Essential role of nucleotide diphosphates in nicorandil-mediated activation of cardiac ATP-sensitive K+ channel. A comparison with pinacidil and lemakalim

The new antihypertensive drug iptakalim activates ATP-sensitive potassium channels in the endothelium of resistance blood vessels

AIM

To investigate the mechanisms underlying the activation of ATP-sensitive potassium channels (K(ATP)) by iptakalim in cultured rat mesenteric microvascular endothelial cells (MVECs).

METHODS

Whole-cell KATP currents were recorded in MVECs using automated patch clamp devices. Nucleotides (ATP, ADP and UDP) were added to the internal perfusion system, whereas other drugs were added to the cell suspension on NPC-1 borosilicate glass chips.

RESULTS

Application of iptakalim (10 and 100 μmol/L) significantly increased the whole-cell K(ATP) currents, which were prevented by the specific K(ATP) blocker glibenclamide (1.0 μmol/L). The opening of K(ATP) channels by iptakalim depended upon the intracellular concentrations of ATP or NDPs: iptakalim activated K(ATP) channels when the intracellular ATP or NDPs were at 100 or 1000 μmol/L, and was ineffective when the non-hydrolysable ATP analogue ATPγS (1000 μmol/L) was infused into the cells. In contrast, the K(ATP) opener pinacidil activated K(ATP) channels when the intracellular concentrations of ATP or NDPs ranged from 10 to 5000 μmol/L, and even ATPγS (1000 μmol/L) was infused into the cells.

CONCLUSION

Iptakalim activates K(ATP) channels in the endothelial cells of resistance blood vessels with a low metabolic status, and this activation is dependent on both ATP hydrolysis and ATP ligands.

Effects of ketamine on nicorandil induced ATP-sensitive potassium channel activity in cell line derived from rat aortic smooth muscle

PURPOSE

Nicorandil opens adenosine triphosphate-sensitive potassium (K(ATP)) channels in the cardiovascular system and is being increasingly used for the treatment of angina pectoris. In the present study, we tested whether intravenous anesthetic agent ketamine affected nicorandil-induced native vascular K(ATP) channel activation.

METHODS

We used excised inside-out patch clamp configurations to investigate the direct effects of ketamine racemate and S-(+)-ketamine on the activities of K(ATP) channels in cultured rat aortic smooth muscle cells. Furthermore, we also investigated whether intracellular MgADP could modulate ketamine inhibition.

RESULTS

Nicorandil significantly activated K(ATP) channel activity, whereas this channel activity was completely blocked by glibenclamide, a specific K(ATP) channel blocker. Ketamine racemate inhibited the nicorandil induced K(ATP) channel activity (IC(50)=34±1 µM, n=14), but S-(+)-ketamine was less potent than ketamine racemate in blocking nicorandil induced K(ATP) channel activities (IC(50)=226±7 µM, n=10). Application of MgADP to the intracellular side of the channel was able to decrease the inhibitory potency of ketamine racemate on nicorandil induced K(ATP) channel activities.

CONCLUSIONS

Our results indicate that ketamine inhibits nicorandil induced K(ATP) channel activities in a dose dependent and stereoselective manner. Furthermore, increase of intracellular MgADP attenuates the inhibitory potency of ketamine racemate. J. Med. Invest. 57: 237-244, August, 2010.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Effects of pinacidil on reentrant arrhythmias generated during acute regional ischemia: a simulation study

Many experimental studies have pointed out the controversy involving the arrhythmogenic effects of potassium channel openers (KCOs) in ischemia. KCOs activate the ATP-sensitive potassium current [IK(ATP)], resulting in action potential duration (APD) shortening, especially under pathological conditions such as ischemia. Acute myocardial ischemia leads to electrophysiological inhomogeneities in APD, conduction velocity, and refractoriness, which provide the substrate for reentry initiation and maintenance and may lead to malignant arrhythmias. The aim of this work is to analyze the effect of the KCO pinacidil on vulnerability to reentry during acute regional ischemia using computer simulations. We use a two-dimensional virtual heart tissue with implementation of acute regional ischemia conditions. Membrane kinetics are represented by a modified version of Luo-Rudy (phase II) action potential model that incorporates the effect of pinacidil on IK(ATP). The vulnerable window (VW) for reentry is quantified for different doses of pinacidil. Our results show that for doses below 3 micromol/l the VW widens with increasing pinacidil concentration, whereas for higher doses of pinacidil the VW decreases, becoming zero for concentrations above 10 micromol/l. The ionic mechanisms involved in this behavior are explored. This study demonstrates that the effect of pinacidil on arrhythmogenesis is strongly dose-dependent, and that high doses of pinacidil exert a strong antiarrhythmic effect.

