proteins both exhibit multispecific recognition properties, we have tested whether SUR ligands could be substrates of P-gp. Interaction with P-gp was assayed by monitoring ATPase activity of P-gp-enriched vesicles. The blockers glibenclamide, tolbutamide, and meglitinide increased ATPase activity, with a rank order of potencies that correlated with their capacity to block K(ATP) channels. P-gp ATPase activity was also increased by the openers SR47063 (a cromakalim analog), P1075 (a pinacidil analog), and diazoxide. Thus, these molecules bind to P-gp (although with lower affinities than for SUR) and are possibly transported by P-gp. Competition experiments among these molecules as well as with typical P-gp substrates revealed a structural similarity between drug binding domains in the two proteins. To rationalize the observed data, we addressed the molecular features of these proteins and compared structural models, computerized by homology from the recently solved structures of murine P-gp and bacterial ABC transporters MsbA and Sav1866. Considering the various residues experimentally assigned to be involved in drug binding, we uncovered several hot spots, which organized spatially in two main binding domains, selective for SR47063 and for glibenclamide, in matching regions of both P-gp and SUR.

Risk of bleeding with single, dual, or triple therapy with warfarin, aspirin, and clopidogrel in patients with atrial fibrillation

BACKGROUND

Patients with atrial fibrillation (AF) often require anticoagulation and platelet inhibition, but data are limited on the bleeding risk of combination therapy.

METHODS

We performed a cohort study using nationwide registries to identify all Danish patients surviving first-time hospitalization for AF between January 1, 1997, and December 31, 2006, and their posthospital therapy of warfarin, aspirin, clopidogrel, and combinations of these drugs. Cox proportional hazards models were used to estimate risks of nonfatal and fatal bleeding.

RESULTS

A total of 82,854 of 118,606 patients (69.9%) surviving AF hospitalization had at least 1 prescription filled for warfarin, aspirin, or clopidogrel after discharge. During mean (SD) follow-up of 3.3 (2.6) years, 13,573 patients (11.4%) experienced a nonfatal or fatal bleeding. The crude incidence rate for bleeding was highest for dual clopidogrel and warfarin therapy (13.9% per patient-year) and triple therapy (15.7% per patient-year). Using warfarin monotherapy as a reference, the hazard ratio (95% confidence interval) for the combined end point was 0.93 (0.88-0.98) for aspirin, 1.06 (0.87-1.29) for clopidogrel, 1.66 (1.34-2.04) for aspirin-clopidogrel, 1.83 (1.72-1.96) for warfarin-aspirin, 3.08 (2.32-3.91) for warfarin-clopidogrel, and 3.70 (2.89-4.76) for warfarin-aspirin-clopidogrel.

CONCLUSIONS

In patients with AF, all combinations of warfarin, aspirin, and clopidogrel are associated with increased risk of nonfatal and fatal bleeding. Dual warfarin and clopidogrel therapy and triple therapy carried a more than 3-fold higher risk than did warfarin monotherapy.

Effects of oral glucose-lowering drugs on long term outcomes in patients with diabetes mellitus following myocardial infarction not treated with emergent percutaneous coronary intervention--a retrospective nationwide cohort study

Background

The optimum oral pharmacological treatment of diabetes mellitus to reduce cardiovascular disease and mortality following myocardial infarction has not been established. We therefore set out to investigate the association between individual oral glucose-lowering drugs and cardiovascular outcomes following myocardial infarction in patients with diabetes mellitus not treated with emergent percutaneous coronary intervention.

Materials and methods

All patients aged 30 years or older receiving glucose-lowering drugs (GLDs) and admitted with myocardial infarction (MI) not treated with emergent percutaneous coronary intervention in Denmark during 1997-2006 were identified by individual-level linkage of nationwide registries of hospitalizations and drug dispensing from pharmacies. Multivariable Cox regression models adjusted for age, sex, calendar year, comorbidity, and concomitant pharmacotherapy were used to assess differences in the composite endpoint of non-fatal MI and cardiovascular mortality between individual GLDs, using metformin monotherapy as reference.

Results

The study comprised 9876 users of GLDs admitted with MI. The mean age was 72.3 years and 56.5% of patients were men. A total of 3649 received sulfonylureas and 711 received metformin at admission. The average length of follow-up was 2.2 (SD 2.6) years. A total of 6,171 patients experienced the composite study endpoint. The sulfonylureas glibenclamide, glimepiride, glipizide, and tolbutamide were associated with increased risk of cardiovascular mortality and/or nonfatal MI with hazard ratios [HRs] of 1.31 (95% confidence interval [CI] 1.17-1.46), 1.19 (1.06-1.32), 1.25 (1.11-1.42), and 1.18 (1.03-1.34), respectively, compared with metformin. Gliclazide was the only sulfonylurea not associated with increased risk compared with metformin (HR 1.03 [0.88-1.22]).

Conclusions

In patients with diabetes mellitus admitted with MI not treated with emergent percutaneous coronary intervention, monotherapy treatment with the sulfonylureas glibenclamide, glimepiride, glipizide, and tolbutamide was associated with increased cardiovascular risk compared with metformin monotherapy.

The risk of overall mortality in patients with type 2 diabetes receiving glipizide, glyburide, or glimepiride monotherapy: a retrospective analysis

OBJECTIVE

Sulfonylureas have historically been analyzed as a medication class, which may be inappropriate given the differences in properties inherent to the individual sulfonylureas (hypoglycemic risk, sulfonylurea receptor selectivity, and effects on myocardial ischemic preconditioning). The purpose of this study was to assess the relationship of individual sulfonylureas and the risk of overall mortality in a large cohort of patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

A retrospective cohort study was conducted using an academic health center enterprise-wide electronic health record (EHR) system to identify 11,141 patients with type 2 diabetes (4,279 initiators of monotherapy with glyburide, 4,325 initiators of monotherapy with glipizide, and 2,537 initiators of monotherapy with glimepiride), >or=18 years of age with and without a history of coronary artery disease (CAD) and not on insulin or a noninsulin injectable at baseline. The patients were followed for mortality by documentation in the EHR and Social Security Death Index. Multivariable Cox models were used to compare cohorts.

