KATP Channels in the Cardiovascular System

Kir6.2-D323 and SUR2A-Q1336: an intersubunit interaction pairing for allosteric information transfer in the KATP channel complex

ATP-sensitive potassium (K_ATP) channels are widely expressed and play key roles in many tissues by coupling metabolic state to membrane excitability. The SUR subunits confer drug and enhanced nucleotide sensitivity to the pore-forming Kir6 subunit, and so information transfer between the subunits must occur. In our previous study, we identified an electrostatic interaction between Kir6 and SUR2 subunits that was key for allosteric information transfer between the regulatory and pore-forming subunit. In this study, we demonstrate a second putative interaction between Kir6.2-D323 and SUR2A-Q1336 using patch clamp electrophysiological recording, where charge swap mutation of the residues on either side of the potential interaction compromise normal channel function. The Kir6.2-D323K mutation gave rise to a constitutively active, glibenclamide and ATP-insensitive K_ATP complex, further confirming the importance of information transfer between the Kir6 and SUR2 subunits. Sensitivity to modulators was restored when Kir6.2-D323K was co-expressed with a reciprocal charge swap mutant, SUR-Q1336E. Importantly, equivalent interactions have been identified in both Kir6.1 and Kir6.2 suggesting this is a second important interaction between Kir6 and the proximal C terminus of SUR.

TRPM4 channel inhibitors 9-phenanthrol and glibenclamide differentially decrease guinea pig detrusor smooth muscle whole-cell cation currents and phasic contractions

Nonselective cation channels, consistent with transient receptor potential melastatin-4 (TRPM4), regulate detrusor smooth muscle (DSM) function. TRPM4 channels can exist as homomers or assemble with sulfonylurea receptors (SURs) as complexes. We evaluated contributions of TRPM4/SUR-TRPM4 channels to DSM excitability and contractility by examining the effects of TRPM4/SUR-TRPM4 channel modulators 9-phenanthrol, glibenclamide, and diazoxide on freshly-isolated guinea pig DSM cells (amphotericin-B perforated patch-clamp electrophysiology) and mucosa-free DSM strips (isometric tension recordings). In DSM cells, complete removal of extracellular Na⁺ decreased voltage-step-induced cation (non-K⁺ selective) currents. At high positive membrane potentials, 9-phenanthrol at 100 μM attenuated voltage step-induced currents more effectively than at 30 μM, revealing concentration-dependent, voltage-sensitive inhibition. In comparison to 9-phenanthrol, glibenclamide (100 μM) displayed lower inhibition of cation currents. In the presence of glibenclamide (100 μM), 9-phenanthrol (100 μM) further decreased the currents. The SUR-TRPM4 complex activator diazoxide (100-300 μM) weakly inhibited the currents. 9-Phenanthrol, but not glibenclamide or diazoxide, increased cell capacitance (a cell surface area indicator). In contractility studies, glibenclamide displayed lower potencies than 9-phenanthrol attenuating spontaneous and 20 mM KCl-induced DSM phasic contractions. While both compounds showed similar maximum inhibitions on DSM spontaneous phasic contractions, glibenclamide was generally less efficacious on 20 mM KCl-induced phasic contractions. In summary, the observed differential effects of 9-phenanthrol and glibenclamide on DSM excitability and contractility support unique mechanisms for the two compounds. The data suggest that SUR-TRPM4 complexes do not contribute to DSM function. This study advances our understanding of pharmacological effects of glibenclamide and 9-phenanthrol on DSM cell cation currents.

Update on antiarrhythmic drug pharmacology

Cardiac arrhythmias constitute a major public health problem. Pharmacological intervention remains mainstay to their clinical management. This, in turn, depends upon systematic drug classification schemes relating their molecular, cellular, and systems effects to clinical indications and therapeutic actions. This approach was first pioneered in the 1960s Vaughan-Williams classification. Subsequent progress in cardiac electrophysiological understanding led to a lag between the fundamental science and its clinical translation, partly addressed by The working group of the European Society of Cardiology (1991), which, however, did not emerge with formal classifications. We here utilize the recent Revised Oxford Classification Scheme to review antiarrhythmic drug pharmacology. We survey drugs and therapeutic targets offered by the more recently characterized ion channels, transporters, receptors, intracellular Ca²⁺ handling, and cell signaling molecules. These are organized into their strategic roles in cardiac electrophysiological function. Following analysis of the arrhythmic process itself, we consider (a) pharmacological agents directly targeting membrane function, particularly the Na⁺ and K⁺ ion channels underlying depolarizing and repolarizing events in the cardiac action potential. (b) We also consider agents that modify autonomic activity that, in turn, affects both the membrane and (c) the Ca²⁺ homeostatic and excitation-contraction coupling processes linking membrane excitation to contractile activation. Finally, we consider (d) drugs acting on more upstream energetic and structural remodeling processes currently the subject of clinical trials. Such systematic correlations of drug actions and arrhythmic mechanisms at different molecular to systems levels of cardiac function will facilitate current and future antiarrhythmic therapy.

Ankyrin-G mediates targeting of both Na+ and KATP channels to the rat cardiac intercalated disc

We investigated targeting mechanisms of Na⁺ and K_ATP channels to the intercalated disk (ICD) of cardiomyocytes. Patch clamp and surface biotinylation data show reciprocal downregulation of each other’s surface density. Mutagenesis of the Kir6.2 ankyrin binding site disrupts this functional coupling. Duplex patch clamping and Angle SICM recordings show that I_Na and I_KATP functionally co-localize at the rat ICD, but not at the lateral membrane. Quantitative STORM imaging show that Na⁺ and K_ATP channels are localized close to each other and to AnkG, but not to AnkB, at the ICD. Peptides corresponding to Nav1.5 and Kir6.2 ankyrin binding sites dysregulate targeting of both Na⁺ and K_ATP channels to the ICD, but not to lateral membranes. Finally, a clinically relevant gene variant that disrupts K_ATP channel trafficking also regulates Na⁺ channel surface expression. The functional coupling between these two channels need to be considered when assessing clinical variants and therapeutics.

Mechanisms of endothelium-dependent vasorelaxation induced by procyanidin B2 in venous bypass graft

Cardioprotective abilities of procyanidins, might, at least in part, attribute to their vasodilator properties. The present study was undertaken to assess the vasorelaxant effect of procyanidin B2 on isolated human saphenous vein (HSV) and its underlying mechanisms. Procyanidin B2 relaxed phenylephrine-induced contraction of HSV rings in concentration-dependent manner. The relaxation was dependent on the presence of endothelium and was strongly affected by l-NAME, hydroxocobalamin or ODQ, the inhibitors of NO/cGMP pathway. Indomethacin significantly affected only the relaxation produced by the highest concentrations of procyanidin B2. Apamin and TRAM-34 combination, in the presence of l-NAME and indomethacin, did not additionally decreased procyanidin B2-induced relaxation. In the presence of K⁺ channel blockers, relaxation induced by procyanidin B2 was partially attenuated by 4-aminopyridine, significantly inhibited by glibenclamide and almost abolished by iberiotoxin. Procyanidin B2 also relaxed the contractions induced by phenylephrine or caffeine in Ca²⁺-free solution. Finally, nifedipine slightly, while thapsigargin strongly antagonized HSV relaxation. Our results indicate that procyanidin B2 induces endothelium-dependent relaxation of HSV, which results primarily from stimulation of NO production, as well K⁺ channels opening, especially BK_Ca, and partially K_ATP and K_V. Regulation of the intracellular Ca²⁺ release and inhibition of Ca²⁺ influx probably contribute to procyanidin B2-induced relaxation.

Cardiac Na+/Ca2+ exchange stimulators among cardioprotective drugs

We previously reviewed our study of the pharmacological properties of cardiac Na+/Ca2+ exchange (NCX1) inhibitors among cardioprotective drugs, such as amiodarone, bepridil, dronedarone, cibenzoline, azimilide, aprindine, and benzyl-oxyphenyl derivatives (Watanabe et al. in J Pharmacol Sci 102:7-16, 2006). Since then we have continued our studies further and found that some cardioprotective drugs are NCX1 stimulators. Cardiac Na+/Ca2+ exchange current (INCX1) was stimulated by nicorandil (a hybrid ATP-sensitive K+ channel opener), pinacidil (a non-selective ATP-sensitive K+ channel opener), flecainide (an antiarrhythmic drug), and sodium nitroprusside (SNP) (an NO donor). Sildenafil (a phosphodiesterase-5 inhibitor) further increased the pinacidil-induced augmentation of INCX1. In paper, here I review the NCX stimulants that enhance NCX function among the cardioprotective agents we examined such as nicorandil, pinacidil, SNP, sildenafil and flecainide, in addition to atrial natriuretic (ANP) and dofetilide, which were reported by other investigators.