The nucleotide-binding domains of sulfonylurea receptor 2A and 2B play different functional roles in nicorandil-induced activation of ATP-sensitive K+ channels

Nicorandil activates ATP-sensitive K(+) channels composed of Kir6.2 and either sulfonylurea receptor (SUR) 2A or 2B. Although SUR2A and SUR2B differ only in their C-terminal 42 amino acids (C42) and possess identical drug receptors and nucleotide-binding domains (NBDs), nicorandil more potently activates SUR2B/Kir6.2 than SUR2A/Kir6.2 channels. Here, we analyzed the roles of NBDs in these channels' response to nicorandil with the inside-out configuration of the patch-clamp method. Binding and hydrolysis of nucleotides by NBDs were impaired by mutations in the Walker A motif of NBD1 (K708A) and NBD2 (K1349A) and in the Walker B motif of NBD2 (D1470N). Experiments were done with internal ATP (1 mM). In SUR2A/Kir6.2 channels, the K708A mutation abolished, and the K1349A but not D1470N mutation reduced the sensitivity to nicorandil. ADP (100 microM) significantly increased the wild-type channels' sensitivity to nicorandil, which was abolished by the K1349A or D1470N mutation. Thus, the SUR2A/Kir6.2 channels' response to nicorandil critically depends on ATP-NBD1 interaction and is facilitated by interactions of ATP or ADP with NBD2. In SUR2B/Kir6.2 channels, either the K708A or K1349A mutation partially suppressed the response to nicorandil, and double mutations abolished it. The D1470N mutation also significantly impaired the response. ADP did not sensitize the channels. Thus, NBD2 hydrolyzes ATP, and NBD1 and NBD2 equally contribute to the response by interacting with ATP and ADP, accounting for the higher nicorandil sensitivity of SUR2B/Kir6.2 than SUR2A/Kir6.2 channels in the presence of ATP alone. Thus, C42 modulates the interaction of both NBDs with intracellular nucleotides.

Cellular remodeling in heart failure disrupts K(ATP) channel-dependent stress tolerance

ATP-sensitive potassium (K(ATP)) channels are required for maintenance of homeostasis during the metabolically demanding adaptive response to stress. However, in disease, the effect of cellular remodeling on K(ATP) channel behavior and associated tolerance to metabolic insult is unknown. Here, transgenic expression of tumor necrosis factor alpha induced heart failure with typical cardiac structural and energetic alterations. In this paradigm of disease remodeling, K(ATP) channels responded aberrantly to metabolic signals despite intact intrinsic channel properties, implicating defects proximal to the channel. Indeed, cardiomyocytes from failing hearts exhibited mitochondrial and creatine kinase deficits, and thus a reduced potential for metabolic signal generation and transmission. Consequently, K(ATP) channels failed to properly translate cellular distress under metabolic challenge into a protective membrane response. Failing hearts were excessively vulnerable to metabolic insult, demonstrating cardiomyocyte calcium loading and myofibrillar contraction banding, with tolerance improved by K(ATP) channel openers. Thus, disease-induced K(ATP) channel metabolic dysregulation is a contributor to the pathobiology of heart failure, illustrating a mechanism for acquired channelopathy.

Effects of acidosis and NO on nicorandil-activated K(ATP) channels in guinea-pig ventricular myocytes

1. Nicorandil is a hybrid compound of K(+) channel opener and nitrate. We investigated a possible interaction of acidosis and nitric oxide (NO)-donors on the nicorandil-activated ATP-sensitive K(+) channel (K(ATP)) in guinea-pig ventricular myocytes using the patch-clamp technique. 2. In whole-cell recordings, external application of 300 microM nicorandil activated K(ATP) in the presence of 2 mM intracellular ATP concentration ([ATP](i)) at external pH (pH(o)) 7. 4, but the activated current was decreased by reducing pH(o) to 6.5 - 6.0. 3. Single-channel recordings of inside-out patches revealed decreased open-state probability (P(o)) of K(ATP) activated by nicorandil with reducing internal pH (pH(i)) from 7.2 to 6.0, whilst the channel activity increased at low pH(i) in the absence of nicorandil. 4. Application of NO donors, 1 mM-sodium nitroprusside (SNP) or -NOR-3 to the membrane cytoplasmic side at pH(i) 7.2 increased the channel activity but decreased it at pH(i) 6.5 - 6.0. Neither removal of the drugs nor application of NO-scavengers reversed depression of channel activity induced by NO-donors. 5. We conclude that an increase in pH(o) and pH(i) depresses rather than stimulates the nicorandil-activated K(ATP). Since NO-donors at low pH(i) exhibited a similar trend, involvement of H(+) and NO interaction can be considered as a mechanism of decreased K(ATP) activated by nicorandil.

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.

Anti-ulcer activity of SKP-450, a novel potassium channel activator, in rats

The anti-ulcer effects of SKP-450, a new potassium channel activator, were evaluated on basal and histamine-induced gastric acid secretion, and against experimentally-induced ulcers such as ethanol-induced and NaOH-induced gastric ulcers. In the pylorus-ligated rat, SKP-450 (0.1-0.5 mg kg(-1)) significantly decreased volume and concentration of gastric juice, and total acid output (ED(50): 0.12 mg kg(-1)). SKP-450 (0.3-3.0 mg kg(-1)) also inhibited histamine-induced gastric acid secretion, maximal effects being achieved at 1.0 mg kg(-1)(37.9% inhibition). In the 95% ethanol-treated rats, SKP-450 significantly reduced the mucosal lesions (46.9 and 31.4% inhibition at 0.1 and 0.2 mg kg(-1), respectively). A significant reduction in the ulcer index by SKP-450 was also observed in 0.3 n NaOH-treated rats (31.5 and 64.3% inhibition at 0.5 and 1.0 mg kg(-1), respectively). The effects of SKP-450 on histamine-induced acid secretion and on NaOH-induced ulcers were inhibited by glibenclamide (20 mg kg(-1), i.v.), a selective blocker of ATP-sensitive potassium channel. These results indicate that SKP-450 possesses anti-ulcer effects and its effects may be mediated by activation of ATP-sensitive potassium channels.