RESULTS

No statistically significant difference in the risk of overall mortality was observed among these agents in the entire cohort, but we did find evidence of a trend toward an increased overall mortality risk with glyburide versus glimepiride (hazard ratio 1.36 [95% CI 0.96-1.91]) and glipizide versus glimepiride (1.39 [0.99-1.96]) in those with documented CAD.

CONCLUSIONS

Our results did not identify an increased mortality risk among the individual sulfonylureas but did suggest that glimepiride may be the preferred sulfonylurea in those with underlying CAD.

Presence and vascular pharmacology of KATP channel subtypes in rat central and peripheral tissues

K(ATP) channel openers are vasodilators and induce headache in normal subjects. We previously identified the Kir6.1/SUR2B K(ATP) channel subtype in major cerebral and dural arteries of rat, pig and man. We hypothesized that craniovascular Kir6.1/SUR2B K(ATP) channels mediate the headache-inducing effects of K(ATP) channel openers and that a Kir6.1/SUR2B specific blocker might be effective in the treatment of primary headaches such as migraine. Since K(ATP) channels are ubiquitous, we characterized the K(ATP) channel subtypes in major rat cranial and peripheral arteries and organs in order to understand the possible adverse effects of a Kir6.1/SUR2B blocker. We studied the mRNA expression of K(ATP) channel subunits in rat femoral, mesenteric, renal, coronary, basilar, middle cerebral and middle meningeal arteries and in tissue from rat heart, brain, liver, colon, lung, kidney and pancreas. We also studied the effects and potencies of a panel of synthetic K(ATP) channel openers and their potential inhibition by the Kir6.1 subunit-specific K(ATP) channel blocker PNU-37883A in segments of the arteries mounted in a wire myograph. Our studies suggest that Kir6.1/SUR2B forms the major functional K(ATP) channel complex in rat cranial and peripheral arteries. The mRNA transcripts of SUR1 and Kir6.2 subunits were predominantly found in brain, pancreas and heart, while SUR2A mRNA was merely detected within the heart. K(ATP) channel blockers highly specific for the SUR2B subunit may have no adverse CNS and cardiac effects and will not affect insulin release in the pancreas. However, a SUR2B blocker may not discriminate between cranial and peripheral arteries.

Novel glimepiride derivatives with potential as double-edged swords against type II diabetes

Sulphonylurea drugs have been widely used in the safe and efficacous therapy of type II diabetes during the past five decades. They lower blood glucose predominantly via the stimulation of insulin release from pancreatic beta-cells. However, a moderate insulin-independent regulation of fatty acid esterification and release in adipose tissue cells has been reported for certain sulphonylureas, in particular for glimepiride. On basis of the known pleiotropic pathogenesis of type II diabetes with a combination of beta-cell failure and peripheral, including adipocyte, insulin resistance, anti-diabetic drugs exerting both insulin releasing- and fatty acid-metabolizing activities in a more balanced and potent fashion may be of advantage. However, the completely different molecular mechanisms underlying the insulin-releasing and fatty acid-metabolizing activities, as have been delineated so far for glimepiride, may hamper their optimization within a single sulphonylurea molecule. By analyzing conventional sulphonylureas and novel glimepiride derivatives for their activities at the primary targets and downstream steps in both beta-cells and adipocytes in vitro we demonstrate here that the insulin-releasing and fatty acid-metabolizing activities are critically dependent on both overlapping and independent structural determinants. These were unravelled by the parallel losses of these two activities in a subset of glimepiride derivatives and the impairment in the insulin-releasing activity in parallel with elevation in the fatty acid-metabolizing activity in a different subset. Together these findings may provide a basis for the design of novel sulphonylureas with blood glucose-lowering activity relying on less pronounced stimulation of insulin release from pancreatic beta-cells and more pronounced insulin-independent stimulation of esterification as well as inhibition of release of fatty acids by adipocytes than provoked by the sulphonylureas currently used in therapy.

Second-generation sulfonylureas preserve inhibition of mitochondrial permeability transition by the mitochondrial K+(ATP) opener nicorandil in experimental myocardial infarction

Openers of K+(ATP) channels protect the myocardium from I/R injury. Sulfonylureas are known as potent blockers of K(ATP) channels. We investigated whether 1) mitochondrial permeability transition pore may be involved in the protection afforded by the mitoK+(ATP) opener nicorandil and 2) whether sulfonylureas may prevent this beneficial effect. Anesthetized New Zealand White rabbits underwent 30 min of coronary artery occlusion, followed by 60 (isolated mitochondria) or 240 min (infarct size) of reperfusion. They received an administration of either saline (control) or nicorandil (0.5 mg kg(-1), i.v.) 15 min before ischemia. Each control and nicorandil group was divided in four subgroups pretreated by either saline, glibenclamide (Glib; 1 mg kg(-1)), gliclazide (Glic; 1 mg kg(-1)), or glimepiride (Glim; 5 microg kg(-1)) 10 min before this. Infarct size was assessed by triphenyltetrazolium chloride staining. Mitochondria were isolated from the area at risk for further assessment of the calcium retention capacity. Glibenclamide (35 +/- 8), but neither Glic (61 +/- 9) nor Glim (48 +/- 7), reversed the improvement in calcium retention capacity due to nicorandil (58 +/- 10 vs. 27 +/- 8 nmoles CaCl2 mg(-1) proteins in control). Infarct size reduction by nicorandil (32% +/- 6% vs. 65% +/- 6% of area at risk) was abolished by Glib (55 +/- 5) but not by Glic (37 +/- 3) or Glim (31 +/- 5). These data suggest that 1) the protective effect of nicorandil involves the inhibition of the mitochondrial permeability transition pore and 2) that unlike Glib, second-generation sulfonylureas preserve this cardioprotection.