Pregnancy-induced hypertension is associated with down-regulation of Kir6.1 in human myometrium

It is generally accepted that activity of K⁺ channels maintain resting membrane potential and uterine quiescence during pregnancy, which is, at least in part, mediated by down-regulation of ATP-sensitive K⁺ (K_ATP) channels. Pregnancy-induced hypertension (PIH) is associated with pre-term and late pre-term labour. Here, we have used real time RT-PCR to compare mRNA levels of K_ATP channel subunits in PIH parturient and control parturient. We have found that Kir6.1, a pore forming, myometrial K_ATP channel subunit is down-regulated in PIH patients. This could perfectly explain increased rate of pre-term labour in patients suffering from PIH.

ABCC9-related Intellectual disability Myopathy Syndrome is a KATP channelopathy with loss-of-function mutations in ABCC9

Mutations in genes encoding KATP channel subunits have been reported for pancreatic disorders and Cantú syndrome. Here, we report a syndrome in six patients from two families with a consistent phenotype of mild intellectual disability, similar facies, myopathy, and cerebral white matter hyperintensities, with cardiac systolic dysfunction present in the two oldest patients. Patients are homozygous for a splice-site mutation in ABCC9 (c.1320 + 1 G > A), which encodes the sulfonylurea receptor 2 (SUR2) subunit of KATP channels. This mutation results in an in-frame deletion of exon 8, which results in non-functional KATP channels in recombinant assays. SUR2 loss-of-function causes fatigability and cardiac dysfunction in mice, and reduced activity, cardiac dysfunction and ventricular enlargement in zebrafish. We term this channelopathy resulting from loss-of-function of SUR2-containing KATP channels ABCC9-related Intellectual disability Myopathy Syndrome (AIMS). The phenotype differs from Cantú syndrome, which is caused by gain-of-function ABCC9 mutations, reflecting the opposing consequences of KATP loss- versus gain-of-function.

Evidence for ATP Interaction with Phosphatidylcholine Bilayers

ATP is a fundamental intracellular molecule and is thought to diffuse freely throughout the cytosol. Evidence obtained from nucleotide-sensing sarcolemmal ion channels and red blood cells, however, suggest that ATP is compartmentalized or buffered, especially beneath the sarcolemma, but no definitive mechanism for restricted diffusion or potential buffering system has been postulated. In this study, we provide evidence from alterations to membrane dipole potential, membrane conductance, changes in enthalpy of phospholipid phase transition, and from free energy calculations that ATP associates with phospholipid bilayers. Furthermore, all-atom molecular dynamics simulations show that ATP can form aggregates in the aqueous phase at high concentrations. ATP interaction with membranes provides a new model to understand the diffusion of ATP through the cell. Coupled with previous reports of diffusion restriction in the subsarcolemmal space, these findings support the existence of compartmentalized or buffered pools of ATP.

Mechanism of pharmacochaperoning in a mammalian KATP channel revealed by cryo-EM

ATP-sensitive potassium (K_ATP) channels composed of a pore-forming Kir6.2 potassium channel and a regulatory ABC transporter sulfonylurea receptor 1 (SUR1) regulate insulin secretion in pancreatic β-cells to maintain glucose homeostasis. Mutations that impair channel folding or assembly prevent cell surface expression and cause congenital hyperinsulinism. Structurally diverse K_ATP inhibitors are known to act as pharmacochaperones to correct mutant channel expression, but the mechanism is unknown. Here, we compare cryoEM structures of a mammalian K_ATP channel bound to pharmacochaperones glibenclamide, repaglinide, and carbamazepine. We found all three drugs bind within a common pocket in SUR1. Further, we found the N-terminus of Kir6.2 inserted within the central cavity of the SUR1 ABC core, adjacent the drug binding pocket. The findings reveal a common mechanism by which diverse compounds stabilize the Kir6.2 N-terminus within SUR1’s ABC core, allowing it to act as a firm ‘handle’ for the assembly of metastable mutant SUR1-Kir6.2 complexes.

Myocardial death and dysfunction after ischemia-reperfusion injury require CaMKIIδ oxidation

Reactive oxygen species (ROS) contribute to myocardial death during ischemia-reperfusion (I/R) injury, but detailed knowledge of molecular pathways connecting ROS to cardiac injury is lacking. Activation of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKIIδ) is implicated in myocardial death, and CaMKII can be activated by ROS (ox-CaMKII) through oxidation of regulatory domain methionines (Met281/282). We examined I/R injury in mice where CaMKIIδ was made resistant to ROS activation by knock-in replacement of regulatory domain methionines with valines (MMVV). We found reduced myocardial death, and improved left ventricular function 24 hours after I/R injury in MMVV in vivo and in vitro compared to WT controls. Loss of ATP sensitive K⁺ channel (KATP) current contributes to I/R injury, and CaMKII promotes sequestration of KATP from myocardial cell membranes. KATP current density was significantly reduced by H₂O₂ in WT ventricular myocytes, but not in MMVV, showing ox-CaMKII decreases KATP availability. Taken together, these findings support a view that ox-CaMKII and KATP are components of a signaling axis promoting I/R injury by ROS.

The Selective Rat Toxicant Norbormide Blocks KATP Channels in Smooth Muscle Cells But Not in Insulin-Secreting Cells

Norbormide is a toxicant selective for rats to which it induces a widespread vasoconstriction. In a recent paper, we hypothesized a role of ATP-sensitive potassium (K_ATP) channels in norbormide-induced vasoconstriction. The current study was undertaken to verify this hypothesis by comparing the effects of norbormide with those of glibenclamide, a known K_ATP channel blocker. The whole-cell patch-clamp method was used to record K_ATP currents in myocytes freshly isolated from the rat and mouse caudal artery and from the rat gastric fundus, as well as in insulin-secreting pancreatic beta cells (INS-1 cells). Smooth muscle contractile function was assessed on either rat caudal artery rings or gastric fundus strips. Molecular modeling and docking simulation to K_ATP channel proteins were investigated in silico. Both norbormide (a racemic mixture of endo and exo isomers) and glibenclamide inhibited K_ATP currents in rat and mouse caudal artery myocytes, as well as in gastric fundus smooth muscle cells. In rat INS-1 cells, only glibenclamide blocked K_ATP channels, whereas norbormide was ineffective. The inhibitory effect of norbormide in rat caudal artery myocytes was not stereo-specific as both the endo isomers (active as vasoconstrictor) and the exo isomers (inactive as vasoconstrictor) had similar inhibitory activity. In rat caudal artery rings, norbormide-induced contraction was partially reverted by the K_ATP channel opener pinacidil. Computational approaches indicated the SUR subunit of K_ATP channels as the binding site for norbormide. K_ATP channel inhibition may play a role in norbormide-induced vasoconstriction, but does not explain the species selectivity, tissue selectivity, and stereoselectivity of its constricting activity. The lack of effect in INS-1 cells suggests a potential selectivity of norbormide for smooth muscle K_ATP channels.

Glibenclamide and HMR1098 normalize Cantú syndrome-associated gain-of-function currents

Cantú syndrome (CS) is caused by dominant gain-of-function mutation in ATP-dependent potassium channels. Cellular ATP concentrations regulate potassium current thereby coupling energy status with membrane excitability. No specific pharmacotherapeutic options are available to treat CS but I_KATP channels are pharmaceutical targets in type II diabetes or cardiac arrhythmia treatment. We have been suggested that I_KATP inhibitors, glibenclamide and HMR1098, normalize CS channels. I_KATP in response to Mg-ATP, glibenclamide and HMR1098 were measured by inside-out patch-clamp electrophysiology. Results were interpreted in view of cryo-EM I_KATP channel structures. Mg-ATP IC₅₀ values of outward current were increased for D207E (0.71 ± 0.14 mmol/L), S1020P (1.83 ± 0.10), S1054Y (0.95 ± 0.06) and R1154Q (0.75 ± 0.13) channels compared to H60Y (0.14 ± 0.01) and wild-type (0.15 ± 0.01). HMR1098 dose-dependently inhibited S1020P and S1054Y channels in the presence of 0.15 mmol/L Mg-ATP, reaching, at 30 μmol/L, current levels displayed by wild-type and H60Y channels in the presence of 0.15 mmol/L Mg-ATP. Glibenclamide (10 μmol/L) induced similar normalization. S1054Y sensitivity to glibenclamide increases strongly at 0.5 mmol/L Mg-ATP compared to 0.15 mmol/L, in contrast to D207E and S1020P channels. Experimental findings agree with structural considerations. We conclude that CS channel activity can be normalized by existing drugs; however, complete normalization can be achieved at supraclinical concentrations only.