Comparison of the vascular relaxant effects of ATP-dependent K+ channel openers on aorta and pulmonary artery isolated from spontaneously hypertensive and Wistar-Kyoto rats

The vasorelaxant actions of adenosine 5'-triphosphate (ATP)-dependent K+ channel openers and sodium nitroprusside in isolated thoracic aorta and pulmonary artery of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats (14-18 weeks old) were investigated. Cumulative addition of sodium nitroprusside and different ATP-dependent K+ channel openers (pinacidil, cromakalim, nicorandil, 2-(2"(1",3"-dioxolone)-2-methyl-4-(2'-oxo-1'-pyrrolidinyl)-6-nitro -2H-1-benzopyren (KR-30450) and aprikalim) to these preparations caused a concentration-dependent relaxation of noradrenaline-pre-contracted aorta and pulmonary artery from both strains. The relative order of relaxation potency, estimated by comparing the IC50, was sodium nitroprusside > KR-30450 > aprikalim > or = cromakalim > pinacidil > nicorandil in pulmonary artery and aorta from both strains. At high concentrations (> or =1 microM), cromakalim, aprikalim and KR-30450 produced a greater percentage relaxation in SHR aorta than in WKY aorta. However, there was no apparent difference between SHR and WKY in the relaxation response to all drugs tested on the pulmonary artery. The effects of cromakalim, aprikalim, pinacidil and KR-30450 observed in aorta and pulmonary artery were significantly attenuated by 3 microM glibenclamide. 6-Anilino-5,8-quinolinequinone (LY 83583, 1 microM), a soluble guanylate cyclase inhibitor, abolished the vasorelaxant effects of nicorandil and sodium nitroprusside. In conclusion, sodium nitroprusside and ATP-dependent K+ channel openers cause relaxation of noradrenaline-pre-contracted aorta and pulmonary artery from both strains. However, all the drugs tested failed to cause selective relaxation of the pulmonary artery relative to the thoracic aorta.

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.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Hemodynamic profile of SKP-450, a new potassium-channel activator

Hemodynamic profiles of SKP-450, a newly synthesized potassium-channel activator, were evaluated in conscious hypertensive rats of several types, and in anesthetized and conscious beagle dogs. In freely moving conscious rats, orally administered SKP-450 (0.03-0.3 mg/kg) dose-dependently decreased arterial pressure in spontaneously hypertensive rats (SHRs), renally hypertensive rats (RHRs), DOCA/salt-induced hypertensive rats (DHRs), and normotensive rats (NRs) with a greater potency than lemakalim except in DHRs (ED20 values: SKP-450, 0.021, 0.013, 0.024, and 0.034 mg/kg; lemakalim, 0.107, 0.018, 0.016, and 0.063 mg/kg, respectively). The blood pressure-reducing effects of SKP-450 reached their maximum within 30 min and lasted for approximately 4 h in all rats, and >6 h, particularly, in SHRs. In NRs, pretreatment with glibenclamide (20 mg/kg, i.v.) antagonized the hypotensive effect of SKP-450, whereas propranolol (2 mg/kg, i.v.) antagonized the tachycardiac response of SKP-450 (0.03 mg/kg, i.v.) without affecting its hypotensive response in NRs. In anesthetized beagle dogs, intraduodenally administered SKP-450 (0.003-0.03 mg/kg) dose-relatedly decreased arterial pressure (ED20 value, 0.007 mg/kg) for > or =3 h with its peak effects reached within 15 min and without significant changes in heart rate (HR). Antihypertensive effects of SKP-450 were accompanied by concurrent reduction in total peripheral resistance and dose-dependent increase in cardiac output. Indirect measures of myocardial oxygen demand such as rate-pressure product, tension-time index, and systolic time interval were dose-dependently decreased by SKP-450 without significant change in left ventricular dP/dt(max). SKP-450 significantly increased coronary blood flow and decreased coronary vascular resistance dose-dependently with a rapid onset of action and long duration of >4 h (maximal changes, 276 and 83.7% at 0.03 mg/kg, respectively). In conscious dogs, orally administered SKP-450 (0.03-0.3 mg/kg) produced a dose-related decrease in arterial pressure for > or =3 h, with its peak effects reached within 20 min (ED20 value, 0.030 mg/kg) accompanied by tachycardia. These results suggest that SKP-450 is a potent, orally active peripheral vasodilator activating ATP-sensitive potassium channels.

Cardioprotective effects of KR-30450, a novel K+(ATP) opener, and its major metabolite KR-30818 on isolated rat hearts

The cardiac effects of KR-30450 ((-)-(2R)-2-([1,3]-dioxolan-2-yl)-2-methyl-4-(2-oxopyrrolidin++ +-1-yl)-6-nitro-2H-1-benzopyran), a newly synthesized potassium channel activator, and its major metabolite KR-30818 ((-)-(2R)-2-hydroxymethyl-2-methyl-4-(2-oxopyrrolidin-1-yl)-6-nitr o-2H-1-benzopyran) were compared with those of lemakalim, a prototype of this class, in isolated globally ischemic rat hearts. KR-30450 and KR-30818 significantly improved reperfusion cardiac function (LVDP, left ventricular developed pressure; double product, LVDP x heart rate/1000), their potency being 5.2-fold and 0.7-fold greater than lemakalim (ED50 for recovering predrug double product: 0.10, 0.80 and 0.54 microM, respectively). KR-30450 and KR-30818 significantly attenuated reperfusion contracture and lactate dehydrogenase release with potency greater than and equal to lemakalim, respectively. They significantly increased time to contracture (TTC) during ischemia in a dose-dependent manner with a greater potency than lemakalim (EC25 for increasing TTC: 1.2, 2.1 and 3.2 microM, respectively). The protective effects of three compounds on the measured parameters were reversed by glyburide, a selective K+(ATP) blocker. In non-ischemic hearts, KR-30450 and lemakalim exerted weak negative inotropism at high concentrations and KR-30818 had no effects, whereas the three compounds significantly increased coronary flow at doses studied. Glyburide completely reversed preischemic cardiodepressant effects of these compounds but not their effects on coronary flow. In conclusion, KR-30450, a recently developed K+(ATP) opener, exerted more potent cardioprotective effects than lemakalim, and its major metabolite KR-30818 may play a significant role in its action in vivo.