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.

Influence of non-nucleoside reverse transcriptase inhibitors (efavirenz and nevirapine) on the pharmacodynamic activity of gliclazide in animal models

BACKGROUND

Type 2 diabetes may occur as a result of HIV infection and/or its treatment. Gliclazide is a widely used drug for the treatment of type 2 diabetes. Efavirenz and nevirapine are widely used non-nucleoside reverse transcriptase inhibitors for the treatment of HIV infection. The role of Efavirenz and nevirapine on the pharmacodynamic activity of gliclazide is not currently known. The objective of this study was to examine the effect of oral administration of efavirenz and nevirapine on blood glucose and investigate their effect on the activity of gliclazide in rats (normal and diabetic) and rabbits to evaluate the safety and effectiveness of the combination.

METHODS

Studies in normal and alloxan induced diabetic rats were conducted with oral doses of 2 mg/kg bd. wt. of gliclazide, 54 mg/kg bd. wt. of efavirenz or 18 mg/kg bd. wt. of nevirapine and their combination with adequate washout periods in between treatments. Studies in normal rabbits were conducted with 5.6 mg/1.5 kg bd. wt. of gliclazide, 42 mg/1.5 kg bd. wt. of efavirenz or 14 mg/1.5 kg bd. wt. of nevirapine and their combination given orally. Blood samples were collected at regular time intervals in rats from retro orbital puncture and by marginal ear vein puncture in rabbits. All the blood samples were analysed for blood glucose by GOD/POD method.

RESULTS

Efavirenz and nevirapine alone have no significant effect on the blood glucose level in rats and rabbits. Gliclazide produced hypoglycaemic/antidiabetic activity in normal and diabetic rats with peak activity at 2 h and 8 h and hypoglycaemic activity in normal rabbits at 3 h. In combination, efavirenz reduced the effect of gliclazide in rats and rabbits, and the reduction was more significant with the single dose administration of efavirenz than multiple dose administration. In combination, nevirapine has no effect on the activity of gliclazide in rats and rabbits.

CONCLUSION

Thus, it can be concluded that the combination of efavirenz and gliclazide may need dose adjustment and care should be taken when the combination is prescribed for their clinical benefit in diabetic patients. The combination of nevirapine and gliclazide was safe. However, further studies are warranted.

Applying the Taguchi design for optimized formulation of sustained release gliclazide chitosan beads: an in vitro/in vivo study

Gliclazide is a second generation of hypoglycemic sulfonylurea and acts selectively on pancreatic beta cell to control diabetes mellitus. The objective of this study was to produce a controlled release system of gliclazide using chitosan beads. Chitosan beads were produced by dispersion technique using tripolyphosphate (TPP) as gelating agent. The effects of process variables including chitosan molecular weight, concentration of chitosan and TPP, pH of TPP, and cross-linking time after addition of chitosan were evaluated by Taguchi design on the rate of drug release, mean release time (MRT), release efficiency (RE(8)%), and particle size of the beads. The blood glucose lowering effect of the beads was studied in normal and streptozotocin-diabetic rats. The optimized formulation CL(2)T(5)P(2)t(10) with about 31% drug loading, 2.4 h MRT, and 69.16% RE(8)% decreased blood glucose level in normal rats for 24 h compared to pure powder of gliclazide that lasted for just 10 h.

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.

Role for SUR2A ED domain in allosteric coupling within the K(ATP) channel complex

Allosteric regulation of heteromultimeric ATP-sensitive potassium (K_ATP) channels is unique among protein systems as it implies transmission of ligand-induced structural adaptation at the regulatory SUR subunit, a member of ATP-binding cassette ABCC family, to the distinct pore-forming K⁺ (Kir6.x) channel module. Cooperative interaction between nucleotide binding domains (NBDs) of SUR is a prerequisite for K_ATP channel gating, yet pathways of allosteric intersubunit communication remain uncertain. Here, we analyzed the role of the ED domain, a stretch of 15 negatively charged aspartate/glutamate amino acid residues (948–962) of the SUR2A isoform, in the regulation of cardiac K_ATP channels. Disruption of the ED domain impeded cooperative NBDs interaction and interrupted the regulation of K_ATP channel complexes by MgADP, potassium channel openers, and sulfonylurea drugs. Thus, the ED domain is a structural component of the allosteric pathway within the K_ATP channel complex integrating transduction of diverse nucleotide-dependent states in the regulatory SUR subunit to the open/closed states of the K⁺-conducting channel pore.

No differences in mortality between users of pancreatic-specific and non-pancreatic-specific sulphonylureas: a cohort analysis

To assess whether users of pancreatic-specific sulphonylureas are at reduced risk of mortality and cardiovascular mortality compared with users of non-specific sulphonylureas, we conducted a cohort study in the population of Tayside, Scotland. We identified 3331 patients with type 2 diabetes who were newly treated with sulphonylureas between 1994 and 2001 and categorized them into those treated with only pancreatic-specific sulphonylureas and those treated with only non-specific sulphonylureas. The risks of mortality and cardiovascular mortality were compared in a survival analysis. There were 2914 patients treated with pancreatic-specific sulphonylureas only, of which 683 (23.4%) died. Of 186 patients treated with non-specific drugs only, 40 (21.5%) died. After adjusting for confounding factors, the adjusted risk ratios (with 95% CI) for mortality and cardiovascular mortality were 0.84 (0.61 to 1.17) and 0.81 (0.59 to 1.11) among the non-specific users compared with the pancreatic-specific users. This provides no evidence that there are differences between the two sulphonylureas types.