Fetal Cerebral Artery Mitochondrion as Target of Prenatal Alcohol Exposure

Prenatal alcohol exposure results in an array of developmental abnormalities known as fetal alcohol spectrum disorders (FASDs). Despite the high prevalence of FASDs, therapeutic interventions against accidental or intended exposure of developing fetuses to alcohol are limited. This review outlines current knowledge about mitochondria in cerebral blood vessels as a potential target for anti-FASDs intervention. First, it describes the multifaceted role of mitochondria in maintaining the cerebral artery diameter as shown in adult tissue. Second, current literature on alcohol-driven damage of mitochondrial morphology and function in several fetal tissues, including liver, heart, and brain is summarized. The functional consequences of alcohol exposure in these organs include morphological enlargement of mitochondria, increased oxidative stress, and alteration of cellular respiration. These studies point to a tissue-specific effect of alcohol on mitochondrial function and a particular vulnerability of fetal mitochondria to alcohol exposure when compared to adult counterparts. Third, recent work from our group describing persistent changes in fetal baboon cerebral artery proteome following three episodes of prenatal alcohol exposure is reviewed. In conclusion, the consequences of prenatal alcohol exposure on cerebral artery mitochondria constitute an open field of investigation and, eventually, a point of therapeutic intervention against FASDs.

Pinacidil, a KATP channel opener, stimulates cardiac Na+/Ca2+ exchanger function through the NO/cGMP/PKG signaling pathway in guinea pig cardiac ventricular myocytes

Pinacidil, a nonselective ATP-sensitive K⁺ (KATP) channel opener, has cardioprotective effects for hypertension, ischemia/reperfusion injury, and arrhythmia. This agent abolishes early afterdepolarizations, delayed afterdepolarizations (DADs), and abnormal automaticity in canine cardiac ventricular myocytes. DADs are well known to be caused by the Na⁺/Ca²⁺ exchange current (I_NCX). In this study, we used the whole-cell patch-clamp technique and Fura-2/AM (Ca²⁺-indicator) method to investigate the effect of pinacidil on I_NCX in isolated guinea pig cardiac ventricular myocytes. In the patch-clamp study, pinacidil enhanced I_NCX in a concentration-dependent manner. The half-maximal effective concentration values were 23.5 and 23.0 μM for the Ca²⁺ entry (outward) and Ca²⁺ exit (inward) components of I_NCX, respectively. The pinacidil-induced I_NCX increase was blocked by L-NAME, a nitric oxide (NO) synthase inhibitor, by ODQ, a soluble guanylate cyclase inhibitor, and by KT5823, a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor, but not by N-2-mercaptopropyonyl glycine (MPG), a reactive oxygen species (ROS) scavenger. Glibenclamide, a nonselective KATP channel inhibitor, blocked the pinacidil-induced I_NCX increase, while 5-HD, a selective mitochondria KATP channel inhibitor, did not. In the Fura-2/AM study pinacidil also enhanced intracellular Ca²⁺ concentration, which was inhibited by L-NAME, ODQ, KT5823, and glibenclamide, but not by MPG and 5-HD. Sildenafil, a phosphodiesterase 5 inhibitor, increased further the pinacidil-induced I_NCX increase. Sodium nitroprusside, a NO donor, also increased I_NCX. In conclusion, pinacidil may stimulate cardiac Na⁺/Ca²⁺ exchanger (NCX1) by opening plasma membrane KATP channels and activating the NO/cGMP/PKG signaling pathway.

Involvement of sex hormonal regulation of K+ channels in electrophysiological and contractile functions of muscle tissues

Sex hormones, such as testosterone, progesterone, and 17β-estradiol, control various physiological functions. This review focuses on the sex hormonal regulation of K⁺ channels and the effects of such regulation on electrophysiological and contractile functions of muscles. In the cardiac tissue, testosterone and progesterone shorten action potential, and estrogen lengthens QT interval, a marker of increased risk of ventricular tachyarrhythmias. We have shown that testosterone and progesterone in physiological concentration activate KCNQ1 channels via membrane-delimited sex hormone receptor/eNOS pathways to shorten the action potential duration. Mitochondrial K⁺ channels are also involved in the protection of cardiac muscle. Testosterone and 17β-estradiol directly activate mitochondrial inner membrane K⁺ channels (Ca²⁺ activated K⁺ channel (K_Ca channel) and ATP-sensitive K⁺ channel (K_ATP channel)) that are involved in ischemic preconditioning and cardiac protection. During pregnancy, uterine blood flow increases to support fetal growth and development. It has been reported that 17β-estradiol directly activates large-conductance Ca²⁺-activated K⁺ channel (BK_Ca channel) attenuating arterial contraction. Furthermore, 17β-estradiol increases expression of BK_Ca channel β1 subunit which enhances BK_Ca channel activity by DNA demethylation. These findings are useful for understanding the mechanisms of sex or generation-dependent differences in the physiological and pathological functions of muscles, and the mechanisms of drug actions.

Functional characterization of ABCC9 variants identified in sudden unexpected natural death

BACKGROUND

Genetic variation in ion channel genes ('channelopathies') are often associated with inherited arrhythmias and sudden death. Genetic testing ('molecular autopsies') of channelopathy genes can be used to assist in determining the likely causes of sudden unexpected death. However, different in silico approaches can yield conflicting pathogenicity predictions and assessing their impact on ion channel function can assist in this regard.

METHODS AND RESULTS

We performed genetic testing of cases of sudden expected death in the New York City metropolitan area and found four rare or novel variants in ABCC9, which codes for the regulatory SUR2 subunit of K_ATP channels. All were missense variants, causing amino acid changes in the protein. Three of the variants (A355S, M941V, and K1379Q) were in cases of infants less than six-months old and one (H1305Y) was in an adult. The predicted pathogenicities of the variants were conflicting. We have introduced these variants into a human SUR2A cDNA, which we coexpressed with the Kir6.2 pore-forming subunit in HEK-293 cells and subjected to patch clamp and biochemical assays. Each of the four variants led to gain-of-function phenotypes. The A355S and M941V variants increased in the overall patch current. The sensitivity of the K_ATP channels to inhibitory 'cytosolic' ATP was repressed for the M941V, H1305Y and K1379Q variants. None of the variants had any effect on the unitary K_ATP channel current or the surface expression of K_ATP channels, as determined with biotinylation assays, suggesting that all of the variants led to an enhanced open state.

CONCLUSIONS

All four variants caused a gain-of-function phenotype. Given the expression of SUR2-containing K_ATP channels in the heart and specialized cardiac conduction, vascular smooth muscle and respiratory neurons, it is conceivable that electrical silencing of these cells may contribute to the vulnerability element, which is a component of the triple risk model of sudden explained death in infants. The gain-of-function phenotype of these ABCC9 variants should be considered when assessing their potential pathogenicity.

2-Deoxy-D-Glucose Exhibits Anti-seizure Effects by Mediating the Netrin-G1-KATP Signaling Pathway in Epilepsy

Epilepsy is a disorder of the brain characterized by an enduring predisposition to generate epileptic seizures. The glycolytic inhibitor 2-deoxy-D-glucose (2-DG) has been reported to exert antiepileptic effects by upregulating KATP subunits (kir6.1 and kir6.2). We evaluated whether 2-DG exhibits anti-seizure effect by mediating the netrin-G1-KATP signaling pathway in epilepsy. In a mouse epilepsy model induced by lithium chloride-pilocarpine, 2-DG intervention increased the mRNA and protein expression levels of kir6.1 and kir6.2, and these increases were significantly reversed after knocking down netrin-G1 expression. Similarly, in cultured neurons with a magnesium-free medium, we found that the frequency of spontaneous postsynaptic potentials (SP) was increased, and in the meanwhile, expression levels of kir6.1 and kir6.2 were increased after pretreatment with 2DG. These effects were remarkably reversed after knocking down netrin-G1. Thus, our findings show that 2DG exhibits anti-seizure effects through the netrin-G1-KATP signaling pathway.