Comparison of coronary flow reserve between focal and diffuse vasoconstriction induced by ergonovine in patients with vasospastic angina

Decreased coronary flow reserve has been reported in patients with ergonovine-induced coronary vasoconstriction by the thermodilution method. To assess the difference of coronary flow reserve between patients with focal and diffuse vasospasm, after the vasospasm is discontinued by injection 3 mg of isosorbide dinitrate, phasic flow velocities of the diseased coronary artery were recorded at rest and during hyperemia (140 microg/kg/min of adenosine infusion intravenously) using a 0.014-inch, 15-MHz Doppler guidewire in 26 patients with ergonovine-induced coronary vasospasm (0.2-mg ergonovine injection intravenously), including 12 patients with focal (>90% stenosis), 14 patients with diffuse vasospasm (>50%), and 11 controls with normal coronary arteries without vasospasm. Although time-averaged peak velocity in cases with diffuse and focal vasospasm was not significantly different compared with that in controls at baseline (22 +/- 7, 18 +/- 5 vs 20 +/- 7 cm/s, respectively, NS), it was significantly lower in patients with diffuse vasospasm than in cases with focal vasospasm and in controls during hyperemia (43 +/- 13 vs 64 +/- 18, 61 +/- 19 cm/s, respectively, p <0.01). As a result, coronary flow reserve obtained from the ratio of hyperemic/baseline time-averaged peak velocity was significantly lower in patients with diffuse vasospasm than that in controls (1.9 +/- 0.4 vs 3.1 +/- 0.4, p <0.01), although it was not significantly different between the subjects with focal vasospasm and controls (3.5 +/- 0.7 vs 3.1 +/- 0.4, NS). Thus, coronary flow reserve is maintained normally in patients with focal vasospasm and limited in those with diffuse vasospasm. Microvascular impairment could exist further in cases with diffuse vasospasm, although similar endothelial dysfunction of the epicardial coronary artery is observed in focal and diffuse vasospasm.

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.

Potassium channel openers prevent potassium-induced calcium loading of cardiac cells: possible implications in cardioplegia

Hyperkalemic solutions that are used as cardioplegic agents, while effective in inducing electromechanical arrest, are only partially cardioprotective, and ventricular dysfunction has been observed. The underlying pathophysiology of cardioplegia-associated ventricular dysfunction is complex and not fully understood, but it could be related, in part, to intracellular Ca2+ loading induced by high K+ concentrations present in cardioplegic solutions. Yet no effective cytoprotective means against possible intracellular Ca2+ loading, under these conditions, has been described. Recently, potassium channel openers, which open adenosine triphosphate-sensitive K+ channels, have been reported to possess cardioprotective properties under global ischemic conditions. However, it is not known whether these novel agents could prevent intracellular Ca2+ loading that could occur during cardioplegia. Intracellular Ca2+ was monitored in ventricular myocytes, loaded with the Ca(2+)-sensitive fluorescent probe Fluo-3AM, using epifluorescent digital imaging and laser confocal microscopy. Exposure of a myocyte to a 16 mmol/L concentration of K+, a concentration of K+ commonly used in cardioplegic solutions, induced a nonhomogeneous increase in intracellular Ca2+. Potassium channel opening drugs, such as aprikalim or nicorandil, effectively prevented these solutions from increasing intracellular Ca2+. The preventive effect of potassium channel opening drugs was antagonized by glyburide, a selective blocker of adenosine triphosphate-sensitive K+ channels. This study demonstrates, at the single cardiac cell level, that solutions containing a 16 mmol/L concentration of K+ promote intracellular Ca2+ loading, which can be prevented by potassium channel opening drugs. Therefore, potassium channel opening drugs should be considered to prevent intracellular Ca2+ loading associated with the use of cardioplegic solutions.

Two types of ATP-sensitive potassium channels in rat portal vein smooth muscle cells

1. Single-channel recordings were made from single, enzymatically isolated smooth muscle cells of rat portal vein by the patch-clamp technique. 2. Unitary potassium currents were identified through two types of K-channels with conductances in 60:130 mM K-gradient of 50 and 22 pS; these are referred to as LK and MK channels respectively. 3. The LK channels became extremely active if isolated patches were created into nucleotide-free solution; activity was inhibited by ATP applied to the inner surface of the patch with a half maximal inhibition (Ki) of 11-23 microM. Channel activity declined and disappeared with time and could be regenerated by a brief application of Mg-ATP or a nucleoside diphosphate such as UDP (in the presence of Mg). LK channel activity was rarely stimulated by levcromakalim and not by pinacidil (K-channel openers, KCOs) but was blocked by glibenclamide. 4. Activity of MK channels declined if isolated patches were created into nucleotide free solution; activity reappeared if UDP or ATP alone (in the presence of Mg) was applied; pinacidil or levcromakalim in the presence of ATP or UDP further increased channel activity which was blocked by glibenclamide. 5. The LK channel inhibited by ATPi is very similar in its conductance and other properties to the KATP channel described in tissues other than smooth muscle, in its conductance and properties the MK channel resembles the KNDP channel we have previous described as present in other smooth muscles and opening in responses to KCOs.

