Potassium channel openers act through an activation of ATP-sensitive K+ channels in guinea-pig cardiac myocytes

Muscarinic suppression of ATP-sensitive K+ channels mediated by the M3/Gq/11/phospholipase C pathway contributes to mouse ileal smooth muscle contractions

ATP-sensitive K⁺ (K_ATP) channels are expressed in gastrointestinal smooth muscles, and their activity is regulated by muscarinic receptor stimulation. However, the physiological significance and mechanisms of muscarinic regulation of K_ATP channels are not fully understood. We examined the effects of the K_ATP channel opener cromakalim and the K_ATP channel blocker glibenclamide on electrical activity of single mouse ileal myocytes and on mechanical activity in ileal segment preparations. To explore muscarinic regulation of K_ATP channel activity and its underlying mechanisms, the effect of carbachol (CCh) on cromakalim-induced K_ATP channel currents ( I_KATP) was studied in myocytes of M₂ or M₃ muscarinic receptor-knockout (KO) and wild-type (WT) mice. Cromakalim (10 µM) induced membrane hyperpolarization in single myocytes and relaxation in segment preparations from WT mice, whereas glibenclamide (10 µM) caused membrane depolarization and contraction. CCh (100 µM) induced sustained suppression of I_KATP in cells from both WT and M₂KO mice. However, CCh had a minimal effect on I_KATP in M₃KO and M₂/M₃ double-KO cells. The G_q/11 inhibitor YM-254890 (10 μM) and PLC inhibitor U73122 (1 μM), but not the PKC inhibitor calphostin C (1 μM), markedly decreased CCh-induced suppression of I_KATP in WT cells. These results indicated that K_ATP channels are constitutively active and contribute to the setting of resting membrane potential in mouse ileal smooth muscles. M₃ receptors inhibit the activity of these channels via a G_q/11/PLC-dependent but PKC-independent pathways, thereby contributing to membrane depolarization and contraction of smooth muscles. NEW & NOTEWORTHY We systematically investigated the regulation of ATP-sensitive K⁺ channels by muscarinic receptors expressed on mouse ileal smooth muscles. We found that M₃ receptors inhibit the activity of ATP-sensitive K⁺ channels via a G_q/11/PLC-dependent, but PKC-independent, pathway. This muscarinic suppression of ATP-sensitive K⁺ channels contributes to membrane depolarization and contraction of smooth muscles.

Drug-induced QT interval shortening: potential harbinger of proarrhythmia and regulatory perspectives

ATP-dependent potassium channel openers such as pinacidil and levcromakalim have long been known to shorten action potential duration and to be profibrillatory in non-clinical models, raising concerns on the clinical safety of drugs that shorten QT interval. Routine non-clinical evaluation of new drugs for their potential to affect cardiac repolarization has revealed that drugs may also shorten QT interval. The description of congenital short QT syndrome in 2000, together with the associated arrhythmias, suggests that drug-induced short QT interval may be proarrhythmic, and an uncanny parallel is evolving between our appreciation of the short and the long QT intervals. Epidemiological studies report an over-representation of short QT interval values in patients with idiopathic ventricular fibrillation. Therefore, as new compounds that shorten QT interval are progressed further into clinical development, questions will inevitably arise on their safety. Arising from the current risk-averse clinical and regulatory environment and concerns on proarrhythmic safety of drugs, together with our lack of a better understanding of the clinical significance of short QT interval, new drugs that substantially shorten QT interval will likely receive an unfavourable regulatory review unless these drugs fulfil an unmet clinical need. This review provides estimates of parameters of QT shortening that may be of potential clinical significance. Rufinamide, a recently approved anticonvulsant, illustrates the current regulatory approach to drugs that shorten QT interval. However, to further substantiate or confirm the safety of these drugs, their approval may well be conditional upon large-scale post-marketing studies with a focus on cardiac safety.

Predicting drug-induced changes in QT interval and arrhythmias: QT-shortening drugs point to gaps in the ICHS7B Guidelines

Background and purpose:

The regulatory guidelines (ICHS7B) recommending inhibition of the delayed rectifier K⁺ current (I_Kr), carried by human ether-a-go-go-related gene (hERG) channels in cardiac cells (the hERG test), as a ‘first line' test for identifying compounds inducing QT prolongation, have limitations, some of which are outlined here.

Experimental approach:

hERG current was measured in HEK293 cells, stably transfected with hERG channels; action potential duration (APD) and arrhythmogenic effects were measured in isolated Purkinje fibres and perfused hearts from rabbits.

Key results:

576 compounds were screened in the hERG test: 58% were identified as hERG inhibitors, 39% had no effect and 3% were classified as stimulators. Of the hERG inhibitors, 92 were tested in the APD assay: 55.4% of these prolonged APD, 28.3% had no effect and 16.3% shortened APD. Of the 70 compounds without effect on hERG channels, 54.3% did not affect APD, 25.7% prolonged, while 20% significantly shortened APD. Dofetilide (hERG inhibitor; IC₅₀, 29 nM) prolonged QT and elicited early after-depolarizations and/or torsade de pointes (TdP) in isolated hearts. Mallotoxin and NS1643 (hERG current stimulators at 3 μM), levcromakalim and nicorandil (no effect on hERG current), all significantly shortened APD and QT, and elicited ventricular fibrillation (VF) in isolated hearts.

Conclusion and implications:

The hERG assay alone did not adequately identify drugs inducing QT prolongation. It is also important to detect drug-induced QT shortening, as this effect is associated with a potential risk for ventricular tachycardia and VF, the latter being invariably fatal, whereas TdP has an ∼15–25% incidence of death.

Cardiac repolarisation and drug regulation: assessing cardiac safety 10 years after the CPMP guidance

December 2007 marks the 10-year anniversary of the first regulatory guidance for evaluation of drug-induced QT interval prolongation. A decade on, it seems surprising that this document, which was released by the Committee on Proprietary Medicinal Products, caused such acrimony in the industry. Sponsors now routinely evaluate their new drugs for an effect on cardiac electrophysiology in preclinical studies, in addition to obtaining ECGs in all phases of drug development and conducting a formal thorough QT study in humans.However, concurrently, new concerns have also emerged on broader issues related to the cardiovascular safety of drugs because of their potential to shorten the QT interval as well as to induce proischaemic, profibrotic or prothrombotic effects. Drugs may also have an indirect effect by adversely affecting one or more of the cardiovascular risk factors (e.g. through fluid retention or induction of dyslipidaemia). In addition to peroxisome proliferator-activated receptor agonists and cyclo-oxygenase 2 selective inhibitors, three other drugs, darbepoetin alfa, pergolide and tegaserod, provide a more contemporary regulatory stance on tolerance of cardiovascular risk of drugs and their benefit-risk assessment. This recent, more assertive, risk-averse stance has significant implications for future drug development. These include the routine evaluation of cardiovascular safety for certain classes of drugs. Drugs that are intended for long-term use will almost certainly require long-term clinical evaluation in studies that enrol populations that most closely resemble the ultimate target population. Novel mechanisms of action and biomarkers by themselves are no guarantee of improved safety or benefits. Even some traditional biomarkers have come to be viewed with scepticism. Requirements for more extensive and earlier postmarketing assessment of clinical benefits and rare, but serious risks associated with new medicinal products should create a new standard of evidence for industry and regulators and almost certainly result in better assessment of benefit/risk, more effective and balanced regulatory actions and better care for patients.

Pharmacological Activation of Rapid Delayed Rectifier Potassium Current Suppresses Bradycardia-Induced Triggered Activity in the Isolated Guinea Pig Heart

Recently, attention has been drawn to compounds that activate the human ether-a-go-go channel potassium channel (hERG), which is responsible for the repolarizing rapid delayed rectifier potassium current (I(Kr)) in the mammalian myocardium. The compound NS3623 [N-(4-bromo-2-(1H-tetrazol-5-yl)-phenyl)-N'-(3'-trifluoromethylphenyl) urea] increases the macroscopic current conducted by the hERG channels by increasing the time constant for channel inactivation, which we have reported earlier. In vitro studies suggest that pharmacological activation is an attractive approach for the treatment of some arrhythmias. We present here data that support that NS3623 affects native I(Kr) and report the effects that activating this potassium current have in the intact guinea pig heart. In Langendorff-perfused hearts, the compound showed a concentration-dependent shortening of action potential duration, which was also detected as concentration-dependent shorter QT intervals. There was no sign of action potential triangulation or reverse use dependence. NS3623 decreased QT variability and distinctly decreased the occurrence of extrasystoles in the acutely bradypaced hearts. Taken together, the present data strongly support the concept of using hERG activators as a treatment for certain kinds of arrhythmias and suggest further investigation of this new approach.