In vitro evidence that endothelium-dependent vasodilatation induced by clozapine is mediated by an ATP-sensitive potassium channel

BACKGROUND

There is a definite association between antipsychotic drugs and arterial hypertension. However, endothelium functions are scarcely considered. This investigation was carried out to study the mechanisms involved in clozapine endothelium-dependent vascular reactivity.

METHODS

The experimental animals were male Wistar rats with a mean age of 70-90 days (250-300 g). The endothelium-dependent vascular reactivity was studied by measuring the isometric force and then constructing clozapine concentration-response curves. The force registrations were obtained in the aorta rings with and without the endothelium precontracted with phenylephrine (PE10^-6M) treatment; this followed incubation for 30 min in "organ chambers" with different inhibitors: l- NAME (nitric oxide/cGMP); indomethacin (PGI2/cAMP); tetraethylammonium (TEA), and specific hyperpolarization blockers (paxillin, apamin, glibenclamide). The data were presented as the mean ± standard error of the mean (SEM) and were compared by one-way ANOVA or two-way ANOVA followed by the Bonferroni post-test.

RESULTS

The primary outcomes were: 1) Clozapine-induced endothelium-dependent relaxation was not inhibited by indomethacin, l-NAME, ODQ, and methylene blue (MB); 2) The combination of l-NAME + indomethacin partially prevented the relaxation; 3) Clozapine did not induce relaxation in vessels contracted with KCl; 4) TEA did not block the clozapine-induced relaxation in vessels precontracted with PE (10^-6 M); 5) The potassium channel blockers paxillin and apamin did not prevent relaxation but glibenclamide did.

CONCLUSION

Concerning the mechanisms involved in clozapine endothelium-dependent vascular reactivity, the present study suggests that there is synergistic participation that probably occurs through a crosstalk mechanism of the cAMP, cGMPpathways and hyperpolarization.

Preprodynorphin gene mutation causes progressive cardiac conduction disease: A whole-exome analysis of a pedigree

AIMS

Progressive cardiac conduction disease (PCCD) is a rare heart disease that usually shows familial inheritance. Potential genetic risk factors for PCCD have been mostly limited to genes that encode ion channels, cardiac transcription factors, T-box transcription factors, gap junction proteins, energy metabolism regulators and structural proteins.

MAIN METHODS

Subjects in the present study came from a family who exhibited the autosomal dominant inheritance of PCCD. The primary proband had syncope and an electrocardiogram typical for PCCD, which started in the left bundle branch block, and passed to the atrioventricular block. The patient received a permanent pacemaker in 2013. Pathogenic mutations in the proband's family were identified using whole-exome sequencing and Sanger sequencing.

KEY FINDINGS

The results for the family members were verified using Sanger sequencing, while the results for healthy unrelated individuals were verified using SNaPShot. All patients in the family shared two adjacent missense mutations in the preprodynorphin (PDYN) gene (c.581A > T, c.580G > C; p.D194L).

SIGNIFICANCE

The PDYN double mutation c.581A > T and c.580G > C (p.D194L) may be linked to the onset of familial PCCD. The effects of these mutations on electrophysiology require further investigation.

Negative feedback regulation of vasocontraction by potassium channels in 10- to 15-day-old rats: Dominating role of Kv 7 channels

AIM

Potassium channels are key regulators of smooth muscle membrane potential and arterial tone. However, the roles of potassium channels in vascular tone regulation in the systemic circulation during early postnatal development are poorly understood. Therefore, this study tested the hypothesis that the negative feedback regulation of vasocontraction by potassium channels changes during maturation.

METHODS

Experiments were performed on endothelium-denuded saphenous arteries from 10- to 15-day-old and 2- to 3-month-old male rats. Isometric force and membrane potential were recorded using wire myography and the sharp microelectrode technique respectively; mRNA and protein contents were determined by qPCR and Western blotting.

RESULTS

The effects of K_v 1, K_ir and K_v 7 channel blockers (DPO-1, BaCl₂ , XE991) on methoxamine-induced contraction were larger in arteries of 10- to 15-day-old compared to 2- to 3-month-old animals. In contrast, the BK_{C a} channel blocker iberiotoxin had a stronger influence in 2- to 3- month-old rats. The effects of K_ATP and K_v 2 channel blockers (glibenclamide, stromatoxin) were not pronounced at both ages. The larger influence of K_v 7 and K_ir channel blockade on arterial contraction in 10- to 15-day-old rats was associated with more prominent smooth muscle depolarization. The developmental alterations in potassium channel functioning were generally consistent with their mRNA and protein expression levels in arterial smooth muscle.

CONCLUSION

The negative feedback regulation of vasocontraction by potassium channels varies during maturation depending on the channel type. A dominating contribution of K_v 7 channels to the regulation of basal tone and agonist-induced contraction was observed in arteries of 10- to 15-day-old animals.

Kir6.2, the Pore-Forming Subunit of ATP-Sensitive K+ Channels, Is Overexpressed in Human Posttraumatic Brain Contusions

Brain contusions (BCs) are one of the most frequent lesions in patients with moderate and severe traumatic brain injury (TBI). BCs increase their volume due to peri-lesional edema formation and/or hemorrhagic transformation. This may have deleterious consequences and its mechanisms are still poorly understood. We previously identified de novo upregulation sulfonylurea receptor (SUR) 1, the regulatory subunit of adenosine triphosphate (ATP)-sensitive potassium (KATP) channels and other channels, in human BCs. Our aim here was to study the expression of the pore-forming subunit of KATP, Kir6.2, in human BCs, and identify its localization in different cell types. Protein levels of Kir6.2 were detected by western blot (WB) from 33 contusion specimens obtained from 32 TBI patients aged 14-74 years. The evaluation of Kir6.2 expression in different cell types was performed by immunofluorescence in 29 contusion samples obtained from 28 patients with a median age of 42 years. Control samples were obtained from limited brain resections performed to access extra-axial skull base tumors or intraventricular lesions. Contusion specimens showed an increase of Kir6.2 expression in comparison with controls. Regarding cellular location of Kir6.2, there was no expression of this channel subunit in blood vessels, either in control samples or in contusions. The expression of Kir6.2 in neurons and microglia was also analyzed, but the observed differences were not statistically significant. However, a significant increase of Kir6.2 was found in glial fibrillary acidic protein (GFAP)-positive cells in contusion specimens. Our data suggest that further research on SUR1-regulated ionic channels may lead to a better understanding of key mechanisms involved in the pathogenesis of BCs, and may identify novel targeted therapeutic strategies.

Exercise and Cardioprotection: A Natural Defense Against Lethal Myocardial Ischemia-Reperfusion Injury and Potential Guide to Cardiovascular Prophylaxis

Similar to ischemic preconditioning, high-intensity exercise has been shown to decrease infarct size following myocardial infarction. In this article, we review the literature on beneficial effects of exercise, exercise requirements for cardioprotection, common methods utilized in laboratories to study this phenomenon, and discuss possible mechanisms for exercise-mediated cardioprotection.

Modernized Classification of Cardiac Antiarrhythmic Drugs

BACKGROUND

Among his major cardiac electrophysiological contributions, Miles Vaughan Williams (1918-2016) provided a classification of antiarrhythmic drugs that remains central to their clinical use.

METHODS

We survey implications of subsequent discoveries concerning sarcolemmal, sarcoplasmic reticular, and cytosolic biomolecules, developing an expanded but pragmatic classification that encompasses approved and potential antiarrhythmic drugs on this centenary of his birth.

RESULTS

We first consider the range of pharmacological targets, tracking these through to cellular electrophysiological effects. We retain the original Vaughan Williams Classes I through IV but subcategorize these divisions in light of more recent developments, including the existence of Na+ current components (for Class I), advances in autonomic (often G protein-mediated) signaling (for Class II), K+ channel subspecies (for Class III), and novel molecular targets related to Ca2+ homeostasis (for Class IV). We introduce new classes based on additional targets, including channels involved in automaticity, mechanically sensitive ion channels, connexins controlling electrotonic cell coupling, and molecules underlying longer-term signaling processes affecting structural remodeling. Inclusion of this widened range of targets and their physiological sequelae provides a framework for a modernized classification of established antiarrhythmic drugs based on their pharmacological targets. The revised classification allows for the existence of multiple drug targets/actions and for adverse, sometimes actually proarrhythmic, effects. The new scheme also aids classification of novel drugs under investigation.