The G53D mutation in Kir6.2 (KCNJ11) is associated with neonatal diabetes and motor dysfunction in adulthood that is improved with sulfonylurea therapy

CONTEXT

Mutations in the Kir6.2 subunit (KCNJ11) of the ATP-sensitive potassium channel (KATP) underlie neonatal diabetes mellitus. In severe cases, Kir6.2 mutations underlie developmental delay, epilepsy, and neonatal diabetes (DEND). All Kir6.2 mutations examined decrease the ATP inhibition of KATP, which is predicted to suppress electrical activity in neurons (peripheral and central), muscle, and pancreas. Inhibitory sulfonylureas (SUs) have been used successfully to treat diabetes in patients with activating Kir6.2 mutations. There are two reports of improved neurological features in SU-treated DEND patients but no report of such improvement in adulthood.

OBJECTIVE

The objective of the study was to determine the molecular basis of intermediate DEND in a 27-yr-old patient with a KCNJ11 mutation (G53D) and the patient's response to SU therapy.

DESIGN

The G53D patient was transferred from insulin to gliclazide and then to glibenclamide over a 160-d period. Motor function was assessed throughout. Electrophysiology assessed the effect of the G53D mutation on KATP activity.

RESULTS

The G53D patient demonstrated improved glycemic control and motor coordination with SU treatment, although glibenclamide was more effective than gliclazide. Reconstituted G53D channels exhibit reduced ATP sensitivity, which is predicted to suppress electrical activity in vivo. G53D channels coexpressed with SUR1 (the pancreatic and neuronal isoform) exhibit high-affinity block by gliclazide but are insensitive to block when coexpressed with SUR2A (the skeletal muscle isoform). High-affinity block by glibenclamide is present in G53D channels coexpressed with either SUR1 or SUR2A.

CONCLUSION

The results demonstrate that SUs can resolve motor dysfunction in an adult with intermediate DEND and that this improvement is due to inhibition of the neuronal but not skeletal muscle KATP.

Neonatal hyperglycaemia and abnormal development of the pancreas

Transient and permanent neonatal diabetes mellitus (TNDM and PNDM) are rare conditions occurring in around 1 per 300,000 live births. In TNDM, growth-retarded infants develop diabetes in the first few weeks of life, only to go into remission after a few months with possible relapse to permanent diabetes usually around adolescence or in adulthood. In PNDM, insulin secretory failure occurs in the late fetal or early postnatal period. The very recently elucidated mutations in KCNJ11 and ABCC8 genes, encoding the Kir6.2 and SUR1 subunits of the pancreatic K(ATP) channel involved in regulation of insulin secretion, account for a third to a half of the PNDM cases. Molecular analysis of chromosome 6 anomalies and the KCNJ11 and ABCC8 genes encoding Kir6.2 and SUR1 provides a tool for distinguishing transient from permanent neonatal diabetes mellitus in the neonatal period. Some patients (those with mutations in KCNJ11 and ABCC8) may be transferred from insulin therapy to sulphonylureas.

Effect of adenosine triphosphate-sensitive potassium channel inhibitors on coronary metabolic vasodilation

The ATP-sensitive potassium (K(ATP)) channel is a distinct type of potassium ion channel that is found in the vascular smooth muscle cells of a variety of mammalian species, including humans. The activity of K(ATP) channels is determined by many factors including cellular ATP and ADP levels, thus providing a link between cellular metabolism and vascular tone through its effects on membrane potential. Experimental studies using inhibitors of K(ATP) channels, such as the sulfonuylurea class of drugs, indicate that these channels modulate coronary vascular tone including the hyperaemia induced by increased myocardial metabolism. This review examines the evidence linking K(ATP) channels to the regulation of coronary vascular tone and the potential clinical implications of pharmacologic therapies that act on K(ATP) channels.

Both G i and G o heterotrimeric G proteins are required to exert the full effect of norepinephrine on the beta-cell K ATP channel

The effects of norepinephrine (NE), an inhibitor of insulin secretion, were examined on membrane potential and the ATP-sensitive K+ channel (K ATP) in INS 832/13 cells. Membrane potential was monitored under the whole cell current clamp mode. NE hyperpolarized the cell membrane, an effect that was abolished by tolbutamide. The effect of NE on K ATP channels was investigated in parallel using outside-out single channel recording. This revealed that NE enhanced the open activities of the K ATP channels approximately 2-fold without changing the single channel conductance, demonstrating that NE-induced hyperpolarization was mediated by activation of the K ATP channels. The NE effect was abolished in cells preincubated with pertussis toxin, indicating coupling to heterotrimeric G i/G o proteins. To identify the G proteins involved, antisera raised against alpha and beta subunits (anti-G alpha common, anti-G beta, anti-G alpha i1/2/3, and anti-G alpha o) were used. Anti-G alpha common totally blocked the effects of NE on membrane potential and K ATP channels. Individually, anti-G alpha i1/2/3 and anti-G alpha o only partially inhibited the action of NE on K ATP channels. However, the combination of both completely eliminated the action. Antibodies against G beta had no effects. To confirm these results and to further identify the G protein subunits involved, the blocking effects of peptides containing the sequence of 11 amino acids at the C termini of the alpha subunits were used. The data obtained were similar to those derived from the antibody work with the additional information that G alpha i3 and G alpha o1 were not involved. In conclusion, both G i and G o proteins are required for the full effect of norepinephrine to activate the K ATP channel.