Physiological and pathophysiological roles of ATP-sensitive K+ channels

ATP-sensitive potassium (K(ATP)) channels are present in many tissues, including pancreatic islet cells, heart, skeletal muscle, vascular smooth muscle, and brain, in which they couple the cell metabolic state to its membrane potential, playing a crucial role in various cellular functions. The K(ATP) channel is a hetero-octamer comprising two subunits: the pore-forming subunit Kir6.x (Kir6.1 or Kir6.2) and the regulatory subunit sulfonylurea receptor SUR (SUR1 or SUR2). Kir6.x belongs to the inward rectifier K(+) channel family; SUR belongs to the ATP-binding cassette protein superfamily. Heterologous expression of differing combinations of Kir6.1 or Kir6.2 and SUR1 or SUR2 variant (SUR2A or SUR2B) reconstitute different types of K(ATP) channels with distinct electrophysiological properties and nucleotide and pharmacological sensitivities corresponding to the various K(ATP) channels in native tissues. The physiological and pathophysiological roles of K(ATP) channels have been studied primarily using K(ATP) channel blockers and K(+) channel openers, but there is no direct evidence on the role of the K(ATP) channels in many important cellular responses. In addition to the analyses of naturally occurring mutations of the genes in humans, determination of the phenotypes of mice generated by genetic manipulation has been successful in clarifying the function of various gene products. Recently, various genetically engineered mice, including mice lacking K(ATP) channels (knockout mice) and mice expressing various mutant K(ATP) channels (transgenic mice), have been generated. In this review, we focus on the physiological and pathophysiological roles of K(ATP) channels learned from genetic manipulation of mice and naturally occurring mutations in humans.

Effects of pinacidil on electrophysiological properties of epicardial border zone of healing canine infarcts: possible effects of K(ATP) channel activation

BACKGROUND

K(ATP) channels, activated by ischemia, participate in the arrhythmogenic response to acute coronary occlusion. The function of these channels in border zones of healing infarcts, where arrhythmias also arise, has not been investigated. Do these channels remain maximally activated during infarct healing, or do they downregulate after a period of time? Both might preclude further activation.

METHODS AND RESULTS

Myocardial infarction was produced in dogs by ligation of the left anterior descending coronary artery. Impulse propagation in the epicardial border zone (EBZ) of 4-day-old healing infarcts was mapped during administration of pinacidil, a K(ATP) channel activator, directly into the EBZ coronary blood supply. Pinacidil restored conduction and excitability when the EBZ was initially inexcitable and had large regions of block (6 of 8 experiments). This allowed reentrant circuits to form in the EBZ, causing tachycardia (4 of 8 experiments). In hearts with an initially excitable EBZ, pinacidil shortened the effective refractory period and abolished conduction block at short cycle lengths (7 experiments). This effect prevented initiation of reentry (1 of 2 experiments).

CONCLUSIONS

The response to pinacidil indicates that K(ATP) channels in the EBZ remain functional and can be activated to influence electrophysiological properties and arrhythmogenesis.

Molecular aspects of ATP-sensitive K+ channels in the cardiovascular system and K+ channel openers

ATP-sensitive K+ (K(ATP)) channels are inhibited by intracellular ATP (ATPi) and activated by intracellular nucleoside diphosphates and thus, provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions, and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective, as well as proarrhythmic, effects. These channels are clearly heterogeneous. The cardiac K(ATP) channel is the prototype of K(ATP) channels possessing approximately 80 pS of single-channel conductance in the presence of approximately 150 mM extracellular K+ and opens spontaneously in the absence of ATPi. A vascular K(ATP) channel called a nucleoside diphosphate-dependent K+ (K(NDP)) channel exhibits properties significantly different from those of the cardiac K(ATP) channel. The K(NDP) channel has the single-channel conductance of approximately 30-40 pS in the presence of approximately 150 mM extracellular K+, is closed in the absence of ATPi, and requires intracellular nucleoside di- or triphosphates, including ATPi to open. Nevertheless, K(ATP) and K(NDP) channels are both activated by K+ channel openers, including pinacidil and nicorandil, and inhibited by sulfonylurea derivatives such as glibenclamide. It recently was found that the cardiac K(ATP) channel is composed of a sulfonylurea receptor (SUR)2A and a two-transmembrane-type K+ channel subunit Kir6.2, while the vascular K(NDP) channel may be the complex of SUR2B and Kir6.1. By precisely comparing the functional properties of the SUR2A/Kir6.2 and the SUR2B/Kir6.1 channels, we shall show that the single-channel characteristics and pharmacological properties of SUR/Kir6.0 channels are determined by Kir and SUR subunits, respectively, while responses to intracellular nucleotides are determined by both SUR and Kir subunits.

Glimepiride (Hoe490) inhibits the rilmakalim induced decrease in intracellular free calcium and contraction of isolated heart muscle cells from guinea pigs to a lesser extent than glibenclamide

Glibenclamide is a potent inhibitor of the ATP-dependent potassium channel. Opening of the ATP-dependent potassium channel is regarded as a mechanism of ischemic preconditioning. This in vitro study examines the influence of glibenclamide and glimepiride, a new sulfonylurea, on the negative inotropic action of the potassium channel opener rilmakalim in isolated ventricular myocytes. Cardiac myocytes were isolated from adult guinea pig hearts by collagenase perfusion and incubated with rilmakalim (concentration range 0.1-12.0 microM), glibenclamide (concentration range 0.03-3.0 microM) plus rilmakalim (3.0 or 7.5 microM), and glimepiride (0.03-9.0 microM) plus rilmakalim (3.0 or 7.5 microM) and paced by electrical field stimulation. Contractility of the myocytes was evaluated by digital image analysis, intracellular free calcium was determined by means of fura-2 fluorescence measurements, and cell viability was assessed morphologically as well as by measurement of lactate dehydrogenase activity. Rilmakalim reduced the systolic intracellular free calcium and contractility of ventricular myocytes in a concentration dependent manner. This effect was antagonized by glibenclamide at lower concentrations (0.3 microM) than glimepiride (3.0 microM). The smaller antagonistic action of glimepiride on the negative inotropic effect of rilmakalim as compared with glibenclamide most likely reflects a less potent inhibition of ATP-dependent potassium channels by glimepiride.

Dispersion-based reentry: mechanism of initiation of ventricular tachycardia in isolated rabbit hearts

The aim of the study was to determine whether facilitation of reentry by potassium-channel openers is related to dispersion of refractoriness and/or modification of anisotropic properties of ventricular myocardium. The dispersion of ventricular effective refractory period (VERP), longitudinal and transverse ventricular conduction velocities (thetaL and thetaT, respectively), and wavelength [lambda = VERP x theta(L or T)] were studied in Langendorff-perfused left ventricular epicardium in 20 rabbits during infusion of incremental doses of levcromakalim or nicorandil. Dispersion of refractoriness was assessed using standard deviation of VERP mean (SD-VERP), dispersion index (DI; SD-VERP/mean VERP), and maximum dispersion (Dmax = VERPmax - VERPmin). Ventricular conduction velocities and anisotropic ratio were not modified, whatever the dose used. VERP and lambda were significantly shortened at high concentrations of levcromakalim and nicorandil. At these doses, SD-VERP, DI, and Dmax were increased significantly. Analysis of ventricular tachycardia induction, performed using a high-resolution ventricular mapping system, confirmed that heterogeneity and shortening of VERP were factors inducing functional conduction block. Our data suggest that, in rabbit left ventricular epicardium, functional conduction block facilitating the occurrence of reentry could be initiated by shortening and, especially, by dispersion of refractoriness during infusion of potassium-channel openers.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

Comparison of the cellular electrophysiological characteristics of canine left ventricular epicardium, M cells, endocardium and Purkinje fibres

Electrophysiological differences among M cells, epicardium, endocardium and Purkinje fibres of the canine ventricle were studied over a wide range of stimulation cycle lengths, and the pharmacological response of these cell types to the sodium channel blocker tetrodotoxin, calcium channel blocker nifedipine and ATP-sensitive potassium channel activator pinacidil was compared. The experiments were carried out by applying standard intracellular microelectrode technique in isolated dog left ventricular preparations. The results confirmed the existence of M cells in the canine ventricle, in addition, the distribution of the rate of rise of the action potential upstroke and action potential amplitude values reflecting probably the inhomogeneity of the fast sodium current in these cells was revealed. It was also demonstrated that M cells differ from Purkinje fibres in some aspects which were not expected from previous investigations: (1) The early portion of the action potential duration restitution curve in M cells is more similar to that of endocardial and epicardial cells than to Purkinje fibres. (2) The plateau phase of the action potentials in Purkinje fibres developed at a more negative potential range than that in the other cell types studied. (3) The pharmacological response to tetrodotoxin and pinacidil in M cells resembles to that in the endocardial and epicardial cells more than in the Purkinje fibres. Our results provide further evidence in support of the existence of M cells but also indicate that there are important electrophysiological as well as pharmacological differences between M cells and Purkinje fibres.

Modulation of mitochondrial ATP-dependent K+ channels by protein kinase C

Pharmacological openers of mitochondrial ATP-dependent K+ (mitoKATP) channels mimic ischemic preconditioning, and such cardioprotection can be prevented by mitoKATP channel blockers. It is also known that protein kinase C (PKC) plays a key role in the induction and maintenance of preconditioning. To look for possible mechanistic links between these 2 sets of observations, we measured mitochondrial matrix redox potential as an index of mitoKATP channel activity in rabbit ventricular myocytes. The mitoKATP channel opener diazoxide (100 micromol/L) partially oxidized the matrix redox potential. Exposure to phorbol 12-myristate 13-acetate (PMA, 100 nmol/L) potentiated and accelerated the effect of diazoxide. These effects of PMA were blocked by the mitoKATP channel blocker 5-hydroxydecanoate, which we verified to be a selective blocker of the mitoKATP channel in simultaneous recordings of membrane current and flavoprotein fluorescence. The inactive control compound 4alpha-phorbol (100 nmol/L) did not alter the effects of diazoxide. We conclude that the activity of mitoKATP channels can be regulated by PKC in intact heart cells. Potentiation of mitoKATP channel opening by PKC provides a direct mechanistic link between the signal transduction of ischemic preconditioning and pharmacological cardioprotection targeted at ATP-dependent K+ channels.