CONCLUSIONS

We emerge with a modernized classification preserving the simplicity of the original Vaughan Williams framework while aiding our understanding and clinical management of cardiac arrhythmic events and facilitating future developments in this area.

Effects of regular exercise on ventricular myocyte biomechanics and KATP channel function

Exercise training is known to protect the heart from ischemia and improve function during exercise by reducing cardiomyocyte action potential duration (APD) and increasing contractility. The cellular mechanisms involve β-adrenergic regulation and the ATP-sensitive K+ (KATP) channel, but how each alters function of the left ventricle and sex specificity is unknown. To address this, female and male Sprague-Dawley rats were randomly assigned to wheel-running (TRN) or sedentary (SED) groups. After 6–8 wk of training, myocytes were isolated from the left ventricle and field stimulated at 1, 2, and 5 Hz. TRN significantly increased cardiomyocyte contractility, the kinetics of the Ca2+ transient, and responsiveness to the adrenergic receptor agonist isoproterenol (ISO), as reflected by an increased sarcomere shortening. Importantly, we demonstrated a TRN-induced upregulation of KATP channels, which was reflected by elevated content, current density, and the channel’s contribution to APD shortening at high activation rates and in the presence of the activator pinacidil. TRN induced increase in KATP current occurred throughout the left ventricle, but channel subunit content showed regional specificity with increases in Kir6.2 in the apex and SUR2A in base regions. In summary, TRN elevated cardiomyocyte cross-bridge kinetics, Ca2+ sensitivity, and the responsiveness of contractile function to β-adrenergic receptor stimulation in both sexes. Importantly, upregulation of the KATP channel accelerates repolarization and shortens APD during stress and exercise. These adaptations have clinical importance, as increased contractility and reduced APD would help protect cardiac output and reduce intracellular Ca2+ overload during stresses such as regional ischemia.

NEW & NOTEWORTHY Our results demonstrate that regular exercise significantly increased ventricular myocyte shortening and relaxation velocity and the rate of rise in intracellular Ca2+ transient and enhanced the response of biomechanics and Ca2+ reuptake to β-adrenergic stimulation. Importantly, exercise training upregulated the cardiomyocyte sarcolemma ATP-sensitive K+ channel across the left ventricle in both sexes, as reflected by elevated channel subunit content, current density, and the channel’s contribution to reduced action potential duration at high activation rates.

Insulin down-regulates cardioprotective SUR2A in the heart-derived H9c2 cells: A possible explanation for some adverse effects of insulin therapy

Some recent studies associated insulin therapy with negative cardiovascular events and shorter lifespan. SUR2A, a K_ATP channel subunit, regulate cardioprotection and cardiac ageing. Here, we have tested whether glucose and insulin regulate expression of SUR2A/K_ATP channel subunits and resistance to metabolic stress in heart H9c2 cells. Absence of glucose in culture media decreased SUR2A mRNA, while mRNAs of Kir6.2, Kir6.1, SUR1 and IES SUR2B were increased. 2-deoxyglucose (50 mM) decreased mRNAs of SUR2A, SUR2B and SUR1, did not affect IES SUR2A and IES SUR2B mRNAs and increased Kir6.2 mRNA. No glucose and 2-deoxyglucose (50 mM) decreased resistance to an inhibitor of oxidative phosphorylation, DNP (10 mM). 50 mM glucose did not alter K_ATP channel subunits nor cellular resistance to DNP (10 mM). Insulin (20 ng/ml) in both physiological and high glucose (50 mM) down-regulated SUR2A while upregulating Kir6.1 and Kir6.2 (in high glucose only). Insulin (20 ng/ml) in physiological and high glucose decreased cell survival in DNP (10 mM). As opposed to Kir6.2, infection with SUR2A resulted in titre-dependent cytoprotection. We conclude that insulin decreases resistance to metabolic stress in H9c2 cells by decreasing SUR2A expression. Lower cardiac SUR2A levels underlie increased myocardial susceptibility to metabolic stress and shorter lifespan.

Transgenic overexpression of the SUR2A-55 splice variant in mouse heart reduces infract size and promotes protective mitochondrial function

ATP-sensitive potassium channels found in both the sarcolemma (sarcK_ATP) and mitochondria (mitoK_ATP) of cardiomyocytes are important mediators of cardioprotection during ischemic heart disease. Sulfonylurea receptor isoforms (SUR2), encoded by Abcc9, an ATP-binding cassette family member, form regulatory subunits of the sarcK_ATP channel and are also thought to regulate mitoK_ATP channel activity. A short-form splice variant of SUR2 (SUR2A-55) was previously shown to target mitochondria and display diaxoxide and ATP insensitive K_ATP activity when co-expressed with the inward rectifier channels Kir6.2 and Kir6.1. We hypothesized that mice with cardiac specific overexpression of SUR2A-55 would mediate cardioprotection from ischemia by altering mitoK_ATP properties. Mice overexpressing SUR2A-55 (TG^SUR2A-55) in cardiomyocytes were generated and showed no significant difference in echocardiographic measured chamber dimension, percent fractional shortening, heart to body weight ratio, or gross histologic features compared to normal mice at 11–14 weeks of age. TG^SUR2A-55 had improved hemodynamic functional recovery and smaller infarct size after ischemia reperfusion injury compared to WT mice in an isolated hanging heart model. The mitochondrial membrane potential of TG^SUR2A-55 mice was less sensitive to ATP, diazoxide, and Ca²⁺ loading. These data suggest that the SUR2A-55 splice variant favorably affects mitochondrial function leading to cardioprotection. These data support a role for the regulation of mitoK_ATP activity by SUR2A-55.

Glibenclamide Prevents Hypoglycemia-Induced Fatal Cardiac Arrhythmias in Rats

Sulfonylureas increase the incidence of severe hypoglycemia in people with type 2 diabetes and might increase the risk of sudden cardiac death. Sulfonylureas stimulate insulin secretion by closing pancreatic ATP-sensitive potassium ion (KATP) channels. To investigate the role of KATP channel modulators on cardiac arrhythmias and mortality in the setting of severe hypoglycemia, adult Sprague-Dawley rats underwent hyperinsulinemic (0.2 U/kg/min) severe hypoglycemic (10 to 15 mg/dL) clamps with continuous electrocardiography. The rats were randomized for treatment with intravenous vehicle (VEH), the sulfonylurea glibenclamide (GLIB; KATP channel blocker; 5 mg/kg/h), or diazoxide (DIAZ; KATP channel opener; 5 mg/kg/h). The results demonstrated that GLIB completely prevented first-degree heart block compared with VEH (0.18 ± 0.09/min) and DIAZ (0.2 ± 0.05/min). Second-degree heart block was significantly reduced with GLIB (0.12 ± 0.1/min) compared with VEH (0.6 ± 0.2/min) and DIAZ (6.9 ± 3/min). The incidence of third-degree heart block was completely prevented by GLIB compared with VEH (67%) and DIAZ (87.5%). Hypoglycemia-induced mortality was completely prevented by GLIB compared with VEH (60%) and DIAZ (82%). In conclusion, although GLIB increases the risk of hypoglycemia by increasing insulin secretion, these results have demonstrated a paradoxical protective role of GLIB against severe hypoglycemia-induced fatal cardiac arrhythmias.

Cardioprotective signalling: Past, present and future

A few decades ago, cardiac muscle was discovered to possess signalling pathways that, when activated, protect the myocardium against the damage induced by ischaemia-reperfusion. The ability of cardiac muscle to protect itself against injury has been termed 'cardioprotection'. Many compounds and procedures can trigger cardioprotection including conditionings (exposure to brief episodes of ischaemia-reperfusion to protect against sustained ischaemia-reperfusion), hypoxia, adenosine, acetylcholine, adrenomedullin, angiotensin, bradykinin, catecholamines, endothelin, estrogens, phenylephrine, opioids, testosterone, and many more. These triggers activate many intracellular signalling factors including protein kinases, different enzymes, transcription factors and defined signalling pathways to target structures in mitochondria, sarcoplasmic reticulum, nucleus and sarcolemma to mediate cardioprotection. Although a lot of information about cardioprotection has been acquired, there are still two major outstanding issues to be addressed in the future 1) better understanding of spatio-temporal relationships between signalling elements, and; 2) devising therapeutic strategies against myocardial diseases based on cardioprotective signalling. Further research is required to paint integral picture of cardioprotective signalling and more clinical studies are required to properly test clinical efficacy and safety of potential cardioprotective strategies. Therapies against cardiac diseases based on cardioprotective strategies would be a perfect adjunct to current therapeutic strategies based on restitution of coronary blood flow and regulation of myocardial metabolic demands.

