ATP-sensitive potassium channels and myocardial ischemia: why do they open?

Activation of pannexin-1 mediates triglyceride-induced macrophage cell death

The accumulation of triglycerides (TGs) in macrophages induces cell death, a risk factor in the pathogenesis of atherosclerosis. We had previously reported that TG-induced macrophage death is triggered by caspase-1 and -2, therefore we investigated the mechanism underlying this phenomenon. We found that potassium efflux is increased in TG-treated THP-1 macrophages and that the inhibition of potassium efflux blocks TG-induced cell death as well as caspase-1 and -2 activation. Furthermore, reducing ATP concentration (known to induce potassium efflux), restored cell viability and caspase-1 and -2 activity. The activation of pannexin-1 (a channel that releases ATP), was increased after TG treatment in THP-1 macrophages. Inhibition of pannexin-1 activity using its inhibitor, probenecid, recovered cell viability and blocked the activation of caspase-1 and -2 in TG-treated macrophages. These results suggest that TG-induced THP-1 macrophage cell death is induced via pannexin-1 activation, which increases extracellular ATP, leading to an increase in potassium efflux.

Heteromeric Slick/Slack K+ channels show graded sensitivity to cell volume changes

Slick and Slack high-conductance K⁺ channels are found in the CNS, kidneys, pancreas, among other organs, where they play an important role in cell excitability as well as in ion transport processes. They are both activated by Na⁺ and Cl^- but show a differential regulation by cell volume changes. Slick has been shown to be regulated by cell volume changes, whereas Slack is insensitive. α-subunits of these channels form homomeric as well as heteromeric channels. It is the aim of this work to explore whether the subunit composition of the Slick/Slack heteromeric channel affects the response to osmotic challenges. In order to provide with the adequate water permeability to the cell membrane of Xenopus laevis oocytes, mRNA of aquaporin 1 was co-expressed with homomeric or heteromeric Slick and Slack α-subunits. Oocytes were superfused with hypotonic or hypertonic buffers and changes in currents were measured by two-electrode voltage clamp. This work presents the first heteromeric K⁺ channel with a characteristic graded sensitivity to small and fast changes in cell volume. Our results show that the cell volume sensitivity of Slick/Slack heteromeric channels is dependent on the number of volume sensitive Slick α-subunits in the tetrameric channels, giving rise to graded cell volume sensitivity. Regulation of the subunit composition of a channel may constitute a novel mechanism to determine volume sensitivity of cells.

Influence of Thromboxane A2 on the Regulation of Adenosine Triphosphate-Sensitive Potassium Channels in Mouse Ventricular Myocytes

Background and Objectives

Adenosine triphosphate (ATP)-sensitive potassium (K_ATP) channels play an important role in myocardial protection. We examined the effects of thromboxane A₂ on the regulation of K_ATP channel activity in single ventricular myocytes.

Subjects and Methods

Single ventricular myocytes were isolated from the hearts of adult Institute of Cancer Research (ICR) mice by enzymatic digestion. Single channel activity was recorded by excised inside-out and cell-attached patch clamp configurations at −60 mV holding potential during the perfusion of an ATP-free K-5 solution.

Results

In the excised inside-out patches, the thromboxane A₂ analog, U46619, decreased the K_ATP channel activity in a dose-dependent manner; however, the thromboxane A₂ receptor antagonist, SQ29548, did not significantly attenuate the inhibitory effect of U46619. In the cell-attached patches, U46619 inhibited dinitrophenol (DNP)-induced K_ATP channel activity in a dose-dependent manner, and SQ29548 attenuated the inhibitory effects of U46619 on DNP-induced K_ATP channel activity.

Conclusion

Thromboxane A₂ may inhibit K_ATP channel activity, and may have a harmful effect on ischemic myocardium.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

The electrophysiological effects of racemic ketamine and etomidate in an in vitro model of "border zone" between normal and ischemic/reperfused guinea pig myocardium

BACKGROUND

Etomidate and ketamine are used during induction of anesthesia in high-risk patients. However, their effects on action potential (AP) variables and ischemia/reperfusion-induced arrhythmias and conduction blocks are unknown.

METHODS

Guinea pig right ventricular muscle strips were mounted in a 5-mL double chamber bath with the strips separated into two zones by an impermeable latex membrane. One-half (normal zone) was exposed to normal perfusate while the other half (altered zone) was exposed to hypoxia, hyperkalemia, acidosis, and lack of glucose. AP variables were recorded continuously in the normal and altered zones. Spontaneous arrhythmias and conduction blocks were noted. Etomidate (10(-7), 10(-6), and 10(-5) M) and ketamine (10(-6), 10(-5), and 10(-4) M) were superfused into the bath throughout the experiment and the electrophysiologic effects compared with the control group.