Sulfonylurea receptors type 1 and 2A randomly assemble to form heteromeric KATP channels of mixed subunit composition

ATP-sensitive potassium (K_ATP) channels play important roles in regulating insulin secretion, controlling vascular tone, and protecting cells against metabolic stresses. K_ATP channels are heterooctamers of four pore-forming inwardly rectifying (Kir6.2) subunits and four sulfonylurea receptor (SUR) subunits. K_ATP channels containing SUR1 (e.g. pancreatic) and SUR2A (e.g. cardiac) display distinct metabolic sensitivities and pharmacological profiles. The reported expression of both SUR1 and SUR2 together with Kir6.2 in some cells raises the possibility that heteromeric channels containing both SUR subtypes might exist. To test whether SUR1 can coassemble with SUR2A to form functional K_ATP channels, we made tandem constructs by fusing SUR to either a wild-type (WT) or a mutant N160D Kir6.2 subunit. The latter mutation greatly increases the sensitivity of K_ATP channels to block by intracellular spermine. We expressed, individually and in combinations, tandem constructs SUR1-Kir6.2 (S1-WT), SUR1-Kir6.2[N160D] (S1-ND), and SUR2A-Kir6.2[N160D] (S2-ND) in Xenopus oocytes, and studied the voltage dependence of spermine block in inside-out macropatches over a range of spermine concentrations and RNA mixing ratios. Each tandem construct expressed alone supported macroscopic K⁺ currents with pharmacological properties indistinguishable from those of the respective native channel types. Spermine sensitivity was low for S1-WT but high for S1-ND and S2-ND. Coexpression of S1-WT and S1-ND generated current components with intermediate spermine sensitivities indicating the presence of channel populations containing both types of Kir subunits at all possible stoichiometries. The relative abundances of these populations, determined by global fitting over a range of conditions, followed binomial statistics, suggesting that WT and N160D Kir6.2 subunits coassemble indiscriminately. Coexpression of S1-WT with S2-ND also yielded current components with intermediate spermine sensitivities, suggesting that SUR1 and SUR2A randomly coassemble into functional K_ATP channels. Further pharmacological characterization confirmed coassembly of not only S1-WT and S2-ND, but also of coexpressed free SUR1, SUR2A, and Kir6.2 into functional heteromeric channels.

The dipeptidyl peptidase 4 inhibitor vildagliptin does not accentuate glibenclamide-induced hypoglycemia but reduces glucose-induced glucagon-like peptide 1 and gastric inhibitory polypeptide secretion

BACKGROUND/AIMS

Inhibition of dipeptidyl peptidase 4 by vildagliptin enhances the concentrations of the active form of the incretin hormones glucagon-like peptide 1 (GLP-1) and gastric inhibitory polypeptide (GIP). The present study asked whether vildagliptin accentuates glibenclamide-induced hypoglycemia or affects endogenous secretion of GLP-1 and GIP after an oral glucose tolerance test.

METHODS

There were 16 healthy male subjects studied on four occasions after an overnight fast in a double-blind, four-way crossover study. In random order, vildagliptin (100 mg) or placebo, with and without glibenclamide (5 mg), was administered 30 min before 75 g oral glucose. Blood was sampled to measure glucose, and total (sum of active and inactive) GLP-1 and GIP. Statistical evaluation was done using repeated-measures ANOVA.

RESULTS

Glibenclamide provoked hypoglycemia (<or=1.9 mm), but this was not accentuated by the simultaneous administration of vildagliptin (P = 0.25). The integrated incremental responses of total GLP-1 were reduced by vildagliptin by 72% (with glibenclamide) and 48% (without glibenclamide) (effect of vildagliptin: P < 0.0001; glibenclamide: P = 0.31; interaction: P = 0.26). Similarly, integrated incremental responses of total GIP were reduced by vildagliptin by 26 and 21%, with and without glibenclamide, respectively (vildagliptin: P = 0.017; glibenclamide: P = 0.44; interaction: P = 0.69).

CONCLUSIONS

Sulfonylurea-induced hypoglycemia after the oral administration of glibenclamide is not accentuated by the coadministration of vildagliptin. This may be explained by a negative feedback regulation of GLP-1 and GIP secretion that limits the degree to which the active incretin levels are enhanced.

Effects of intracellular MgADP and acidification on the inhibition of cardiac sarcolemmal ATP-sensitive potassium channels by propofol

PURPOSE

Propofol inhibits adenosine triphosphate-sensitive potassium (K(ATP)) channels, which may result in the blocking of ischemic preconditioning in the heart. During cardiac ischemia, sarcolemmal K(ATP) channel activity is regulated by the increased levels of cytosolic metabolites, such as adenosine diphosphate (ADP) and protons. However, it remains unclear whether these cytosolic metabolites modulate the inhibitory action of propofol. The aim of this study was to investigate the effects of intracellular MgADP and acidification on K(ATP) channel inhibition by propofol.

METHODS

We used inside-out patch-clamp configurations to investigate the effects of propofol on the activities of recombinant cardiac sarcolemmal K(ATP) channels, which are reassociated by expressed subunits, sulfonylurea receptor (SUR) 2A, and inwardly rectifying potassium channels (Kir6.2).

RESULTS

In the absence of MgADP, propofol inhibited the SUR2A/Kir6.2 channel currents in a concentration-dependent manner, and an IC(50) of 78 microM. Increasing the intracellular MgADP concentrations to 0.1 and 0.3 mM markedly attenuated the inhibitory potency of propofol, and shifted the IC(50) to 183 and 265 microM, respectively. Moreover, decreasing the intracellular pH from 7.4 to 6.5 attenuated the inhibitory potency of propofol, and shifted the IC(50) to 277 microM. In addition, propofol-induced inhibition of truncated Kir6.2DeltaC36 currents, which form a functional channel without SUR2A, was not affected by an increase in intracellular MgADP. However, intracellular acidification (pH 6.5) significantly reduced the propofol sensitivity of Kir6.2DeltaC36 channels.