SUR2 subtype (A and B)-dependent differential activation of the cloned ATP-sensitive K+ channels by pinacidil and nicorandil

1. The classical ATP sensitive K+ (K(ATP)) channels are composed of a sulphonylurea receptor (SUR) and an inward rectifying K+ channel subunit (BIR/Kir6.2). They are the targets of vasorelaxant agents called K+ channel openers, such as pinacidil and nicorandil. 2. In order to examine the tissue selectivity of pinacidil and nicorandil, in vitro, we compared the effects of these agents on cardiac type (SUR2A/Kir6.2) and vascular smooth muscle type (SUR2B/Kir6.2) of the K(ATP) channels heterologously expressed in HEK293T cells, a human embryonic kidney cell line, by using the patch-clamp method. 3. In the cell-attached recordings (145 mM K+ in the pipette), pinacidil and nicorandil activated a weakly inwardly-rectifying, glibenclamide-sensitive 80 pS K+ channel in both the transfected cells. 4. In the whole-cell configuration, pinacidil showed a similar potency in activating the SUR2B/Kir6.2 and SUR2A/Kir6.2 channels (EC50 of approximately 2 and approximately 10 microM, respectively). On the other hand, nicorandil activated the SUR2B/Kir6.2 channel > 100 times more potently than the SUR2A/Kir6.2 (EC50 of approximately 10 microM and > 500 microM, respectively). 5. Thus, nicorandil, but not pinacidil, preferentially activates the K(ATP) channels containing SUR2B. Because SUR2A and SUR2B are diverse only in 42 amino acids at their C-terminal ends, it is strongly suggested that this short part of SUR2B may play a critical role in the action of nicorandil on the vascular type classical K(ATP) channel.

Glibenclamide inhibits thromboxane A2-induced contraction in human internal mammary artery and saphenous vein

Glibenclamide, like other hypoglycemic sulfonylurea derivatives, is a potent blocker of ATP-regulated K+ channels. In addition, it is reported to inhibit prostanoid-induced contractions of isolated vascular smooth muscle from different animal species. We investigated the effect of glibenclamide on the thromboxane A2-mimetic U-46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy-prostaglandin F2alpha)-induced contractions in human isolated internal mammary arteries and saphenous veins. In the two vascular preparations, glibenclamide (3, 10 and 30 microM) caused a concentration-dependent shift to the right of the U-46619 contraction-response curve with a reduction, at the highest concentrations, in the maximal responses. This inhibitory effect appears selective for thromboxane A2-induced contractions since glibenclamide (30 microM) did not alter the contraction of internal mammary arteries in response to norepinephrine and of saphenous veins in response to 5-hydroxytryptamine (5-HT) and endothelin-1. However, glibenclamide reduced the endothelin-1-induced contraction in internal mammary arteries. The endothelin-1-induced contractions were similarly inhibited by GR 32191 ([1R-[1alpha(Z),2beta,3beta,5alpha]]-(+)-7-[5-([1,1'-b iphenyl]-4-ylmethoxy)-3-hydroxy-2-(1-piperidinyl)cyclopentyl]-4-++ +heptonoic acid, a thromboxane A2 receptor antagonist. These results suggest that glibenclamide also reduced the endothelin-1-induced contractions by inhibiting a thromboxane A2 receptor-mediated component of the contraction elicited by this peptide. In conclusion, glibenclamide clearly appears to exert a specific inhibitory influence on prostanoid-induced contractions in human internal mammary arteries and saphenous veins.

KvLQT1 potassium channel but not IsK is the molecular target for trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethyl- chromane

Mutations in the KvLQT1 gene are the cause for the long QT syndrome [Circulation 94:1996-2012 (1996)]. Coexpression of KvLQT1 in association with the channel regulator protein IsK produces a K+ current with characteristics reminiscent of the slow component of the delayed rectifier in cardiac myocytes. We explored the pharmacological properties of trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dime thyl- chromane (293B), a chromanol compound, on the K+ current produced by direct intranuclear injection of KvLQT1 and IsK cDNA plasmids in COS-7 cells. Injected cells were recorded by means of the whole-cell and cell-attached patch-clamp configurations under chloride-free conditions. Cells injected with KvLQT1 cDNA alone exhibited a fast-activating outward K+ current, whereas cells coinjected with KvLQT1 plus IsK cDNAs exhibited a time-dependent outward current with slower activation kinetics. The chromanol 293B blocked the K+ current related to KvLQT1 expression in both the absence or presence of IsK. The IC50 value for 293B to block KvLQT1-related current was not significantly modified by the presence of IsK (9.9 microM in the absence of IsK versus 9.8 microM in its presence). The block produced by 293B was strongly voltage-dependent inasmuch as it was close to 0 at -80 mV and occurred during a depolarizing voltage step. The time constants for the drug to block the current were in the same order of magnitude as activation kinetics of the current. Kinetics for drug unblock at the holding potential were much faster, in the order of a few tenths of a msec. KvLQT1 currents recorded in the cell-attached configuration were also blocked by externally applied 293B, suggesting that the compound penetrated the cell to block the channel. Cromakalim, another chromanol compound, also blocked KvLQT1 currents. Our results show that the chromanol compound 293B is targeted to KvLQT1 channels but not to the IsK regulator.

Frequency-dependent Cardiac Electrophysiologic Effects of Tedisamil: Comparison With Quinidine and Sotalol

BACKGROUND: Tedisamil is a potent bradycardiac/antiischemic drug known to lengthen cadiac repolarization by blocking various potassium channels. Recent in vivo experiments revealed that it is an antiarrhythmic agent. It was therefore of interest to compare the cellular electrophysiologic effects of tedisamil with those of quinidine and sotalol in isolated cardiac preparations. METHODS AND RESULTS: The conventional microelectrode technique was applied in isolated dog cardiac Purkinje and ventricular muscle fibers and in rabbit left atrial muscle. Tedisamil (1 µM) and sotalol (30 µM) lengthened, while quinidine (10 µM) shortened action potential duration in dog Purkinje fibers. The phase 1 repolarization was delayed by tedisamil and quinidine and not changed by sotalol. In dog ventricular muscle and in rabbit atrial muscle, all three drugs studied lengthened repolarization. In dog Purkinje fiber, tedisamil and sotalol lengthened action potential duration more at slow than at high stimulation frequency (reverse use-dependence). In dog ventricular muscle fibers, the effect of the drugs was not clearly frequency dependent. In rabbit atrial muscle fibers, the quinidine-evoked repolarization lengthening was most pronounced at intermediate cycle lengths (500-1000 ms). Tedisamil and quinidine but not sotalol depressed the maximal rate of depolarization (V(max)), which depended on the stimulation frequency (use-dependence). The nature of the use-dependent V(max) block differed between quinidine and tedisamil. Quinidine decreased V(max) at a relatively wide range of stimulation frequencies whle tedisamil. Quinidine decreased V(max) at a relatively wide range of stimulation frequencies while tedisamil decreased V(max) largely at high rate of stimulation. Tedisamil and quiinidine prevented or decreased the pinacidil-evoked action potential shortening in dog ventricular muscle, suggesting block of the ATP-dependent potassium channels (I(KATP)), while with sotalol such effect was not observed. CONCLUSIONS: Although tedisamil, quinidine, and sotalol are known to lengthen the QT interval, their cellular electrophysiologic effects substantially differ. Tedisamil lengthens repolarization and prevents pinacidil-evoked action potential duration shortening, suggesting I(K(ATP)) blockade. Its effect on the V(max) is limited mostly to fast heart rate. These electrophysiologic effects of tedisamil resemble those of chronic amiodarone treatment.

Effects of tilisolol, a nonselective beta-adrenergic blocker, on the membrane currents of isolated guinea pig ventricular myocytes

The effects of tilisolol, a nonselective beta-adrenoceptor blocker, on transmembrane ionic currents were studied in single guinea pig ventricular myocytes by using the whole-cell voltage clamp technique. In the absence of beta-adrenergic stimulation, 10 microM tilisolol, a concentration higher than that used in the clinical therapeutic regimen, did not affect the L-type Ca2+ current (ICa), the inwardly rectifying K+ current (IK1), or the delayed rectifying K+ current (IK). In addition, it did not induce currents through the adenosine triphosphate (ATP)-sensitive K+ channels. However, under the nonselective beta-adrenergic stimulation with 1 microM isoproterenol, 1 microM tilisolol almost completely reversed the agonist-induced increase of IK. The increase of ICa by isoproterenol was blocked only by approximately 30% with tilisolol. We concluded that, at therapeutic concentrations (0.01-0.15 microM), tilisolol is a pure beta-adrenoceptor antagonist that has no direct effects on the transmembrane ionic currents of mammalian ventricular myocytes, such as ICa, IK1, or IK. Comparison of the dose-dependent effects of tilisolol on ICa and IK suggested that tilisolol may selectively inhibit catecholamine-induced increase of IK at the therapeutic concentrations. The virtually selective inhibition of IK, leaving ICa intact, may be favorable to prevent the catecholamine-induced arrhythmia without inhibiting contraction.