[Nicorandil improves cognitive dysfunction in mice with streptozotocin-induced diabetes]

OBJECTIVE

To observe the protective effects of potassium channel opener nicorandil against cognitive dysfunction in mice with streptozotocin (STZ)-induced diabetes.

METHODS

C57BL/6J mouse models of type 1 diabetes mellitus (T1DM) were established by intraperitoneal injection of STZ and received daily treatment with intragastric administration of nicorandil or saline (model group) for 4 consecutive weeks, with normal C57BL/6J mice serving as control. Fasting blood glucose level was recorded every week and Morris water maze was used to evaluate the cognitive behavior of the mice in the 4th week. At the end of the experiment, the mice were sacrificed to observe the ultrastructural changes in the hippocampus and pancreas under transmission electron microscopy; the contents of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) in the hippocampus and SOD activity and MDA level in the brain tissue were determined.

RESULTS

Compared with the control group, the model group showed significantly increased fasting blood glucose (P<0.001), significantly prolonged escape latency (P<0.05) and increased swimming distance (P<0.01) with ultrastructural damage of pancreatic β cells and in the hippocampus; GIP and GLP-1 contents in the hippocampus (P<0.01) and SOD activity in the brain were significantly decreased (P<0.05) and MDA content was significantly increased in the model group (P<0.05). Compared with the model group, nicorandil treatment did not cause significant changes in fasting blood glucose, but significantly reduced the swimming distance (P<0.05); nicorandil did not improve the ultrastructural changes in pancreatic β cells but obviously improved the ultrastructures of hippocampal neurons and synapses. Nicorandil also significantly increased the contents of GIP and GLP-1 in the hippocampus (P<0.05), enhanced SOD activity (P<0.05) and decreased MDA level (P<0.01) in the brain tissue.

CONCLUSION

Nicorandil improves cognitive dysfunction in mice with STZ-induced diabetes by increasing GIP and GLP-1 contents in the hippocampus and promoting antioxidation to relieve hippocampal injury.

Boosting the signal: Endothelial inward rectifier K+ channels

Endothelial cells express a diverse array of ion channels including members of the strong inward rectifier family composed of K_IR 2 subunits. These two-membrane spanning domain channels are modulated by their lipid environment, and exist in macromolecular signaling complexes with receptors, protein kinases and other ion channels. Inward rectifier K⁺ channel (K_IR ) currents display a region of negative slope conductance at membrane potentials positive to the K⁺ equilibrium potential that allows outward current through the channels to be activated by membrane hyperpolarization, permitting K_IR to amplify hyperpolarization induced by other K⁺ channels and ion transporters. Increases in extracellular K⁺ concentration activate K_IR allowing them to sense extracellular K⁺ concentration and transduce this change into membrane hyperpolarization. These properties position K_IR to participate in the mechanism of action of hyperpolarizing vasodilators and contribute to cell-cell conduction of hyperpolarization along the wall of microvessels. The expression of K_IR in capillaries in electrically active tissues may allow K_IR to sense extracellular K⁺ , contributing to functional hyperemia. Understanding the regulation of expression and function of microvascular endothelial K_IR will improve our understanding of the control of blood flow in the microcirculation in health and disease and may provide new targets for the development of therapeutics in the future.

The trafficking protein, EHD2, positively regulates cardiac sarcolemmal KATP channel surface expression: role in cardioprotection

ATP-sensitive K⁺ (K_ATP) channels uniquely link cellular energy metabolism to membrane excitability and are expressed in diverse cell types that range from the endocrine pancreas to neurons and smooth, skeletal, and cardiac muscle. A decrease in the surface expression of K_ATP channels has been linked to various disorders, including dysregulated insulin secretion, abnormal blood pressure, and impaired resistance to cardiac injury. In contrast, up-regulation of K_ATP channel surface expression may be protective, for example, by mediating the beneficial effect of ischemic preconditioning. Molecular mechanisms that regulate K_ATP channel trafficking are poorly understood. Here, we used cellular assays with immunofluorescence, surface biotinylation, and patch clamping to demonstrate that Eps15 homology domain-containing protein 2 (EHD2) is a novel positive regulator of K_ATP channel trafficking to increase surface K_ATP channel density. EHD2 had no effect on cardiac Na⁺ channels (Nav1.5). The effect is specific to EHD2 as other members of the EHD family-EHD1, EHD3, and EHD4-had no effect on K_ATP channel surface expression. EHD2 did not directly affect K_ATP channel properties as unitary conductance and ATP sensitivity were unchanged. Instead, we observed that the mechanism by which EHD2 increases surface expression is by stabilizing K_ATP channel-containing caveolar structures, which results in a reduced rate of endocytosis. EHD2 also regulated K_ATP channel trafficking in isolated cardiomyocytes, which validated the physiologic relevance of these observations. Pathophysiologically, EHD2 may be cardioprotective as a dominant-negative EHD2 mutant sensitized cardiomyocytes to ischemic damage. Our findings highlight EHD2 as a potential pharmacologic target in the treatment of diseases with K_ATP channel trafficking defects.-Yang, H. Q., Jana, K., Rindler, M. J., Coetzee, W. A. The trafficking protein, EHD2, positively regulates cardiac sarcolemmal K_ATP channel surface expression: role in cardioprotection.

Cantu syndrome-associated SUR2 (ABCC9) mutations in distinct structural domains result in KATP channel gain-of-function by differential mechanisms

The complex disorder Cantu syndrome (CS) arises from gain-of-function mutations in either KCNJ8 or ABCC9, the genes encoding the Kir6.1 and SUR2 subunits of ATP-sensitive potassium (K_ATP) channels, respectively. Recent reports indicate that such mutations can increase channel activity by multiple molecular mechanisms. In this study, we determined the mechanism by which K_ATP function is altered by several substitutions in distinct structural domains of SUR2: D207E in the intracellular L0-linker and Y985S, G989E, M1060I, and R1154Q/R1154W in TMD2. We engineered substitutions at their equivalent positions in rat SUR2A (D207E, Y981S, G985E, M1056I, and R1150Q/R1150W) and investigated functional consequences using macroscopic rubidium (⁸⁶Rb⁺) efflux assays and patch-clamp electrophysiology. Our results indicate that D207E increases K_ATP channel activity by increasing intrinsic stability of the open state, whereas the cluster of Y981S/G985E/M1056I substitutions, as well as R1150Q/R1150W, augmented Mg-nucleotide activation. We also tested the responses of these channel variants to inhibition by the sulfonylurea drug glibenclamide, a potential pharmacotherapy for CS. None of the D207E, Y981S, G985E, or M1056I substitutions had a significant effect on glibenclamide sensitivity. However, Gln and Trp substitution at Arg-1150 significantly decreased glibenclamide potency. In summary, these results provide additional confirmation that mutations in CS-associated SUR2 mutations result in K_ATP gain-of-function. They help link CS genotypes to phenotypes and shed light on the underlying molecular mechanisms, including consequences for inhibitory drug sensitivity, insights that may inform the development of therapeutic approaches to manage CS.

New and revisited approaches to preserving the reperfused myocardium

Early coronary artery reperfusion improves outcomes for patients with ST-segment elevation myocardial infarction (STEMI), but morbidity and mortality after STEMI remain unacceptably high. The primary deficits seen in these patients include inadequate pump function, owing to rapid infarction of muscle in the first few hours of treatment, and adverse remodelling of the heart in the months that follow. Given that attempts to further reduce myocardial infarct size beyond early reperfusion in clinical trials have so far been disappointing, effective therapies are still needed to protect the reperfused myocardium. In this Review, we discuss several approaches to preserving the reperfused heart, such as therapies that target the mechanisms involved in mitochondrial bioenergetics, pyroptosis, and autophagy, as well as treatments that harness the cardioprotective properties of inhaled anaesthetic agents. We also discuss potential therapies focused on correcting the no-reflow phenomenon and its effect on healing and adverse left ventricular remodelling.