CONCLUSION

Our results demonstrated that the existence of intracellular MgADP and protons attenuated the direct inhibitory potency of propofol on recombinant cardiac sarcolemmal K(ATP) channels, via SUR2A and Kir6.2 subunits, respectively.

CFTR inhibition by glibenclamide requires a positive charge in cytoplasmic loop three

The sulfonylurea glibenclamide is widely used as an open-channel blocker of the CFTR chloride channel. Here, we used site-directed mutagenesis to identify glibenclamide site of interaction: a positively charged residue K978, located in the cytoplasmic loop 3. Charge-neutralizing mutations K978A, K978Q, K978S abolished the inhibition of forskolin-activated CFTR chloride current by glibenclamide but not by CFTR(inh)-172. The charge-conservative mutation K978R did not alter glibenclamide sensitivity of CFTR current. Mutations of the neighbouring R975 (R975A, R975S, R975Q) did not affect electrophysiological and pharmacological properties of CFTR. No alteration of halide selectivity was observed with any of these CFTR mutant channels. This study identifies a novel potential inhibitor site within the CFTR molecule, and suggests a novel role of cytoplasmic loop three, within the second transmembrane domain of CFTR protein. This work is the first to report on the role of a residue in a cytoplasmic loop in the mechanism of action of the channel blocker glibenclamide.

Testing the bipartite model of the sulfonylurea receptor binding site: binding of A-, B-, and A + B-site ligands

ATP-sensitive K(+) (K(ATP)) channels are composed of pore-forming subunits (Kir6.x) and of regulatory subunits, the sulfonylurea receptors (SURx). Subtypes of K(ATP) channels are expressed in different organs. The sulfonylureas and glinides (insulinotropes) close the K(ATP) channel in pancreatic beta-cells and stimulate insulin secretion. The insulinotrope binding site of the pancreatic channel (Kir6.2/SUR1) consists of two overlapping (sub)-sites, site A, located on SUR1 and containing Ser1237 (which in SUR2 is replaced by Tyr1206), and site B, formed by SUR1 and Kir6.2. Insulinotropes bind to the A-, B-, or A + B-site(s) and are grouped accordingly. A-ligands are highly selective in closing the pancreatic channel, whereas B-ligands are nonselective and insensitive to the mutation S1237Y. We have examined the binding of insulinotropes representative of the three groups in [(3)H]glibenclamide competition experiments to determine the contribution of Kir6.x to binding affinity, the effect of the mutation Y1206S in site A of SUR2, and the subtype selectivity of the compounds. The results show that the bipartite nature of the SUR1 binding site applies also to SUR2. Kir6.2 as part of the B-site may interact directly or allosterically with structural elements common to all insulinotropes, i.e., the negative charge and/or the adjacent phenyl ring. The B-site confers a moderate subtype selectivity on B-ligands. The affinity of B-ligands is altered by the mutation SUR2(Y1206S), suggesting that the mutation affects the binding chamber of SUR2 as a whole or subsite A, including the region where the subsites overlap.

Diabetes and hypoglycaemia in young children and mutations in the Kir6.2 subunit of the potassium channel: therapeutic consequences

ATP-sensitive potassium channels (K(ATP)) couple cell metabolism to electrical activity by regulating potassium movement across the membrane. These channels are octameric complex with two kind of subunits: four regulatory sulfonylurea receptor (SUR) embracing four poreforming inwardly rectifying potassium channel (Kir). Several isoforms exist for each type of subunits: SUR1 is found in the pancreatic beta-cell and neurons, whereas SUR2A is in heart cells and SUR2B in smooth muscle; Kir6.2 is in the majority of tissues as pancreatic beta-cells, brain, heart and skeletal muscle, and Kir6.1 can be found in smooth vascular muscle and astrocytes. The K(ATP) channels play multiple physiological roles in the glucose metabolism regulation, especially in beta-cells where it regulates insulin secretion, in response to an increase in ATP concentration. They also seem to be critical metabolic sensors in protection against metabolic stress as hypo or hyperglycemia, hypoxia, ischemia. Persistent hyperinsulinemic hypoglycaemia (HI) of infancy is a heterogeneous disorder which may be divided into two histopathological forms (diffuse and focal lesions). Different inactivating mutations have been implicated in both forms: the permanent inactivation of the K(ATP) channels provokes inappropriate insulin secretion, despite low ATP. Diazoxide, used efficiently in certain cases of HI, opens the K(ATP) channels and therefore overpass the mutation effect on the insulin secretion. Conversely, several studies reported sequencing of KCNJ11, coding for Kir6.2, in patients with permanent neonatal diabetes mellitus and found different mutations in 30 to 50% of the cases. More than 28 heterozygous activating mutations have now been identified, the most frequent mutation being in the aminoacid R201. These mutations result in reduced ATP-sensitivity of the K(ATP) channels compared with the wild-types and the level of channel block is responsible for different clinical features: the "mild" form confers isolated permanent neonatal diabetes whereas the severe form combines diabetes and neurological symptoms such as epilepsy, deve-lopmental delay, muscle weakness and mild dimorphic features. Sulfonylureas close K(ATP) channels by binding with high affinity to SUR suggesting they could replace insulin in these patients. Subsequently, more than 50 patients have been reported as successfully and safely switched from subcutaneous insulin injections to oral sulfonylurea therapy, with an improvement in their glycated hemoglobin. We therefore designed a protocol to transfer and evaluate children who have insulin treated neonatal diabetes due to KCNJ11 mutation, from insulin to sulfonylurea. The transfer from insulin injections to oral glibenclamide therapy seems highly effective for most patients and safe. This shows how the molecular understan-ding of some monogenic form of diabetes may lead to an unexpected change of the treatment in children. This is a spectacular example by which a pharmacogenomic approach improves the quality of life of our young diabetic patients in a tremendous way.