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.

Inhibition of ATP-sensitive potassium channels in cardiac myocytes by the novel class III antiarrhythmic agent MS-551

The novel class III antiarrhythmic agent, MS-551, has recently been shown to attenuate the decrease in ventricular effective refractory period and to prevent the subsequent ventricular fibrillation induced by pinacidil and hypoxia in isolated perfused rabbit hearts (Friedrichs et al. 1994). We studied the effects of MS-551 on single ATP-sensitive potassium channels in isolated rabbit ventricular myocytes using standard patch-clamp methods. MS-551 in the range from 1 microM to 100 microM produced a concentration-dependent reduction of the open probability of the ATP-sensitive potassium channel, with an apparent ED50 of 30 microM. This reduced channel activity was due to a smaller number of channel openings per unit time, and the average duration of each opening of the channel was unaffected. This property of MS-551 is likely to be of most significance in ischaemic tissue, where the ATP-sensitive channels are thought to carry the predominant current that shortens the duration of the action potential.

Mexiletine-induced shortening of the action potential duration of ventricular muscles by activation of ATP-sensitive K+ channels

A class Ib antiarrhythmic drug, mexiletine (100 microM) significantly shortened the action potential duration (APD) of guinea-pig ventricular muscles and this effect was completely abolished in the presence of glibenclamide (50 microM), a blocker of the ATP-sensitive K+ channel (KATP). Mexiletine significantly increased the open probability of uridine diphosphate-primed KATP channels, recorded in inside-out patches of the ventricular cells. The results suggest that mexiletine shortens the APD of ventricular muscles, at least in part, via activation of KATP.

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.

Electrophysiologic effects of potassium channel openers

Potassium-channel openers or activators have been introduced as a new class of antihypertensive and antianginal agents that act by increasing membrane conductance to potassium, mainly through augmentation of the ATP-sensitive potassium current. Recent in vitro studies have shown that K(+)-channel openers exert concentration-dependent effects on cardiac electrophysiology. A shortening of the cardiac action potential by acceleration of repolarization has been reported in multicellular preparations as well as in isolated myocytes. However, drug concentrations that affect the action potential duration of myocardial cells are considerably higher (10- to 100-fold) than those needed for effects on vascular smooth muscle cells. Studies in which mostly high concentrations of K(+)-channel openers were used have demonstrated that these drugs may accelerate automaticity and may promote reentrant activity. Particular interest has focused on the question whether opening of potassium channels may be potentially arrhythmogenic in the setting of acute myocardial ischemia. On the other hand, recent studies have shown that K(+)-channel openers are effective in suppressing polymorphic ventricular tachyarrhythmias induced by early afterdepolarizations and triggered activity in vivo. The clinical relevance of these experimental studies to the clinical situation is still unclear. Some K(+)-channel openers have been shown to produce electrocardiographic T-wave changes in patients in whom their effectiveness as antihypertensives was tested. However, this effect was not associated with adverse effects and has not been demonstrated for all compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation by intracellular GDP, metabolic inhibition and pinacidil of a glibenclamide-sensitive K-channel in smooth muscle cells of rat mesenteric artery

1. Single-channel recordings were made from cell-attached and isolated patches, and whole-cell currents were recorded under voltage clamp from single smooth muscle cells obtained by enzymic digestion of a small branch of the rat mesenteric artery. 2. In single voltage-clamped cells 1 mM uridine diphosphate (UDP) or guanidine diphosphate (GDP) added to the pipette solution, or pinacidil (100 microM) a K-channel opener (KCO) applied in the bathing solution, evoked an outward current of up to 100pA which was blocked by glibenclamide (10 microM). In single cells from which recordings were made by the 'perforated patch' (nystatin pipette) technique, metabolic inhibition by 1 mM NaCN and 10 mM 2-deoxy-glucose also evoked a similar glibenclamide-sensitive current. 3. Single K-channel activity was observed in cell-attached patches only infrequently unless the metabolism of the cell was inhibited, whereupon channel activity blocked by glibenclamide was seen; pinacidil applied to the cell evoked similar glibenclamide-sensitive channel activity. If the patch was pulled off the cell to form an isolated inside-out patch, similar glibenclamide-sensitive single-channel currents were observed in the presence of UDP and/or pinacidil to those seen in cell-attached mode; channel conductance was 20 pS (60:130 K-gradient) and openings showed no voltage-dependence and noisy inward currents, typical of the nucleoside diphosphate (NDP) activated K-channel (KNDP) seen previously in rabbit portal vein. 4. Formation of an isolated inside-out patch into an ATP-free solution did not increase the probability of channel opening which declined with time even when some single-channel activity had occurred in the cell-attached mode before detachment. However, application of 1 mM UDP or GDP, but not ATP, to inside-out patches evoked single-channel activity. Application of ATP-free solution to isolated patches, previously exposed to ATP and in which channel activity had been seen, did not evoke channel activity. 5. It is concluded that small conductance K-channels (KNDP) open in smooth muscle cells from this small artery in response to UDP or GDP acting from the inside, or pinacidil acting from the outside; the same channels open during inhibition of metabolism presumably mainly due to the rise in nucleoside diphosphates, but a fall in the ATP concentration on the inside of the channel did not by itself evoke channel activity. Failure to respond to a fall in ATP concentration upon formation of an inside-out patch could not be due to dephosphorylation of the channel because sometimes it had been active previously during cell-attached recording. NDPs, instead of ATP, are more important regulators of KNDP channels. It is suggested that the KNDP is the main target K-channel for KCOs.