Myocardial protection in the acutely injured heart: hyperpolarizing versus depolarizing hypothermic cardioplegia

OBJECTIVES

The superiority of hyperpolarized arrest with adenosine triphosphate-sensitive potassium channel openers over standard hyperkalemic depolarizing cardioplegia during normothermic ischemia has been documented. This study examined the hypothesis that pinacidil would provide superior protection in a more clinically relevant model of an acutely injured heart and hypothermic cardioplegic arrest.

METHODS

In a blood-perfused, parabiotic, rabbit heart Langendorff model, hearts underwent 15 minutes of unprotected global normothermic ischemia before the administration of 50 ml of cardioplegic solution at 4 degrees C, followed by 50 minutes of hypothermic (15 degrees C) ischemia and 30 minutes of reperfusion. The cardioplegic solutions administered consisted of Krebs-Henseleit solution alone (N = 6), Krebs-Henseleit solution with pinacidil (50 mumol/L; N = 10), Krebs-Henseleit solution with pinacidil (50 mumol/L) and glibenclamide (a potassium channel blocker, 10 mumol/L; N = 8), or St. Thomas' Hospital solution (N = 8). The percent recovery of developed pressure, linear diastolic pressure-volume relationships, and coronary blood flow were compared.

RESULTS

The percent recovery of developed pressure was 32.8% +/- 2.8%, 43.0% +/- 4.3%, 46.5% +/- 2.2%, and 49.3% +/- 2.7% for the Krebs-Henseleit, the Krebs-Henseleit with pinacidil and glibenclamide, the St. Thomas' Hospital, and the Krebs-Henseleit with pinacidil groups, respectively. No hearts had ventricular fibrillation on reperfusion.

CONCLUSIONS

During hypothermic hyperpolarized arrest, as opposed to normothermic ischemia as in our previous studies, there was neither an increased incidence of ventricular fibrillation nor prolonged electrical activity when compared with results during traditional hyperkalemic arrest. Myocardial protection by St. Thomas' Hospital solution and pinacidil was superior (p = 0.009) to that with Krebs-Henseleit solution alone. The protection provided by pinacidil was lost with the addition of glibenclamide, indicating that the drug has adenosine triphosphate-sensitive potassium channel activity during hypothermia.

Effects of tolbutamide on ATP-sensitive K+ channels from human right atrial cardiac myocytes

In order to gain further insight into possible deleterious effects on ischaemia-induced myocardial damage induced by sulfonylureas when administered to humans, the effects of tolbutamide on ATP-sensitive K+ (KATP) channels from human right atrial myocytes were studied. Single myocytes were enzymatically isolated from human right atrium. The cell-attached and inside-out configuration of the patch-clamp technique were employed at room temperature (both the pipette and the bath solution contained high [K+]). KATP channels in inside-out patches showed slight inward rectification, had a slope conductance of 75.1 +/- 2.4 pS (mean +/- S.E.M.; n = 5) at negative membrane potentials and these channels were blocked by ATP (half-maximal block (EC50) at 39 microM; Hill coefficient = 1.65). In cell-attached recordings, cromakalim (300 microM) opened KATP channels (with a slope conductance of 73.3 +/- 1.8 pS (n = 16) at negative membrane potentials) in previously silent patches. Cromakalim-induced openings of KATP channels were not markedly affected by 100 or 300 microM tolbutamide but were blocked by tolbutamide at millimolar concentrations (1-3 mM). The concentration-response relationship for tolbutamide-induced block of KATP channels in the presence of 300 microM cromakalim in cell-attached patches was calculated to values for the EC50 of 1.325 mM and for the Hill coefficient of 1.0, respectively. 1 mM tolbutamide-induced block of cromakalim-induced KATP channel openings was not different at room temperature when compared to 37 degrees. It is concluded that KATP channels from human right atrial myocytes have a low sensitivity towards tolbutamide-induced block.

Comparison of proarrhythmogenic effects of two potassium channel openers, levcromakalim (BRL 38227) and nicorandil (RP 46417): a high-resolution mapping study on rabbit heart

This study was designed (a) to test and (b) to compare proarrhythmic effects of levcromakalim and nicorandil; and (c) determine the mechanism of arrhythmia initiation by using high-resolution ventricular epicardial mapping on 44 Langendorff-perfused rabbit hearts. Eighteen hearts were kept intact and received incremental doses (1-500 microM) of levcromakalim, nicorandil, and isosorbide dinitrate. In 26 hearts, a thin layer of epicardium was obtained after endocardial cryotechnique (frozen hearts). In intact hearts, isosorbide dinitrate did not produce any arrhythmia. In contrast, levcromakalim induced spontaneous ventricular fibrillation (VF) in all hearts at 50 microM, whereas only one VF occurred at 500 microM nicorandil. These three drugs produced a dose-dependent bradycardia in intact hearts. In frozen hearts, arrhythmias were induced by 5 microM levcromakalim and 50 microM nicorandil. Isosorbide dinitrate had no proarrhythmogenic effect. Epicardial mapping showed that most of induced ventricular tachycardias were based on reentry around an arc of functional conduction block. Ventricular conduction velocities did not change, but levcromakalim and nicorandil shortened ventricular effective refractory period. We conclude that (a) levcromakalim and nicorandil, used in toxic concentrations, have direct proarrhythmic effects; (b) nicorandil proarrhythmogenic effects are 10 times less marked than those of levcromakalim (arrhythmia is solely the result of the potassium channel opener property of nicorandil); and (c) most of ventricular tachycardias induced are based on reentry.

Y-26763 protects the working rat myocardium from ischemia/reperfusion injury through opening of KATP channels

This investigation was undertaken to determine the possible protection against ischemia afforded by Y-26763, [(-)-(3S,4R)-4-(N- acetyl-N-hydroxyamino)-6-cyano-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran- 3- ol], which has K+ channel-opening properties, in isolated rat hearts under working conditions. This preparation was subjected to 28 min of global ischemia followed by 30 min of reperfusion. Drugs were injected into the aortic cannula prior to ischemia. Compared to control, Y-26763 (1 microM) resulted in a significant recovery of post-ischemic cardiac functions, significant reduction of cellular enzyme loss, and preserved significantly the stocks of cellular high-energy phosphates and the myocyte ultrastructure. These effects of Y-26763 were completely prevented by glibenclamide (10 microM), a specific K+ channel blocker of KATP channels. In non-ischemic conditions, Y-26763 significantly increased coronary flow without affecting cardiac output and heart rate. The data were analyzed statistically by analysis of variance. The results clearly demonstrate that Y-26763 protects the myocardium from ischemic injury by opening KATP channels.

Differential Electrophysiologic Effects of Chronically Administered Amiodarone on Canine Purkinje Fibers versus Ventricular Muscle

BACKGROUND: Acute and chronic treatment with amiodarone has been reported to cause different electrocardiographic changes in patients. The cellular electrophysiologic effects of chronic administration (50 mg/kg/day orally for 6 weeks) and acute superfusion (5 µM in the tissue bath) of amiodarone were therefore studied in dog cardiac ventricular muscle and Purkinje fibers using conventional microelectrode techniques. METHODS AND RESULTS: During stimulation at 1 Hz, chronic amiodarone treatment lengthened the ventricular muscle action potential duration (APD) from 227.8 +/- 6.3 ms (n = 20) to 262.3 +/- 5.2 ms (n = 21; P <.01), but shortened that of Purkinje fibers from 337.6 +/- 9.2 (n = 21) to 308.3 +/- 7.1 (n = 19; P <.05). Acute superfusion of 5 µM amiodarone in cardiac tissue obtained from chronically treated dogs did not change ventricular muscle APD but shortened Purkinje fiber AP from 309.7 +/- 13.6 ms to 281.9 +/- 11.9 ms (n = 8; P <.05). Neither the chronic nor the acute amiodarone exposure prevented the APD shortening in ventricular muscle evoked by 10 µM pinacidil, suggesting that amiodarone does not interfere with the ATP-dependent potassium channels. The normal difference in APD between ventricular muscle and Purkinje fibers in untreated, control preparations was 110 ms but decreased to 46 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in the presence of 30 µM sotalol, a class III antiarrhythmic drug used for comparison. Amiodarone (5 µM) applied directly abolished early afterdepolarizations (EADs) (induced by 1 µM dofetilide + 20 µM BaCl(2) + 2 mM CsCl) in 5 of 6 experiments and caused strong use-dependent V(max) block with relatively fast onset kinetics (rate constant = 1.23 +/- 0.13 AP(-1), n = 5) and offset (time constant = 364 +/- 62.5 ms, n = 5). After chronic amiodarone treatment, in contrast with acute sotalol application (30 µM), no reverse use-dependent effect was observed on the APD in Purkinje fibers. CONCLUSIONS: These results provide further evidence that amiodarone differs from other recognized class III antiarrhythmic drugs (ie, it is a mixed type agent with acute fast kinetic class I [type B] and a unique class III antiarrhythmic action characterized by decreased dispersion of APDs between ventricular muscle and Purkinje fibers). Amiodarone can abolish EADs unlike other class III agents that are usually associated to induction of EADs. These features might be responsible not only for the antiarrhythmic efficacy, but also for the relative safety (low incidence of torsade de pointes) of amiodarone in clinical settings.