The Kir6.2-F333I mutation differentially modulates KATP channels composed of SUR1 or SUR2 subunits

Mutations in Kir6.2, the pore-forming subunit of the KATP channel, that reduce the ability of ATP to block the channel cause neonatal diabetes. The stimulatory effect of MgATP mediated by the regulatory sulphonylurea receptor (SUR) subunit of the channel may also be modified. We compared the effect of the Kir6.2-F333I mutation on KATP channels containing SUR1, SUR2A or SUR2B. The open probability of Kir6.2/SUR1 channels, or a C-terminally truncated form of Kir6.2 expressed in the absence of SUR, was unaffected by the mutation. However, that of Kir6.2/SUR2A and Kir6.2/SUR2B channels was increased. In the absence of Mg2+, ATP inhibition of all Kir6.2-F333I/SUR channel types was reduced, although SUR1-containing channels were reduced more than SUR2-containing channels. These results suggest F333 is involved in differential coupling of Kir6.2 to SUR1 and SUR2. When Mg2+ was present, ATP blocked SUR2A channels but activated SUR2B and SUR1 channels. Activation by MgGDP (or MgADP) was similar for wild-type and mutant channels and was independent of SUR. This indicates Mg-nucleotide binding to SUR and the transduction of binding into opening of the Kir6.2 pore are unaffected by the mutation. The data further suggest that MgATP hydrolysis by the nucleotide-binding domains of SUR1 and SUR2B, but not SUR2A, is enhanced by the F333I mutation in Kir6.2. Taken together, our data suggest the region of the C terminus within which F333 lies is involved in more than one type of functional interaction with SUR, and that F333 interacts differentially with SUR1 and SUR2.

A new benzoxazine compound blocks KATP channels in pancreatic beta cells: molecular basis for tissue selectivity in vitro and hypoglycaemic action in vivo

BACKGROUND AND PURPOSE

The 2-propyl-1,4 benzoxazine (AM10) shows a peculiar behaviour in skeletal muscle, inhibiting or opening the ATP-sensitive K(+) (KATP) channel in the absence and presence of ATP, respectively. We focused on tissue selectivity and mechanism of action of AM10 by testing its effects on pancreatic KATP channels by means of both in vitro and in vivo investigations.

EXPERIMENTAL APPROACH

In vitro, patch-clamp recordings were performed in native pancreatic beta cells and in tsA201 cells expressing the Kir6.2 Delta C36 channel. In vivo, an intraperitoneal glucose tolerance test was performed in normal mice.

KEY RESULTS

In contrast with what observed in the skeletal muscle, AM10, in whole cell perforated mode, did not augment KATP current (I(KATP)) of native beta cells but it inhibited it in a concentration-dependent manner (IC(50): 11.5 nM; maximal block: 60%). Accordingly, in current clamp recordings, a concentration-dependent membrane depolarization was observed. On excised patches, AM10 reduced the open-time probability of KATP channels without altering their single channel conductance; the same effect was observed in the presence of trypsin in the bath solution. Moreover, AM10 inhibited, in an ATP-independent manner, the K(+) current resulting from expressed Kir6.2 Delta C36 (maximal block: 60% at 100 microM; IC(50): 12.7 nM) corroborating an interaction with Kir. In vivo, AM10 attenuated the glycemia increase following a glucose bolus in a dose-dependent manner, without, at the dose tested, inducing fasting hypoglycaemia.

CONCLUSION AND IMPLICATIONS

Altogether, these results help to gain insight into a new class of tissue specific KATP channel modulators.

Imaging docking and fusion of insulin granules induced by antidiabetes agents: sulfonylurea and glinide drugs preferentially mediate the fusion of newcomer, but not previously docked, insulin granules

Sulfonylurea and glinide drugs, commonly used for antidiabetes therapies, are known to stimulate insulin release from pancreatic beta-cells by closing ATP-sensitive K+ channels. However, the specific actions of these drugs on insulin granule motion are largely unknown. Here, we used total internal reflection fluorescence (TIRF) microscopy to analyze the docking and fusion of single insulin granules in live beta-cells exposed to either the sulfonylurea drug glibenclamide or the glinide drug mitiglinide. TIRF images showed that both agents caused rapid fusion of newcomer insulin granules with the cell membrane in both control and diabetic Goto-Kakizaki (GK) rat pancreatic beta-cells. However, in the context of beta-cells from sulfonylurea receptor 1 (SUR1) knockout mice, TIRF images showed that only mitiglinide, but not glibenclamide, caused fusion of newcomer insulin granules. Compositely, our data indicate that 1) the mechanism by which both sulfonylurea and glinide drugs promote insulin release entails the preferential fusion of newcomer, rather than previously docked, insulin granules, and that 2) mitiglinide can induce insulin release by a mechanism independent of mitiglinide binding to SUR1.

Effects of sulfonylureas on mitochondrial ATP-sensitive K+ channels in cardiac myocytes: implications for sulfonylurea controversy

BACKGROUND

Mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channel plays a key role in cardioprotection. Hence, a sulfonylurea that does not block mitoK(ATP) channels would be desirable to avoid damage to the heart. Accordingly, we examined the effects of sulfonylureas on the mitoK(ATP) channel and mitochondrial Ca(2+) overload.