The effects of potassium channel openers and blockers on the specific binding sites for [3H]glibenclamide in rat tissues

The effects of K+ channel openers (PCOs), NIP-121, levcromakalim and nicorandil, and the blockers of the specific binding sites for [3H]glibenclamide, ATP-sensitive K+ channel blocker, were investigated in rat brain and cardiac ventricle membrane preparations. When the microsomes were incubated with [3H]glibenclamide, the specific glibenclamide binding was fully inhibited by unlabeled glibenclamide (1 microM) and apamin (100 microM). However, the specific glibenclamide binding was not influenced by excess NIP-121, levcromakalim and nicorandil, although glibenclamide antagonized the increase in the 86Rb+ efflux by PCOs. On the other hand, the binding of [3H]glibenclamide after a long pre-incubation (60 min) at 37 degrees C with NIP-121 and levcromakalim at pharmacological effective concentrations (10 nM to 1 microM) was significantly influenced. Both PCOs partially reduced both Kd and Bmax values of the specific [3H]glibenclamide binding in a concentration-dependent manner that was not regulated by GTP gamma S. The dose-effect relationships for the Bmax's of NIP-121 and levcromakalim seemed similar to those for vasorelaxation. These findings indicate that the pharmacological effect of PCO may be caused by the binding to its own specific sites but not to the specific sulfonylurea sites. The binding of PCOs may inhibit, in a negative allosteric manner the binding of sulfonylureas.

Mechanism of action of a K+ channel activator BRL 38227 on ATP-sensitive K+ channels in mouse skeletal muscle fibres

1. Investigations were made into the effects of BRL 38227, a potassium channel activator, on ATP-sensitive potassium channels (K+ATP channels) in single fibres dissociated from the flexor digitorum brevis muscle of C57BL/6J mice. 2. In cell-attached patches BRL 38227 (100 microM) caused activation of a glibenclamide-sensitive potassium current. Linear slope conductance of the inward current, partial rectification of the outward current and glibenclamide sensitivity indicate that K+ATP channels are the site of action of BRL 38227. 3. In the absence of ATP at the cytoplasmic side of excised inside-out patches, BRL 38227 caused direct and magnesium-dependent activation of K+ATP channels. The degree of activation diminished with successive applications of BRL 38227. 4. BRL 38227 also caused activation of K+ATP channels in the presence of low (< 100 microM) but not high (1.0 mM) ATP, particularly in patches containing large numbers of channels. 5. BRL 38227 and 5 microM MgATP failed to activate channels following complete run-down. 6. Results show that BRL 38227 caused direct activation of K+ATP in skeletal muscle and that this was mediated through a magnesium-dependent binding site rather than alleviation of inhibition by competitive displacement of ATP from the inhibitory site.

Membrane ionic currents and properties of freshly dissociated rat brainstem neurons

It is well known that neuronal firing properties are determined by synaptic inputs and inherent membrane functions such as specific ionic currents. To characterize the ionic currents of brainstem cardio-respiratory neurons, cells from the hypoglossal (XII) nucleus and the dorsal motor nucleus of the vagus (DMX) were freshly dissociated and membrane ionic currents were studied under whole-cell voltage and current clamp. Both of these neurons showed a TTX-sensitive Na+ current with a much larger current density in XII than DMX neurons. This Na+ current had two (fast and slow) distinct inactivation decay components. The ratio of the magnitudes of the fast to slow component was roughly two-fold greater in DMX than in XII cells. Both DMX and XII neurons also showed a high voltage-activated Ca2+ current, but this current density was significantly greater (three-fold) in DMX than XII neurons. A relatively small amount of low-voltage activated Ca2+ current was also observed in DMX neurons, but not in the majority of XII cells. A transient and a sustained outward current components were observed in DMX cells, but only sustained currents were present in XII neurons. These outward currents had a reversal potential of about -70 mV with 3 mM external K+ and -30 mV with 25 mM K+, and substitution of K+ with cesium and tetraethylammonium suppressed more than 90% the outward currents, indicating that most outward currents were carried by K+. The transient outward current consisted of two components with one sensitive to 4-aminopyridine and the other to intracellular Ca2+. In XII neurons, BRL 38227 (lemakalim), an ATP-sensitive K+ (KATP) channel activator, increased the sustained K+ currents by 10% of control, and glibenclamide, a KATP channel blocker, decreased the sustained K+ currents by 20%. Evidence for the presence of an inward rectifier K+ current was also obtained from both XII and DMX neurons. These results on XII and DMX neurons indicate that (1) the methods used to dissociate neurons provide a useful means to overcome voltage clamp technical difficulties; (2) ion channel characteristics such as density and biophysical properties of DMX neurons are very different from those of XII neurons; and (3) several newly discovered membrane ionic currents are present in these cells.