Haemodynamic and other effects of sulphonylurea drugs on the heart

Antidiabetic sulphonylureas have attracted a great deal of interest in experimental cardiology to evaluate the role of ATP-sensitive potassium channels in the cardiovascular system. It is well established that KATP channels are present in cardiac cells and also in vascular smooth muscle cells and are implicated in the regulation of myocardial and vascular function. It follows that drugs which open, or inhibit the opening of these channels, might profoundly modify cardiovascular function both under physiological and pathophysiological conditions. This paper reviews the evidence for the role of KATP channels in the cardiovascular system and discusses how the different generations of sulphonylurea drugs interfere with cardiac function. We will particularly concentrate on the haemodynamic effects of different sulphonylureas and shortly discuss how these drugs modify ischaemia-reperfusion arrhythmias.

Myocardial protection with pinacidil cardioplegia in the blood-perfused heart

BACKGROUND

Adenosine triphosphate-sensitive potassium-channel openers are potent vasodilators that have been found to be cardioprotective during myocardial ischemia. The potassium-channel opener pinacidil was investigated to determine its efficacy as a cardioplegic agent.

METHODS

A blood-perfused, parabiotic, isolated rabbit heart Langendorff preparation was used. Fifty-six hearts underwent 30 minutes of global normothermic ischemia after a 50-mL infusion of cardioplegia, followed by 60 minutes of reperfusion. The cardioplegia consisted of Krebs-Henseleit solution with either vehicle alone (control), 20 mmol KCl, or pinacidil (10, 50, 100, 150, or 200 mumol/L). The developed pressure was measured at baseline and after reperfusion. Coronary blood flow was measured with an in-line ultrasonic probe.

RESULTS

Pinacidil (50 mumol/L), as opposed to potassium cardioplegia, provided significantly better postischemic percentage recovery of developed pressure compared with controls (68.3% +/- 4.0% versus 44.6% +/- 5.5%; p < 0.05). The time until electrical arrest was significantly shorter in the hyperkalemic group than in all other groups. Linear end-diastolic pressure-volume relationships revealed an increase in slope after ischemia in all groups. Coronary flow after 5 minutes of reperfusion was significantly higher in both the 50-mumol/L and 100-mumol/L pinacidil groups compared with traditional hyperkalemic arrest, and this returned to baseline after 15 minutes.

CONCLUSIONS

The potassium channel opener pinacidil provided dose-dependent myocardial protection during global ischemia in the blood-perfused rabbit heart model. Potassium-channel openers are a promising class of drugs that may provide an alternative to traditional hyperkalemic cardioplegia.

Action potential duration and force-frequency relationship in isolated rabbit, guinea pig and rat cardiac muscle

The effect of action potential duration and elevated cytosolic sodium concentration on the force-frequency relationship in isolated rabbit, guinea pig and rat papillary muscle preparations was studied. Shortening of action potential duration in guinea pig and rabbit from 150-200 ms to values characteristic of rat (20-40 ms), using the K(ATP) channel activator levkromakalim (15 mumol.l-1), markedly reduced the force of contraction and converted the positive force-frequency relationship into negative one at longer pacing cycle lengths. This conversion was greatly enhanced in the presence of acetylstrophanthidin (0.2-1 mumol.l-1), an inhibitor of the Na-K pump. Acetylstrophanthidin (1 mumol.l-1) alone, however, had no effect on the force-frequency relationship. Prolongation of action potential duration in rat with inhibitors of cardiac K channels (4-aminopyridine [10 mmol.l-1] plus tetraethylammonium [2 mmol.l-1) increased the force of contraction and abolished the negative force-frequency relationship observed in rat at longer pacing-cycle lengths. It is concluded that both action potential duration and cytosolic sodium concentration are major determinants of the force-frequency relationship in mammalian myocardium.

ATP-sensitive potassium channels are altered in ventricular myocytes from diabetic rats

Hypoxia-induced shortening of the action potential duration, attributed to activation of the ATP-sensitive potassium (KATP) channels, occurs to a much greater extent in ventricular cells from diabetic rats. This study examined whether the KATP channels are altered in streptozotocin-diabetic myocardium. In inside-out patches from ventricular myocytes (with symmetrical 140 mM [K+]), inward KATP currents (at potentials negative to the K+ reversal potential) were similar in amplitude in control and diabetic patches (slope conductances: 69 and 74 pS, respectively). However, outward single-channel currents were larger for channels from diabetic heart cells than from control cells (e.g., at +75 mV the diabetic channel currents were 3.7 +/- 0.3 pA vs. 2.7 +/- 0.1 pA for control currents, p < 0.05), due to reduced inward rectification of diabetic channel currents. There was no difference in open and closed times between control and diabetic channels. The IC50 for ATP inhibition of the KATP channel single-channel currents was 11.4 microM for control currents and 4.7 microM for diabetic channel currents. Thus, the major difference found between KATP channels from control and diabetic hearts was the greater outward diabetic single-channel current, which may contribute to the enhanced sensitivity to hypoxia (or ischemia) in diabetic hearts.

A family of sulfonylurea receptors determines the pharmacological properties of ATP-sensitive K+ channels

We have cloned an isoform of the sulfonylurea receptor (SUR), designated SUR2. Coexpression of SUR2 and the inward rectifier K+ channel subunit Kir6.2 in COS1 cells reconstitutes the properties of K(ATP) channels described in cardiac and skeletal muscle. The SUR2/Kir6.2 channel is less sensitive than the SUR/Kir6.2 channel (the pancreatic beta cell KATP channel) to both ATP and the sulfonylurea glibenclamide and is activated by the cardiac K(ATP) channel openers, cromakalim and pinacidil, but not by diazoxide. In addition, SUR2 binds glibenclamide with lower affinity. The present study shows that the ATP sensitivity and pharmacological properties of K(ATP) channels are determined by a family of structurally related but functionally distinct sulfonylurea receptors.

Mechanism of action of K channel openers on skeletal muscle KATP channels. Interactions with nucleotides and protons

The molecular mechanisms underlying the actions of K channel openers (KCOs) on KATP channels were studied with the patch clamp technique in excised inside-out patches from frog skeletal muscle fibers. Benzopyran KCOs (levcromakalim and SR 47063) opened channels partially blocked by ATP, ADP, or ATP gamma s, with and without Mg2+, but they had no effects in the absence of internal nucleotides, even after channel activity had significantly declined because of rundown. The effects of KCOs could therefore be attributed solely to a competitive interaction between KCOs and nucleotides, as confirmed by observations that ATP decreased the apparent affinity for KCOs and that, conversely, KCOs decreased ATP or ADP sensitivity. Protons antagonized the action of the non-benzopyran KCOs, pinacidil and aprikalim, by enhancing their dissociation rate. This effect resembled the effect of acidification on benzopyran KCOs (Forestier, C., Y. Depresle, and M. Vivaudou. FEBS Lett. 325:276-280, 1993), suggesting that, in spite of their structural diversity, KCOs could act through the same binding sites. Detailed analysis of the inhibitory effects of protons on channel activity induced by levcromakalim or SR 47063 revealed that, in the presence of 100 microM ATP, this effect developed steeply between pH 7 and 6 and was half maximal at pH 6.6. These results are in quantitative agreement with an allosteric model of the KATP channel possessing four protonation sites, two nucleotidic sites accessible preferentially to Mg(2+)-free nucleotides, and one benzopyran KCO site. The structural implications of this model are discussed.