The therapeutic potential of targeting the Kir1.1 (renal outer medullary K+) channel

K_ir1.1 (renal outer medullary K⁺) channels are potassium channels expressed almost exclusively in the kidney and play a role in the body's electrolyte and water balance. Potassium efflux through K_ir1.1 compliments the role of transporters and sodium channels that are the targets of known diuretics. Consequently, loss-of-function mutations in men and rodents are associated with salt wasting and low blood pressure. On this basis, K_ir1.1 inhibitors may have value in the treatment of hypertension and heart failure. Efforts to develop small molecule K_ir1.1 inhibitors produced MK-7145, which entered into clinical trials. The present manuscript describes the structure-activity relationships associated with this scaffold alongside other preclinical K_ir1.1 blockers.

Ion channels, long QT syndrome and arrhythmogenesis in ageing

Ageing is associated with increased prevalences of both atrial and ventricular arrhythmias, reflecting disruption of the normal sequence of ion channel activation and inactivation generating the propagated cardiac action potential. Experimental models with specific ion channel genetic modifications have helped clarify the interacting functional roles of ion channels and how their dysregulation contributes to arrhythmogenic processes at the cellular and systems level. They have also investigated interactions between these ion channel abnormalities and age-related processes in producing arrhythmic tendency. Previous reviews have explored the relationships between age and loss-of-function Na_v 1.5 mutations in producing arrhythmogenicity. The present review now explores complementary relationships arising from gain-of-function Na_v 1.5 mutations associated with long QT3 (LQTS3). LQTS3 patients show increased risks of life-threatening ventricular arrhythmias, particularly after 40 years of age, consistent with such interactions between the ion channel abnormailities and ageing. In turn clinical evidence suggests that ageing is accompanied by structural, particularly fibrotic, as well as electrophysiological change. These abnormalities may result from biochemical changes producing low-grade inflammation resulting from increased production of reactive oxygen species and superoxide. Experimental studies offer further insights into the underlying mechanisms underlying these phenotypes. Thus, studies in genetically modified murine models for LQTS implicated action potential recovery processes in arrhythmogenesis resulting from functional ion channel abnormalities. In addition, ageing wild type (WT) murine models demonstrated both ion channel alterations and fibrotic changes with ageing. Murine models then suggested evidence for interactions between ageing and ion channel mutations and provided insights into potential arrhythmic mechanisms inviting future exploration.

Hearts lacking plasma membrane KATP channels display changes in basal aerobic metabolic substrate preference and AMPK activity

Cardiac ATP-sensitive K+ (KATP) channels couple changes in cellular metabolism to membrane excitability and are activated during metabolic stress, although under basal aerobic conditions, KATP channels are thought to be predominately closed. Despite intense research into the roles of KATP channels during metabolic stress, their contribution to aerobic basal cardiac metabolism has not been previously investigated. Hearts from Kir6.2+/+ and Kir6.2−/− mice were perfused in working mode, and rates of glycolysis, fatty acid oxidation, and glucose oxidation were measured. Changes in activation/expression of proteins regulating metabolism were probed by Western blot analysis. Despite cardiac mechanical function and metabolic efficiency being similar in both groups, hearts from Kir6.2−/− mice displayed an approximately twofold increase in fatty acid oxidation and a 0.45-fold reduction in glycolytic rates but similar glucose oxidation rates compared with hearts from Kir6.2+/+ mice. Kir6.2−/− hearts also possessed elevated levels of activated AMP-activated protein kinase (AMPK), higher glycogen content, and reduced mitochondrial density. Moreover, activation of AMPK by isoproterenol or diazoxide was significantly blunted in Kir6.2−/− hearts. These data indicate that KATP channel ablation alters aerobic basal cardiac metabolism. The observed increase in fatty acid oxidation and decreased glycolysis before any metabolic insult may contribute to the poor recovery observed in Kir6.2−/− hearts in response to exercise or ischemia-reperfusion injury. Therefore, KATP channels may play an important role in the regulation of cardiac metabolism through AMPK signaling.

NEW & NOTEWORTHY In this study, we show that genetic ablation of plasma membrane ATP-sensitive K+ channels results in pronounced changes in cardiac metabolic substrate preference and AMP-activated protein kinase activity. These results suggest that ATP-sensitive K+ channels may play a novel role in regulating metabolism in addition to their well-documented effects on ionic homeostasis during periods of stress.

Gastrodin causes vasodilation by activating KATP channels in vascular smooth muscles via PKA-dependent signaling pathway

Gastrodin, one of the major components extracted from the Chinese herb Gastrodia elata Bl., has been widely used as an anticonvulsant, sedative, analgesic and hypotensive. In our study, we aimed to investigate the effects and possible mechanisms of gastrodin on vascular K_ATP channels. Tension experiments were used on rat mesenteric artery rings without an endothelium. Patch clamp experiments were executed to investigate the influences of gastrodin on the membrane current in mesenteric artery smooth muscle cells. Gastrodin induced vasorelaxation in a concentration dependent manner when rat mesenteric artery rings were pre-contracted with Phenylephrine. The vasorelaxation effect was partially diminished by pre-treating with a K_ATP channel inhibitor, or a PKA inhibitor. With whole-cell patch-clamp recording techniques, we found that gastrodin is a activator of K_ATP in rat mesenteric artery smooth muscle cells, and this effect was eliminate by pre-treating with H89or PKI, PKA inhibitor. In addition, when rat vascular smooth muscle cells were treated with 100 μM gastrodin for 24 h, maximum K_ATP current density increased by 28.1%. The results indicate that gastrodin exerts vasorelaxation effect through activation of PKA and subsequent opening of smooth muscle K_ATP channels.

Signalling assemblies: the odds of symmetry

The assembly of proteins into complexes is fundamental to nearly all biological signalling processes. Symmetry is a dominant feature of the structures of experimentally determined protein complexes, observed in the vast majority of homomers and many heteromers. However, some asymmetric structures exist, and asymmetry also often forms transiently, intractable to traditional structure determination methods. Here, we explore the role of protein complex symmetry and asymmetry in cellular signalling, focusing on receptors, transcription factors and transmembrane channels, among other signalling assemblies. We highlight a recurrent tendency for asymmetry to be crucial for signalling function, often being associated with activated states. We conclude with a discussion of how consideration of protein complex symmetry and asymmetry has significant potential implications and applications for pharmacology and human disease.

Electrophysiological Investigation of the Subcellular Fine Tuning of Sympathetic Neurons by Hydrogen Sulfide

H₂S is well-known as hypotensive agent, whether it is synthetized endogenously or administered systemically. Moreover, the H₂S donor NaHS has been shown to inhibit vasopressor responses triggered by stimulation of preganglionic sympathetic fibers. In contradiction with this latter result, NaHS has been reported to facilitate transmission within sympathetic ganglia. To resolve this inconsistency, H₂S and NaHS were applied to primary cultures of dissociated sympathetic ganglia to reveal how this gasotransmitter might act at different subcellular compartments of such neurons. At the somatodendritic region of ganglionic neurons, NaHS raised the frequency, but not the amplitudes, of cholinergic miniature postsynaptic currents via a presynaptic site of action. In addition, the H₂S donor as well as H₂S itself caused membrane hyperpolarization and decreased action potential firing in response to current injection. Submillimolar NaHS concentrations did not affect currents through K_υ7 channels, but did evoke currents through K_ATP channels. Similarly to NaHS, the K_ATP channel activator diazoxide led to hyperpolarization and decreased membrane excitability; the effects of both, NaHS and diazoxide, were prevented by the K_ATP channel blocker tolbutamide. At postganglionic sympathetic nerve terminals, H₂S and NaHS enhanced noradrenaline release due to a direct action at the level of vesicle exocytosis. Taken together, H₂S may facilitate transmitter release within sympathetic ganglia and at sympatho-effector junctions, but causes hyperpolarization and reduced membrane excitability in ganglionic neurons. As this latter action was due to K_ATP channel gating, this channel family is hereby established as another previously unrecognized determinant in the function of sympathetic ganglia.