Dualistic behavior of ATP-sensitive K+ channels toward intracellular nucleoside diphosphates

ATP-sensitive K+ (KATP) channels are intracellular ligand-gated channels which regulate diverse cellular functions. Intracellular nucleoside diphosphates (NDPs) are essential for the physiological opening of KATP channels which would otherwise be permanently closed by their overt sensitivity to intracellular ATP. We find that KATP channels exhibit dualistic behavior toward NDPs depending on their operative condition. When channels are in the spontaneous operative condition, NDPs antagonize channel inhibition by intracellular ATP. When channels have "run down", NDPs induce channel opening but no longer antagonize intracellular ATP. The switch of the KATP channel response to the same ligand, i.e., NDPs, is controlled by a Mg-ATP-dependent reaction. The condition of the target protein therefore determines the effect of the ligand. This property provides a novel basis to evaluate the dynamic regulation of ion channels by their ligands.

G proteins activate ATP-sensitive K+ channels by antagonizing ATP-dependent gating

To determine whether G proteins activate cardiac ATP-sensitive K+ (KATP) channels by regulating intracellular ATP (ATPi)-dependent gating, currents were measured in inside-out patches. When ATPi closed KATP channels, activators of endogenous G proteins, GTP (plus adenosine or acetylcholine), GTP gamma S, or AlF-4 stimulated channels, an effect prevented by GDP beta S. In the absence of ATPi, G protein activators were ineffective. Intracellular nucleoside diphosphates restored KATP channel openings after the "rundown" of spontaneous activity. Only when ATPi suppressed nucleoside diphosphate-induced openings, GTP gamma S or AlF-4 enhanced KATP channel activity. Active forms of exogenous G protein subunits (G alpha i-1, G alpha i-2, or G alpha o) activated only KATP channels closed by ATPi. G proteins stimulate cardiac KATP channels apparently by antagonizing ATPi-dependent inhibitory gating. Regulation of ligand-dependent gating represents a distinct type of G protein modulation of ion channels.

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.

Enhancement of ATP-sensitive potassium current in cat ventricular myocytes by beta-adrenoreceptor stimulation

1. To address the questions of whether beta-adrenoreceptor stimulation can augment ATP-sensitive potassium current (IK(ATP)), and what the mechanism of such an effect might be, action potentials and whole-cell ionic currents were recorded from adult cat cardiac ventricular myocytes using a conventional whole-cell patch technique. 2. An outwardly directed, ohmic, non-inactivating, glyburide (10 microM)-sensitive current reversing near the reversal potential for potassium (EK) developed slowly (10-25 min) in cells dialysed with an ATP-free pipette (intracellular) solution. During this time, action potential duration markedly decreased while the resting membrane potential hyperpolarized closer to EK. Extended (> 30 min) periods of internal dialysis with ATP-free solution eventually resulted in run-down of the outward current. 3. Externally applied isoprenaline (1 microM) caused a rapidly developing (< or = 60 s), sustained enhancement of a glyburide (10 microM)-sensitive IK(ATP) in cells internally dialysed with ATP-free solution. IK(ATP) remained elevated even after the isoprenaline was removed, and subsequent applications of the beta-agonist failed to increase IK(ATP) further. Half-maximal isoprenaline stimulation of IK(ATP) occurred at a concentration of approximate of 1.5 nM. 4. Pretreatment with propranolol (1 microM) prevented the enhancement of IK(ATP) by a beta-agonist. 5. Isoprenaline-induced IK(ATP) could be blocked by either internal application of GDP-beta-S (2-5 mM) or pretreatment with cholera toxin (1-10 microgram ml-1, > 18 h). Pretreatment with pertussis toxin (1-2 microgram ml-1, > 18 h) did not attenuate the isoprenaline response, whereas internally applied GTP-gamma-S (100 microM) or F- (20 mM) caused IK(ATP) to increase rapidly in the absence of the beta-agonist. 6. Although externally applied forskolin (10 microM) also stimulated IK(ATP), neither 1,9-dideoxyforskolin (10 microM) nor 8-(4-chlorophenylthio)-cAMP (200 microM) had any effect on the current. Internal application of the adenylate cyclase inhibitor 2'-deoxyadenosine-3'-monophosphate (100 microM) resulted in a reduction in the response to isoprenaline, while internal application of a protein kinase A inhibitor (PKI5-24, 22.5 microM) did not attenuate the response to the beta-agonist. 7. IK(ATP) developed slowly during internal dialysis with ATP-free solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Single channel and whole-cell K-currents evoked by levcromakalim in smooth muscle cells from the rabbit portal vein

1. Single channel and whole-cell current recordings were made from single smooth muscle cells isolated from the rabbit portal vein. 2. Application of 10 microM levcromakalim ((-)-Ckm) to single cells held with pipettes containing 1 mM GDP induced a K-current (IK(Ckm)) which occurred in addition to the current caused by GDP alone (IK(GDP)) and averaged 135 pA at -37 mV. We have investigated whether the same K channels underlie the GDP- and Ckm-induced K-currents. 3. If 1 mM GDP was in the pipette but Mg ions were omitted the effect of GDP was absent and IK(Ckm) averaged only 10 pA, suggesting that the action of (-)-Ckm was Mg-dependent. 4. Intracellular ATP was not observed to have much effect on IK(-Ckm). Loading of cells with 10 mM ATP from the recording pipette had no significant effect and flash photolysis of caged-ATP loaded into cells from the pipette, estimated to release about 1 mM free ATP, also had no effect on IK(-Ckm). 5. Bath-applied glibenclamide inhibited IK(-Ckm) with an IC50 of 200 nM, a value 8 times higher than that found for inhibition of IK(GDP). The delayed rectifier K-current (IK(DR)) was also inhibited by glibenclamide but at higher concentrations (IC50 100 microM). Bath-applied tetraethylammonium ions (TEA) inhibited IK(-Ckm) and IK(GDP) to the same extent (IC50 about 7 mM). 6. In inside-out patch recordings (- )-Ckm (10 microM) applied to the intracellular surface of the membrane potentiated the opening of K channels already stimulated by I mM GDP and all of the channel activity was abolished by 10 microM glibenclamide. The unitary conductance of the channels was 24lpS in a 60 mM: 130 mM K-gradient.7. We suggest that (-)-Ckm may hyperpolarize and relax smooth muscle cells by opening KNDP, a class of small conductance K channels that are related to the ATP-sensitive K channels seen in other tissues.