Synergistic modulation of ATP-sensitive K+ currents by protein kinase C and adenosine. Implications for ischemic preconditioning

Ischemic preconditioning has been shown to involve the activation of adenosine receptors, protein kinase C (PKC), and ATP-sensitive K+ (K ATP) channels. We investigated the effects of PKC activation and adenosine on K(ATP) current (I KATP) and action potentials in isolated rabbit ventricular myocytes. Responses to pinacidil (100 to 400 micromol/L), an opener of K(ATP) channels, were markedly increased by preexposure to the PKC activator phorbol 12-myristate 13-acetate (PMA, 100 nmol/L). I(KATP) measured at 0 mV was increased by PMA pretreatment from 0.55 +/- 0.32 to 3.25 +/- 0.47 nA (n=6, P < .01). We next determined whether PKC activation abbreviates the time required to turn on I(KATP) developed after an average of 15.1 +/- 2.4 minutes (n=8). Ten-minute pretreatment with PMA alone (PMA+MI) did not significantly alter this latency (11.9 +/- 2.0 minutes, n=8). Since adenosine receptor activation has been shown to play an important role in the preconditioning response, two groups of myocytes were studied with adenosine (10 micromol/L) included during MI. Without PMA, adenosine alone (MI+Ado) did not affect the latency to develop I(KATP) (12.3 +/- 1.5 minutes, n=8). However, if cells were pretreated with PMA and then subjected to MI in the presence of adenosine (PMA+MI+Ado), the latency was greatly shortened to 5.5 +/- 1.6 minutes (n=8;P < .02 versus MI, PMA+MI, and MI+Ado groups). This effect could not be reproduced by an inactive phorbol but was completely abolished by the adenosine receptor antagonist 8-(p-sulfophenyl)-theophylline. The opening of K(ATP) channels may be cardioprotective because of the abbreviation of action potential duration (APD) during ischemia. Therefore, we tested whether PKC activation could modify the time course of APD shortening during MI. Consistent with the ionic current measurements, PMA pretreatment significantly accelerated APD shortening, but only when adenosine (10 micromol/L) was included during MI. The effects were not attributable to accelerated ATP consumption: PMA pretreatment did not alter the time required to induce rigor during MI, whether or not adenosine was included. Our results indicate that PKC activation increases the I(KATP) Induced by pinacidil or by MI. The latter effect requires concomitant adenosine receptor activation. The synergistic modulation of I(KATP) by PKC and adenosine provides an explicit basis for current paradigms of ischemic preconditioning.

Comparative effects of K+ channel modulating agents on contractions of rat intestinal smooth muscle

The effects of six K+ channel openers were investigated on contractions of the rat ileum longitudinal muscle-myenteric plexus preparation elicited by electrical field stimulation and by K+. Levcromakalim, pinacidil, RP 49356 (N-methyl-2-(3 pyridyl)-tetrahydrothiopyran-2-carbothioamide-1-oxide) and SDZ PCO 400 ((3S,4R)-3, 4-dihydro-3-hydroxy-2, 2-dimethyl-4-[(3-oxo-1-cyclopenten-1-yl)oxy]-2H-1-benzopyran-6-car bonitrile) completely abolished contractions elicited by electrical stimulation and caused complete relaxation of contractions elicited by K+ with comparable IC50 values. Minoxidil sulphate was much less potent and diazoxide was without effect in either protocol. The relaxant effects of these agents were antagonized by glibenclamide, tetraethylammonium and yohimbine in a manner which was not surmountable. The present study indicates that the relaxant effect of these compounds in intestinal smooth muscle is mediated through glibenclamide-sensitive ATP-dependent K+ channels. These compounds did not preferentially inhibit either direct smooth muscle- or nerve-mediated responses. The present data may point to differences in the channels or their regulatory sites, in intestinal, compared with vascular, smooth muscle.

Protective effects of the K+ ATP channel opener, aprikalim, against free radicals in isolated rabbit hearts

Aprikalim, a K+ ATP channel opener, is a potent vasodilator with demonstrated cardioprotective properties against ischemia/reperfusion injury. It is still unknown if K+ ATP channel openers exert their beneficial effects via interaction with oxygen-derived free radicals. Therefore, we investigated the cardioprotective effects of aprikalim against oxygen-derived free radicals. Isolated rabbit hearts were perfused at constant pressure (85 cm H2O) or constant flow (30-35 ml/min). Heart rate, left ventricular developed pressure (LVDP), and either coronary flow or coronary perfusion pressure (CPP) were monitored. Free radicals were produced by electrolysis of the perfusate (0.6 mA, direct current), and 10 microM aprikalim was infused before and after exposure to free radicals. In the constant perfusion pressure experiments, 10 min of exposure to free radicals resulted in a significant reduction of heart rate (137 to 129 beats/min), LVDP (112 to 91 mmHg) and coronary flow (37 to 29 ml/min); coronary flow was more markedly impaired than contractile function. Acetylcholine-induced coronary dilation was also significantly attenuated in the presence of free radicals. After 30 min of recovery, both coronary flow and LVDP were still significantly decreased while acetylcholine-induced coronary dilation had fully recuperated. Aprikalim completely abated the coronary and cardiac depressant actions of free radicals. Constant flow experiments indicated that exposure to free radicals increased CPP (+40%, p < 0.05), an effect totally suppressed by aprikalim. These results demonstrate that aprikalim reverses the cardiodepressant actions of free radicals. The cardioprotection it afforded involves both contractile function and the coronary vasculature. Acetylcholine-induced coronary dilation was blunted by free radicals, an indication of complex interactions at the coronary endothelial level.

The role of ATP-sensitive potassium channels in myocardial ischemia: pharmacology and implications for the future

Modulators of potassium channels are of great interest for their potential scientific as well as clinical value. These agents may be used for a variety of illnesses including asthma, hypertension, myocardial ischemia, and arrhythmias. The development of KATP openers and blockers has opened a large area of research, particularly on their potential role in the pathogenesis of myocardial ischemia. While much work has shown protective effects for KATP openers, it is unknown whether currently existing agents are optimal. It is also possible that KATP openers may be useful for other types of ischemia such as peripheral vascular disease and cerebral ischemia. It would be exciting to develop agents which not only would protect ischemic myocardium, but also reduce the severity of peripheral and cerebral ischemia. The convergence of the KATP opener studies and the preconditioning area of study was a classical intersection of two seemingly independent lines of research. This convergence has been largely responsible for the heightened interest in KATP. Our quest for knowledge on the role of KATP openers in myocardial ischemia and their potential utility has only just begun.

Cardiac ATP-sensitive K+ channels: regulation by intracellular nucleotides and K+ channel-opening drugs

ATP-sensitive K+ (KATP) channels are present at high density in membranes of cardiac cells where they regulate cardiac function during cellular metabolic impairment. KATP channels have been implicated in the shortening of the action potential duration and the cellular loss of K+ that occurs during metabolic inhibition. KATP channels have been associated with the cardioprotective mechanism of ischemia-related preconditioning. Intracellular ATP (ATPi) is the main regulator of KATP channels. ATPi has two functions: 1) to close the channel (ligand function) and 2) in the presence of Mg2+, to maintain the activity of KATP channels (presumably through an enzymatic reaction). KATP channel activity is modulated by intracellular nucleoside diphosphates that antagonize the ATPi-induced inhibition of channel opening or induce KATP channels to open. How nucleotides will affect KATP channels depends on the state of the channel. K+ channel-opening drugs are pharmacological agents that enhance KATP channel activity through different mechanisms and have great potential in the management of cardiovascular conditions. KATP channel activity is also modulated by neurohormones. Adenosine, through the activation of a GTP-binding protein, antagonizes the ATPi-induced channel closure. Understanding the molecular mechanisms that underlie KATP channel regulation should prove essential to further define the function of KATP channels and to elucidate the pharmacological regulation of this channel protein. Since the molecular structure of the KATP channel has now become available, it is anticipated that major progress in the KATP channel field will be achieved.

Time-dependent fading of the activation of KATP channels, induced by aprikalim and nucleotides, in excised membrane patches from cardiac myocytes

1. The effects of the potassium channel opener (KCO) aprikalim (RP 52891) on the nucleotide-induced modulation of ATP-sensitive K+ (KATP) channels in freshly dissociated ventricular myocytes of guinea-pig heart, were studied by use of the inside-out patch-clamp technique. The internal surface of the excised membrane patch was initially bathed with a standard solution (Mg(2+)-free with EDTA), then sequentially superfused with solutions containing nucleoside diphosphates (NDPs: 200 microM ADP and 50 microM GDP) and NDPs plus 1 mM MgCl2 (with EGTA; referred to as Mg-NDP solution). 2. The normalized concentration-response (channel closing) relationship to ATP was shifted to the right when the standard solution was replaced by the Mg-NDP solution. Hence, the internal concentration of ATP ([ATP]i) inhibiting the channel activity by half (Ki) increased from 56 microM to 180 microM, with an apparently constant slope factor (s = 2.37). NDPs in the absence of Mg2+ did not decrease the sensitivity of the channels to ATP. 3. In standard solution, aprikalim (100 microM) activated KATP channels in the presence of a maximally inhibitory [ATP]i (500 microM). This effect was strongly enhanced when aprikalim was applied to patches exposed to Mg-NDP solution, as demonstrated by the 9 fold increase in Ki for [ATP]i (from 180 microM to 1.5 mM and s = 2.37). 4. The ability of aprikalim to overcome the channel closing effects of ATP in Mg-NDP solution waned rapidly. Similarly, the NDP-induced activation of ATP-blocked channels was also time-dependent. Both activation processes disappeared before the channel run-down phenomenon appeared in ATP-free conditions. 5. In conclusion, aprikalim is much more potent in opening KATP channels in membrane patches bathed in Mg-NDP solution than in standard solution. However, under the former experimental conditions, the effect of aprikalim waned rapidly. It is proposed that the waning phenomenon results from changes in the intrinsic enzymatic activity of the KATP channel protein (possibly linked to the experimental conditions) which lead to the channel closure.