Molecular Bases of Brain Preconditioning

Preconditioning of the brain induces tolerance to the damaging effects of ischemia and prevents cell death in ischemic penumbra. The development of this phenomenon is mediated by mitochondrial adenosine triphosphate-sensitive potassium (KATP+) channels and nitric oxide signaling (NO). The aim of this study was to investigate the dynamics of molecular changes in mitochondria after ischemic preconditioning (IP) and the effect of pharmacological preconditioning (PhP) with the KATP+-channels opener diazoxide on NO levels after ischemic stroke in rats. Immunofluorescence-histochemistry and laser-confocal microscopy were applied to evaluate the cortical expression of electron transport chain enzymes, mitochondrial KATP+-channels, neuronal and inducible NO-synthases, as well as the dynamics of nitrosylation and nitration of proteins in rats during the early and delayed phases of IP. NO cerebral content was studied with electron paramagnetic resonance (EPR) spectroscopy using spin trapping. We found that 24 h after IP in rats, there is a two-fold decrease in expression of mitochondrial KATP+-channels (p = 0.012) in nervous tissue, a comparable increase in expression of cytochrome c oxidase (p = 0.008), and a decrease in intensity of protein S-nitrosylation and nitration (p = 0.0004 and p = 0.001, respectively). PhP led to a 56% reduction of free NO concentration 72 h after ischemic stroke simulation (p = 0.002). We attribute this result to the restructuring of tissue energy metabolism, namely the provision of increased catalytic sites to mitochondria and the increased elimination of NO, which prevents a decrease in cell sensitivity to oxygen during subsequent periods of severe ischemia.

Acute Ethanol Increases IGF-I-Induced Phosphorylation of ERKs by Enhancing Recruitment of p52-Shc to the Grb2/Shc Complex

Ethanol plays a detrimental role in the development of the brain. Multiple studies have shown that ethanol inhibits insulin-like growth factor I receptor (IGF-IR) function. Because the IGF-IR contributes to brain development by supporting neural growth, survival, and differentiation, we sought to determine the molecular mechanism(s) involved in ethanol's effects on this membrane-associated tyrosine kinase. Using multiple neuronal cell types, we performed Western blot, immunoprecipitation, and GST-pulldowns following acute (1-24 h) or chronic (3 weeks) treatment with ethanol. Surprisingly, exposure of multiple neuronal cell types to acute (up to 24 h) ethanol (50 mM) enhanced IGF-I-induced phosphorylation of extracellular regulated kinases (ERKs), without affecting IGF-IR tyrosine phosphorylation itself, or Akt phosphorylation. This acute increase in ERKs phosphorylation was followed by the expected inhibition of the IGF-IR signaling following 3-week ethanol exposure. We then expressed a GFP-tagged IGF-IR construct in PC12 cells and used them to perform fluorescence recovery after photobleaching (FRAP) analysis. Using these fluorescently labeled cells, we determined that 50 mM ethanol decreased the half-time of the IGF-IR-associated FRAP, which implied that cell membrane-associated signaling events could be affected. Indeed, co-immunoprecipitation and GST-pulldown studies demonstrated that the acute ethanol exposure increased the recruitment of p52-Shc to the Grb2-Shc complex, which is known to engage the Ras-Raf-ERKs pathway following IGF-1 stimulation. These experiments indicate that even a short and low-dose exposure to ethanol may dysregulate function of the receptor, which plays a critical role in brain development. J. Cell. Physiol. 232: 1275-1286, 2017. © 2016 Wiley Periodicals, Inc.

Bioenergetic profile of human coronary artery smooth muscle cells and effect of metabolic intervention

Bioenergetics of artery smooth muscle cells is critical in cardiovascular health and disease. An acute rise in metabolic demand causes vasodilation in systemic circulation while a chronic shift in bioenergetic profile may lead to vascular diseases. A decrease in intracellular ATP level may trigger physiological responses while dedifferentiation of contractile smooth muscle cells to a proliferative and migratory phenotype is often observed during pathological processes. Although it is now possible to dissect multiple building blocks of bioenergetic components quantitatively, detailed cellular bioenergetics of artery smooth muscle cells is still largely unknown. Thus, we profiled cellular bioenergetics of human coronary artery smooth muscle cells and effects of metabolic intervention. Mitochondria and glycolysis stress tests utilizing Seahorse technology revealed that mitochondrial oxidative phosphorylation accounted for 54.5% of ATP production at rest with the remaining 45.5% due to glycolysis. Stress tests also showed that oxidative phosphorylation and glycolysis can increase to a maximum of 3.5 fold and 1.25 fold, respectively, indicating that the former has a high reserve capacity. Analysis of bioenergetic profile indicated that aging cells have lower resting oxidative phosphorylation and reduced reserve capacity. Intracellular ATP level of a single cell was estimated to be over 1.1 mM. Application of metabolic modulators caused significant changes in mitochondria membrane potential, intracellular ATP level and ATP:ADP ratio. The detailed breakdown of cellular bioenergetics showed that proliferating human coronary artery smooth muscle cells rely more or less equally on oxidative phosphorylation and glycolysis at rest. These cells have high respiratory reserve capacity and low glycolysis reserve capacity. Metabolic intervention influences both intracellular ATP concentration and ATP:ADP ratio, where subtler changes may be detected by the latter.

Rat Models of Ventricular Fibrillation Following Acute Myocardial Infarction

A number of animal models have been designed in order to unravel the underlying mechanisms of acute ischemia-induced arrhythmias and to test compounds and interventions for antiarrhythmic therapy. This is important as acute myocardial infarction (AMI) continues to be the major cause of sudden cardiac death, and we are yet to discover safe and effective treatments of the lethal arrhythmias occurring in the acute setting. Animal models therefore continue to be relevant for our understanding and treatment of acute ischemic arrhythmias. This review discusses the applicability of the rat as a model for ventricular arrhythmias occurring during the acute phase of AMI. It provides a description of models developed, advantages and disadvantages of rats, as well as an overview of the most important interventions investigated and the relevance for human pathophysiology.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

KATP channel inhibition blunts electromechanical decline during hypoxia in left ventricular working rabbit hearts

KEY POINTS

Heart function is critically dependent upon the balance of energy production and utilization. Sarcolemmal ATP-sensitive potassium channels (K_ATP channels) in cardiac myocytes adjust contractile function to compensate for the level of available energy. Understanding the activation of K_ATP channels in working myocardium during high-stress situations is crucial to the treatment of cardiovascular disease, especially ischaemic heart disease. Using a new optical mapping approach, we measured action potentials from the surface of excised contracting rabbit hearts to assess when sarcolemmal K_ATP channels were activated during physiologically relevant workloads and during gradual reductions in myocardial oxygenation. We demonstrate that left ventricular pressure is closely linked to K_ATP channel activation and that K_ATP channel inhibition with a low concentration of tolbutamide prevents electromechanical decline when oxygen availability is reduced. As a result, K_ATP channel inhibition probably exacerbates a mismatch between energy demand and energy production when myocardial oxygenation is low.

ABSTRACT

Sarcolemmal ATP-sensitive potassium channel (K_ATP channel) activation in isolated cells is generally understood, although the relationship between myocardial oxygenation and K_ATP activation in excised working rabbit hearts remains unknown. We optically mapped action potentials (APs) in excised rabbit hearts to test the hypothesis that hypoxic changes would be more severe in left ventricular (LV) working hearts (LWHs) than Langendorff (LANG) perfused hearts. We further hypothesized that K_ATP inhibition would prevent those changes. Optical APs were mapped when measuring LV developed pressure (LVDP), coronary flow rate and oxygen consumption in LANG and LWHs. Hearts were paced to increase workload and perfusate was deoxygenated to study the effects of myocardial hypoxia. A subset of hearts was perfused with 1 μm tolbutamide (TOLB) to identify the level of AP duration (APD) shortening attributed to K_ATP channel activation. During sinus rhythm, APD was shorter in LWHs compared to LANG hearts. APD in both LWHs and LANG hearts dropped steadily during deoxygenation. With TOLB, APDs in LWHs were longer at all workloads and APD reductions during deoxygenation were blunted in both LWHs and LANG hearts. At 50% perfusate oxygenation, APD and LVDP were significantly higher in LWHs perfused with TOLB (199 ± 16 ms; 92 ± 5.3 mmHg) than in LWHs without TOLB (109 ± 14 ms, P = 0.005; 65 ± 6.5 mmHg, P = 0.01). Our results indicate that K_ATP channels are activated to a greater extent in perfused hearts when the LV performs pressure-volume work. The results of the present study demonstrate the critical role of K_ATP channels in modulating myocardial function over a wide range of physiological conditions.