METHODS

Flavoprotein fluorescence in rabbit ventricular myocytes was measured to assay mitoK(ATP) channel activity. The mitochondrial Ca(2+) concentration was measured by loading cells with rhod-2.

RESULTS

The mitoK(ATP) channel opener diazoxide (100 microM) reversibly increased flavoprotein oxidation to 31.8 +/- 4.3% (n = 5) of the maximum value induced by 2,4-dinitrophenol. Glimepiride (10 microM) alone did not oxidize the flavoprotein, and the oxidative effect of diazoxide was unaffected by glimepiride (35.4 +/- 3.2%, n = 5). Similarly, the diazoxide-induced flavoprotein oxidation was unaffected both by gliclazide (10 microM) and by tolbutamide (100 microM). Exposure to ouabain (1 mM) for 30 min produced mitochondrial Ca(2+) overload, and the intensity of rhod-2 fluorescence increased to 197.4 +/- 7.2% of baseline (n = 11). Treatment with diazoxide significantly reduced the ouabain-induced mitochondrial Ca(2+) overload (149.6 +/- 5.1%, n = 11, p < 0.05 versus ouabain alone), and the effect was antagonized by the mitoK(ATP) channel blocker 5-hydroxydecanoate (189.8 +/- 27.8%, n = 5) and glibenclamide (193.1 +/- 7.7%, n = 8). On the contrary, cardioprotective effect of diazoxide was not abolished by glimepiride (141.8 +/- 7.8%, n = 6), gliclazide (139.0 +/- 9.4%, n = 5), and tolbutamide (141.1 +/- 4.5%, n = 7).

CONCLUSIONS

Our results indicate that glimepiride, gliclazide, and tolbutamide have no effect on mitoK(ATP) channel, and do not abolish the cardioprotective effects of diazoxide. Therefore, these sulfonylureas, unlike glibenclamide, do not interfere with the cellular pathways that confer cardioprotection.

Selectivity of repaglinide and glibenclamide for the pancreatic over the cardiovascular K(ATP) channels

AIMS/HYPOTHESIS

Sulfonylureas and glinides close beta cell ATP-sensitive K(+) (K(ATP)) channels to increase insulin release; the concomitant closure of cardiovascular K(ATP) channels, however, leads to complications in patients with cardiac ischaemia. The insulinotrope repaglinide is successful in therapy, but has been reported to inhibit the recombinant K(ATP) channels of beta cells, cardiocytes and non-vascular smooth muscle cells with similar potencies, suggesting that the (patho-)physiological role of the cardiovascular K(ATP) channels may be overstated. We therefore re-examined repaglinide's potency at and affinity for the recombinant pancreatic, myocardial and vascular K(ATP) channels in comparison with glibenclamide.

METHODS

K(ATP) channel subunits (i.e. inwardly rectifying K(+) channels [Kir6.x] and sulfonylurea receptors [SURx]) were expressed in intact human embryonic kidney cells and assayed in whole-cell patch-clamp and [(3)H]glibenclamide binding experiments at 37 degrees C.

RESULTS

Repaglinide and glibenclamide, respectively, were >or=30 and >or=1,000 times more potent in closing the pancreatic than the cardiovascular channels and they did not lead to complete inhibition of the myocardial channel. Binding assays showed that the selectivity of glibenclamide was essentially based on high affinity for the pancreatic SUR, whereas binding of repaglinide to the SUR subtypes was rather non-selective. After coexpression with Kir6.x to form the assembled channels, however, the affinity of the pancreatic channel for repaglinide was increased 130-fold, an effect much larger than with the cardiovascular channels. This selective effect of coexpression depended on the piperidino substituent in repaglinide.

CONCLUSIONS/INTERPRETATION

Repaglinide and glibenclamide show higher potency and efficacy in inhibiting the pancreatic than the cardiovascular K(ATP) channels, thus supporting their clinical use.

Influence of nicorandil on the pharmacodynamics and pharmacokinetics of gliclazide in rats and rabbits

Chronic diabetes precipitates ischaemic heart disease (IHD) and many other disorders. IHD inturn is shown in the form of angina initially. According to EUROPA study, the incidence of angina is high in type II diabetics. Gliclazide, a second generation sulphonylurea derivative is widely used in the treatment of type-II diabetes and is known to release insulin by K(+) channel inhibition. Nicorandil, a newer antianginal drug widely used now a days acts by opening potassium channels in the cardiac muscle cell and also by releasing nitric oxide. However its action on pancreatic cell K(+) channel is not known. Since there is possibility for drug interaction leading to decreased activity of gliclazide the present study was conducted to evaluate the effect of the combination. Studies in normal and alloxan induced diabetic rats were conducted with oral doses of 2 mg/kg bd. wt. of gliclazide, 1.8 mg/kg bd. wt. of nicorandil and their combination with adequate washout periods in between treatments. Studies in normal rabbits were conducted with 5.6 mg/1.5 kg bd. wt. of gliclazide, 1.4 mg/1.5 kg bd. wt. of nicorandil and their combination given orally. Blood samples were collected in rats from retro orbital puncture at 0, 1, 2, 3, 4, 6, 8, 10 and 12 h and by marginal ear vein puncture in rabbits at 0, 1, 2, 3, 4, 6, 8, 12, 16, 20 and 24 h. All the blood samples were analysed for glucose by GOD/POD method. The blood samples of rabbits were analysed by HPLC for gliclazide. Gliclazide produced hypoglycaemic/antidiabetic activity in normal and diabetic rats with peak activity at 1 h and 8 h and hypoglycaemic activity in normal rabbits at 3 h, while nicorandil alone produced significant hyperglycaemia at 4 h and reduced the effect of gliclazide with no significant change in pharmacokinetics when administered in combination. The interaction observed appears to be pharmacodynamic at the receptor level as expected.