Functional evidence for a glibenclamide-sensitive K+ channel in rat ileal smooth muscle

The motor activity of gastrointestinal smooth muscle is closely related to the membrane potential. Controlling the membrane potential via modulation of K+ channels is essential for the action of neurotransmitters on smooth muscle. In the present study the effect of the K+ channel activator, lemakalim, on longitudinal smooth muscle of the rat ileum was investigated. Segments of rat ileum were stimulated by the muscarinic receptor agonist, carbachol (10(-6) M). Lemakalim (10(-10) to 3 x 10(-5) M) induced a dose-dependent inhibition of the carbachol-induced contraction. This inhibitory effect of lemakalim was not modified by neural blockade with tetrodotoxin (10(-6) M, n = 9). Glibenclamide (10(-7) to 10(-5) M), a specific blocker of ATP-dependent K+ channels antagonized dose dependently the relaxant effect of lemakalim (IC50: 3.4 x 10(-6) M, n = 11, P < 0.001). In contrast, apamin (10(-7) M, n = 9, n.s.) and charybdotoxin (10(-7) M, n = 9, n.s.), specific blockers of Ca2+-dependent K+ channels and the non-specific K+ channel blocker, tetraethylammonium (10(-4) to 10(-1) M), had no influence on the inhibitory effect of lemakalim. Contractions induced by the Ca2+ channel activator, Bay-K-8644, were completely inhibited by lemakalim (10(-5) M, n = 12). This inhibitory effect was also selectively antagonized by glibenclamide (10(-5) M). Potential non-adrenergic non-cholinergic (NANC) inhibitory mediators like ATP, nitric oxide (NO) or neurotensin showed no sensitivity to glibenclamide. These functional data indicate that the relaxant effect of lemakalim is due to a specific activation of glibenclamide-sensitive K+ channels, which in turn can modulate the activity of dihydropyridine-sensitive (voltage-dependent) Ca2+ channels. A physiological or pathophysiological role of the glibenclamide-sensitive K+ channels in intestinal smooth muscle is discussed; however, they seem not to be involved in the effect of the NANC inhibitory mediators tested.

Glibenclamide enhances but pinacidil reduces attenuation in sympathetic responsiveness after acute coronary artery occlusion

To investigate the role of ATP-sensitive K+ channels in modulating the efferent autonomic response following acute myocardial ischemia/infarction, we examined the effects of a blocker (glibenclamide) and an opener (pinacidil) of ATP-sensitive K+ channels on the time course and extent of the attenuation in efferent cardiac sympathetic responsiveness in anesthetized dogs. We measured the effective refractory periods (ERPs) at nonischemic sites basal and apical to the area of myocardial ischemia/infarction in the baseline state and during bilateral stimulation of the ansae subclaviae before and after each drug administration and 5, 30, 60, 120, and 180 minutes after latex injection of a diagonal branch of the left anterior descending coronary artery. Animals received either vehicle (n = 12), glibenclamide (0.3 mg.kg-1, n = 10), pinacidil (0.15 mg.kg-1 + 0.2 mg.kg-1 infusion, n = 10), or a combination of these two drugs (n = 9) intravenously. In another group of dogs receiving just pinacidil (n = 10), an intra-aortic balloon was inflated distal to the renal arteries to prevent pinacidil-induced hypotension. Another group of dogs received either high-dose glibenclamide (0.3 mg.kg-1 + 0.15 mg.kg-1, n = 4), low-dose glibenclamide (0.06 mg.kg-1, n = 4), medium-dose pinacidil (0.03 mg.kg-1 + 0.04 mg.kg-1 infusion, n = 4), or low-dose pinacidil (0.0075 mg.kg-1 + 0.01 mg.kg-1 infusion, n = 4). In all dogs, basal sites exhibited no attenuation of sympathetically induced shortening of the ERP throughout the period of acute myocardial ischemia/infarction. Cumulative attenuation in sympathetic responsiveness (shortening of ERP < or = 2 milliseconds induced by bilateral stimulation of the ansae subclaviae) at nonischemic test sites apical to the area of ischemia/infarction during a 3-hour period was greater in the glibenclamide group (26 of 44 sites, P = .008) and less in the pinacidil (2 of 44 sites, P = .002) and pinacidil-balloon (1 of 48 sites, P < .001) groups compared with the vehicle group (14 of 46 sites). Glibenclamide abolished the protective effect of pinacidil so that 10 of 45 sites had < 2-millisecond shortening during a 3-hour period in the glibenclamide + pinacidil group (P = .018 versus pinacidil group, P = .286 versus vehicle group). Such effects of glibenclamide and pinacidil on sympathetic attenuation were dose dependent. Maintaining the blood glucose level during glibenclamide administration did not affect the sympathetic attenuation after acute coronary artery occlusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of minoxidil sulfate and pinacidil on single potassium channel current in cultured human outer root sheath cells and dermal papilla cells

Minoxidil sulfate and pinacidil are K channel openers and are considered to promote hair growth. However, there have been no studies on the single channel current of isolated cells from hair follicles. Therefore, we characterized the single K channel current of outer root sheath cells and dermal papilla cells and the effect of K channel openers on K currents by patch clamp. We also carried out 86Rb efflux studies to observe macroscopic K channel currents. In physiological saline, these two cells showed two types of K channels, large and small conductance Ca(2+)-activated K channels, both intact cell-attached and excised inside-out patches. In symmetrical 150 mM K solution, unitary conductances were 246 and 70 pS, respectively. Intracellular ATP (up to 5 mM) or glibenclamide (20 nM), a specific ATP-sensitive K channel blocker, did not block these channels. Minoxidil sulfate (5 micrograms/ml) or pinacidil (10 microM) did not open these two types of K channels or increase 86Rb efflux. These results suggest that minoxidil sulfate or pinacidil did not activate K channel current in hair follicles, and that the drug effect on hair growth might be mediated by other mechanisms such as increased blood flow.

The effect of glibenclamide on frog skeletal muscle: evidence for K+ATP channel activation during fatigue

1. The purpose of this study was to determine whether ATP-sensitive K+ (K+ATP) channels are activated and contribute to the decrease in force during fatigue development in the sartorius muscle of the frog, Rana pipiens. Tetanic force (elicited by field stimulation), action potential and membrane conductance (using conventional microelectrodes), were measured in the presence and absence of glibenclamide, a K+ATP channel antagonist. Experiments were performed in bicarbonate-buffered solutions at pH 7.2. 2. In unfatigued muscle 100 mumol l-1 glibenclamide had no effect on the resting potential, the overshoot, the half-depolarization time or the maximum rate of depolarization of action potentials, while the mean half-repolarization time increased by 19 +/- 4% (+/- S.E.M.) and the maximum rate of repolarization decreased by 17 +/- 5%. 3. Fatigue was elicited using 100 ms tetanic contractions every 1 s for 3 min. In the absence of glibenclamide the mean half-repolarization time increased from 0.57 +/- 0.05 to 0.89 +/- 0.05 ms during fatigue. The mean half-repolarization times after fatigue, when muscle fibres were exposed to 100 mumol l-1 glibenclamide either 60 min prior to fatigue or 60 s before the end of fatigue, were 1.16 +/- 0.08 and 1.17 +/- 0.07 ms respectively. Application of 100 mumol l-1 glibenclamide after 5 min of recovery did not increase the half-repolarization time, but decreased the rate of recovery compared to control values. 4. In unfatigued muscles, 100 mumol l-1 glibenclamide did not affect the tetanic contraction. In the absence of glibenclamide, the mean tetanic force after fatigue was 11.0 +/- 0.9% of prefatigue values. Application of 100 mumol l-1 glibenclamide 60 min before fatigue increased the rate of fatigue development as the mean tetanic force was 4.8 +/- 0.8% after 3 min of stimulation. The addition of 100 mumol l-1 glibenclamide 60 s before the end of fatigue had no effect on tetanic force during this time compared to control. 5. In the absence of glibenclamide, muscles recovered 90.1 +/- 1.6% of their tetanic force after 100 min. Addition of 100 mumol l-1 glibenclamide 60 min prior to fatigue significantly reduced the capacity of muscles to recover their tetanic force: after 100 min of recovery tetanic force was only 47.3 +/- 9.4% of the pre-fatigue value.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of K+ channel openers on the vascular actions of human gamma globulin

The aim of this study was to determine if the stimulatory action of human gamma globulin on the spontaneous activity of the rat mesenteric portal vein is due to decreased K+ conductance. Glibenclamide potentiated the action of human gamma-globulin on the portal vein by 45% and on its own had a concentration- and time-dependent biphasic (increase followed by a decrease) effect on the spontaneous activity of the portal vein. Diazoxide and pinacidil both inhibited the action of human gamma-globulin on the rat mesenteric portal vein. Levcromakalim (BRL 38227) potentiated the stimulatory action of human gamma-globulin on the integrated force of the spontaneous contractions of the rat mesenteric portal vein by 40% and 49% at concentrations of 0.5 and 5 microM, respectively. These studies suggest that human gamma-globulin can act by directly modulating a K+ channel.

Activating effect of the flavonoid phloretin on Ca(2+)-activated K+ channels in myelinated nerve fibers of Xenopus laevis [corrected]

The effects of the flavonoid phloretin on K+ channels in amphibian myelinated nerve were studied by patch clamping. The open probability of Ca(2+)-activated K+ channels was greatly increased by external phloretin (10-200 microM) due to a shift of the membrane potential for half-maximal activation, E50, of -63.9 mV (80 microM phloretin). Open times were prolonged and closed times shortened. Channel activation by phloretin developed slowly (tau on = 33.4 s) and its washout was even slower (tau off,1 = 4.7 and tau off,2 = 183.2 s). In contrast, submillimolar phloretin blocked two delayed rectifier K+ channels (I and F) whereas the gating of the ATP-sensitive and the flickering K+ channel were unaffected. Phloretin may directly affect the voltage sensor of K+ channels.