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)

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.

Stimulation of the KATP channel by ADP and diazoxide requires nucleotide hydrolysis in mouse pancreatic beta-cells

1. The mechanisms by which ADP and the hyperglycaemic compound diazoxide stimulate the activity of the ATP-regulated K+ channel (KATP channel) were studied using inside-out patches isolated from mouse pancreatic beta-cells maintained in tissue culture. 2. The ability of diazoxide and ADP to increase KATP channel activity declined with time following patch excision and no stimulation was observed after 15-40 min. 3. Activation of KATP channels by ADP required the presence of intracellular Mg2+. The stimulatory effect of ADP was mimicked by AMP but only in the presence of ATP. Replacement of ATP with the non-hydrolysable analogue beta, gamma-methylene ATP did not interfere with the ability of ADP to stimulate KATP channel activity. By contrast, enhancement of KATP channel activity was critically dependent on hydrolysable ADP and no stimulation was observed after substitution of alpha,beta-methylene ADP for standard ADP. 4. The ability of diazoxide to enhance KATP channel activity was dependent on the presence of both internal Mg2+ and ATP. Diazoxide stimulation of KATP channel activity was not observed after substitution of beta,gamma-methylene ATP for ATP. However, in the presence of ADP, at a concentration which in itself had no stimulatory action (10 microM), diazoxide was stimulatory also in the presence of the stable ATP analogue. 5. The stimulatory action of diazoxide on KATP channel activity in the presence of ATP was markedly enhanced by intracellular ADP. This potentiating effect of ADP was not reproduced by the stable analogue alpha,beta-methylene ADP and was conditional on the presence of intracellular Mg2+. A similar enhancement of channel activity was also observed with AMP (0.1 mM). In the absence of ATP, diazoxide was still capable of stimulating channel activity provided ADP was present. This effect was not reproduced by AMP. 6. In both nucleotide-free solution and in the presence of 0.1 mM ATP, the distribution of the KATP channel open times were described by a single exponential with a time constant of approximately 20 ms. Addition of ADP or diazoxide resulted in the appearance of a second component with a time constant of > 100 ms which comprised 40-70% of the total number of events. Under the latter experimental conditions, the open probability of the channel increased more than fivefold relative to that observed in the presence of ATP alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Nucleotide diphosphates activate the ATP-sensitive potassium channel in mouse skeletal muscle

Patch-clamp techniques were used to study the effects of internal nucleotide diphosphates on the KATP channel in mouse skeletal muscle. In inside-out patches, application of GDP (100 microM) and ADP (100 microM) reversibly increased the channel activity. In the presence of internal Mg2+ (1 mM), low concentrations of ADP (< 300 microM) enhanced channel activity and high concentrations of ADP (> 300 microM) limited channel opening while GDP activated the channel at all concentrations tested. In the absence of internal Mg2+, ADP decreased channel activity at all concentrations tested while GDP had no noticeable effect at submillimolar concentrations and inhibited channel activity at millimolar concentrations. GDP [beta S] (100 microM), which behaved as a weak GDP agonist in the presence of Mg2+, stimulated ADP-evoked activation whereas it inhibited GDP-evoked activation. The K+ channel opener pinacidil was found to activate the KATP channel but only in the presence of internal GDP, ADP and GDP [beta S]. The results are discussed in terms of the existence of multiple nucleotide binding sites, in charge of the regulation of the KATP channel.

Aromatic aldehydes and aromatic ketones open ATP-sensitive K+ channels in guinea-pig ventricular myocytes

Patch-clamp techniques were used to study the effects of three carbonyl compounds, 3,4-dihydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, and 2,4-dihydroxyacetophenone, on the adenosine-5'-triphosphate(ATP)-sensitive K+ channel current (IK.ATP) in guinea-pig ventricular myocytes. 3,4-Dihydroxybenzaldehyde (0.5-1 mM) shortened the action potential duration, and this effect was inhibited by application of a specific blocker of IK.ATP, glibenclamide. The shortening of the action potential duration was shown to be caused by a time-independent outward current. In the cell-attached patch configuration, all three compounds activated a kind of single-channel current, which showed an inward rectification at positive potentials and which had a linear current/voltage relation at negative potentials, having a conductance of 90 pS. The current reversed at about 0 mV in symmetrical K+ concentrations on both sides of the membrane. In excised patches this current was blocked by internal application of ATP. Thus we identified this channel as IK.ATP. The activation effects of two aromatic aldehydes were stronger than that of the aromatic ketone. The effect of these compounds on IK.ATP was not reduced by addition of cysteine (10 mM). In inside-out patches, 3,4-dihydroxybenzaldehyde increased the activity of IK.ATP, which had been blocked by 0.5 mM MgATP in the presence of 0.5 mM ADP, but the activation effect was variable and much weaker than that in the cell-attached configuration, and was completely eliminated in the absence of ADP.(ABSTRACT TRUNCATED AT 250 WORDS)