Tolbutamide, but not glyburide, affects the excitability and contractility of unfatigued frog sartorius muscle

The goal of this study was to characterize the effects of tolbutamide and glyburide, two known KATP channel blockers, on intact, unfatigued sartorius muscle fibres of the frog, Rana pipiens. Tetanic contractions were elicited by field stimulation with 200 ms long train of pulses (0.5 ms, 6 V, 140 Hz). Resting and action potentials were measured using conventional microelectrodes. At pHo 7.2 (extracellular pH), the tetanic force was unaffected by 0.5 mM and 1.0 mM tolbutamide, but at 2.0 mM it decreased by 15.5 +/- 1.0%. The effect of tolbutamide on the tetanic force was significantly greater at pHo 6.4: all three tolbutamide concentrations caused a significant decrease in tetanic force, being 62.3 +/- 9.4% at 2 mM. In the presence of tolbutamide a large number of fibres became unexcitable at pHo 6.4, but not at pHo 7.2. Glyburide at 10 microM, on the other hand, caused a 5-7% decrease in tetanic force at both pHo 6.4 and 7.2, but no further decreases in tetanic force were observed when the glyburide concentration was increased up to 100 microM. Unlike tolbutamide, glyburide did not affect the excitability of muscle fibres, but significantly prolonged the repolarization phase of action potentials, especially at pHo 6.4. We suggest that several of the tolbutamide effects reported in this study cannot be accounted for by a direct effect on KATP channels, and that the large decrease in membrane excitability and muscle contractility in the presence of tolbutamide must seriously be taken into consideration when this channel blocker is used to study the physiological role of KATP channels in intact muscle fibres.

Regulation of arterial tone by calcium-dependent K+ channels and ATP-sensitive K+ channels

Resistance arteries depolarize and constrict to elevations in intravascular pressure. However, many of the molecular aspects of this phenomenon are not known. We present evidence that large conductance calcium-dependent potassium (KCa) channels, which are activated by intracellular calcium and membrane depolarization, play a fundamental role in regulating the degree of intravascular pressure-induced, myogenic tone. We found that blockers of KCa channels, charybdotoxin (CTX, < 100 nM) and TEA+ (< 0.5 mM), further depolarized pressurized arteries by as much as 12 mV and decreased diameter by up to 40%. CTX blocked KCa channels in outside-out patches from arterial smooth muscles with half-block constant of 10 nM and external TEA+ caused a flickery block, with a half-block constant of 200 microM. We propose that KCa channels serve as a negative feedback pathway to limit the degree of membrane depolarization and hence vasoconstriction to pressure. In contrast, CTX and TEA+ (< 1 mM) were without effect on membrane hyperpolarization and dilation to a wide variety of synthetic (cromakalim, pinacidil, diazoxide, minoxidil sulfate) and endogenous agents [calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide, an endothelial-derived hyperpolarizing factor]. Glibenclamide and low concentrations of external barium that inhibit ATP-sensitive potassium (KATP) channels, however, blocked the hyperpolarizations and dilations to these substances. We have identified KATP channels as well as high-affinity glibenclamide binding sites in arterial smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation and reactivation of the ATP-sensitive K+ channel of the heart can be modified by drugs

Activation and reactivation of the ATP-sensitive K+ channel (IK.ATP) were studied with the patch-clamp technique in guinea-pig ventricular myocytes. The K+ channel openers, nicorandil and pinacidil, activated IK.ATP in an internal ATP-dependent manner. Both drugs increased the open probability of IK.ATP without changing the channel conductance. They prolonged lifetimes of bursts and shortened interburst intervals without influencing the fast gating within bursts. These effects were the opposite of those of internal ATP. However, the interaction between ATP and either nicorandil or pinacidil appeared not to be simple competition. We found that three carbonyl compounds--3,4-dihydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, and 2,4-dihydroxyacetophenone--could activate IK.ATP through an intracellular mechanism that was dependent upon the presence of ADP and Mg2+. It has been suggested that these three carbonyl compounds bind covalently to proteins to form a Schiff base, which may be responsible for their effects upon IK.ATP. Internal application of the proteolytic enzyme trypsin prevented both the spontaneous and Ca(2+)-induced rundown of the KK.ATP channel. Tryptic digestion did not change either the channel's sensitivity to inhibition by ATP nor the fast gating kinetics of IK.ATP. Internal application of an exopeptidase, carboxypeptidase A, but not leu-aminopeptidase, prevented the spontaneous and Ca(2+)-induced rundown of the IK/ATP channel, effects similar to those of trypsin treatment. These results suggest that the target site of trypsin digestion may be located on the carboxy (C)-terminal of the channel proteins or associated regulatory units.

K channel openers activate different K channels in vascular smooth muscle cells

The properties of K channels activated by K channel openers (nicorandil, cromakalim, pinacidil, etc.) were investigated using conventional microelectrode and patch-clamp methods. In single smooth muscle cells of the rat and rabbit portal veins, K channel openers produced an outward current sensitive to glibenclamide, 4-AP, and TEA (1 mM), but insensitive to apamin and charybdotoxin. Glibenclamide-sensitive K channels in both tissues had a small unitary conductance (10 pS and 15 pS) and were inhibited by intracellular ATP. The activity of the 15 pS channel in the rabbit portal vein was not changed by an increase in the intracellular free Ca concentration, but the activity of the 10 pS channel in the rat portal vein was markedly modified by Ca concentration. These results coincided with previous observations using a conventional microelectrode and whole-cell voltage-clamp experiments. In the inside-out membrane patch, the 10 pS channel in the rat portal vein was activated by the application of K channel openers, while the 15 pS channel in the rabbit portal vein was rapidly inactivated, even in the presence of K channel openers. GDP, but neither GTP gamma S nor GDP beta S, reopened the 15 pS channel in the presence of K channel openers. These results suggested that the 15 pS channel had two channel states, that is, both operative and inoperative states, while the 10 pS channel did not have an inoperative state. The K channel openers open the ATP-sensitive K channel only at the operative state.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP-sensitive K+ channels in cardiac ischemia: an endogenous mechanism for protection of the heart

The Role of ATP-sensitive K+ channels (KATP) in action potential shortening and protection of myocardium in ischemia were explored using isolated ventricular myocytes and arterially perfused right ventricular walls of guinea pigs. Conditions "simulating" some aspects of ischemia--(10.8 mM K+o, 6.9 pHo, 20 mM lactate, no glucose; 10 mM 2-deoxy-D-glucose; and either 1 mM cyanide or no O2 (bubbled with 95/5% N2/CO2)--caused a decline in action potential duration (APD) and the elaboration of time- and voltage-independent, steady-state outward conductance due to KATP, which could be inhibited with glibenclamide (50 microM) in myocytes studied via the perforated patch (nystatin) whole-cell technique. Right ventricular walls subjected to no-flow ischemia +/- glibenclamide (10 microM) to block, or +/- pinacidil (1 and 10 microM) to activate, KATP, respectively, exhibited varied ischemic injury. Glibenclamide caused a greater fall in resting membrane potential, inhibited the decline in APD, caused an early rise in resting tension, and inhibited recovery of contractile function upon reflow. Pinacidil caused a greater decline in APD, inhibited changes in resting tension, and improved recovery during reperfusion. These results indicate that KATP contributes to action potential shortening in isolated myocytes in simulated ischemia and intact myocardium in no-flow ischemia. Activation of this membrane current may be an important adaptive mechanism for protecting the myocardium when blood flow to the tissue is compromised.

Pharmacological properties of ATP-sensitive K+ channels in mammalian skeletal muscle cells

The patch-clamp technique (single-channel recordings) was used to study the effects of glibenclamide and some channel openers on the KATP channel in mouse skeletal muscle. In outside/out membrane patches, glibenclamide reversibly inhibited KATP channel activity in a dose-dependent manner with an apparent Ki of 190 nM. In inside/out membrane patches, RP 61419 increased KATP channel activity both in the absence and in the presence of internal ATP while other K+ channel openers such as nicorandil and cromakalim required the presence of internal ATP to evoke channel activation. The half-maximal activity effect for cromakalim, with 0.5 mM ATP at the cytoplasmic face, was observed at about 220 microM. Pinacidil was unable to activate the KATP channel in the absence of internal ATP and could even reduce channel opening in situations where activity was high in the control. In the presence of internal Mg2+, activation by pinacidil occurred when ATP or low and weakly activating concentrations of ADP were present at the cytoplasmic side. Pinacidil activation could also be observed in the presence of ATP or ADP when Mg2+ was absent from the internal solution. The mechanism of action of pinacidil is discussed in terms of interactions between the different nucleotide regulatory sites and the K+ channel opener binding site of the KATP channel. Half-maximum activation of the KATP channel in the presence of 0.5 mM ATP at the cytoplasmic face was observed at 125 microM pinacidil.