RESULTS

We found that under control conditions, etomidate and ketamine did not modify resting membrane potential, maximal upstroke velocity, AP amplitude, or AP duration at 90% of repolarization (APD90). Ketamine (10(-4) M), but not weaker concentrations and none of the concentration of etomidate, reversed the ischemia-induced shortening of APD90 and APD dispersion. Etomidate and ketamine did not modify the occurrence of conduction block during simulated ischemia. In contrast, ketamine (25% at 10(-6) M, 13% at 10(-5) M, and 13% at 10(-4) M vs 90% in the control group, P < 0.05) but not etomidate (38% at 10(-7) M, 63% at 10(-6) M, and 63% at 10(-5) M vs 90% in the control group, NS) decreased the incidence of reperfusion-induced spontaneous arrhythmias.

CONCLUSIONS

In guinea pig myocardium, our data suggest that ketamine, in clinically relevant concentrations, decreases ischemia-induced AP shortening and spontaneous reperfusion-induced ventricular arrhythmias. Further study is required to precisely determine the effect of etomidate on reperfusion-induced arrhythmias.

Electrophysiological effects of azimilide in an in vitro model of simulated-ischemia and reperfusion in guinea-pig ventricular myocardium

There are few investigations on azimilide effects during ischemia/reperfusion. We have therefore investigated low concentrations of azimilide (0.1 and 0.5 micromol/l) versus Controls on action potential parameters and occurrence of repetitive responses during simulated ischemia and reperfusion. An in vitro model of "border zone" in guinea-pig ventricular myocardium (n=30) was used. Azimilide 0.5 micromol/l lengthened action potential duration in normoxic but not in ischemic-like conditions. Therefore an increased dispersion of action potential duration at 90% of repolarization during simulated ischemia in presence of azimilide was seen. Upon reperfusion, both normal and reperfused myocardium showed azimilide-induced action potential duration increase. There was a neutral effect on the occurrence of arrhythmias during simulated ischemia; however azimilide showed significant (P=0.033) antiarrhythmic properties following reperfusion. To mimic I(Kr) and I(Ks) blocking properties of azimilide we further used dofetilide 10 nmol/l with HMR 1556 1 nmol/l (N=9), which was accompanied by less severe shortening (P<0.05) of action potential duration at 90% of repolarization at 30 min of ischemic-like conditions (-43+/-9%), as compared with azimilide 0.5 micromol/l (-64+/-5%) but similar to what seen with azimilide 0.1 micromol/l (-53+/-5%) and Controls (-52+/-6%). During reperfusion, 2/9 (22%) preparations had sustained activities, which was less than what observed in Controls (5/10, 50%) and with azimilide 0.5 micromol/l (0/10, 0%), although not statistically different (respectively, P=0.35 and P=0.21). Lack versus homogenous class III effects of azimilide in respectively simulated ischemia and reperfusion may explain its different efficacy on arrhythmias, although prevention of reperfusion arrhythmias calls for other than just its I(Kr) and I(Ks) blocking properties.

Electrophysiological effects of morphine in an in vitro model of the 'border zone' between normal and ischaemic-reperfused guinea-pig myocardium

BACKGROUND

Morphine is commonly used in clinical practice in pain management. Although morphine has been shown to precondition the myocardium, its effects on action potential parameters and ischaemia-reperfusion-induced arrhythmias and conduction blocks remain unknown.

METHODS

In a double-chamber bath, guinea-pig right ventricular muscle strips were subjected partly to normal conditions and partly to 30 min of simulated ischaemia (hypoxia, hyperkalaemia, acidosis, and lack of nutritional substrate) followed by 30 min of reperfusion. Action potential parameters were recorded continuously in the normal zone and in the ischaemic- reperfused zone. Spontaneous arrhythmias and conduction blocks were noted. The electro physiological effects of morphine were studied at 0.01 and 0.1 micro M.

RESULTS

In control conditions, morphine did not modify action potential parameters of resting membrane potential, maximal upstroke velocity (V(max)), action potential amplitude (APA) and action potential duration at 50 and 90% of repolarization. Morphine reduced ischaemia-induced depolarization and lessened the ischaemia-induced decrease in APA and V(max). Morphine significantly decreased the occurrence of conduction block during simulated ischaemia (20% at 0.01 and 0.1 micro M vs 67% in the control group, P<0.05) and reperfusion-induced arrhythmias (40% at 0.01 micro M and 30% at 0.1 micro M vs 92% in the control group, P<0.05).

CONCLUSIONS

In ischaemic-reperfused guinea-pig myocardium, morphine at clinically relevant concentrations decreased ischaemia-induced conduction blocks and reperfusion-induced ventricular arrhythmias.

Functional hepatocyte cation compartmentation demonstrated with 133Cs NMR

This study utilized the large intrinsic chemical shift range of (133)Cs, a potassium congener, in an NMR study of intracellular cation distribution. It demonstrates two distinct intracellular environments in isolated perfused hepatocytes from cesium-fed rats, evident as compartments with different (133)Cs chemical shifts and containing different proportions of total detected cesium. The chemical shifts of the two intracellular compartments were 2.44 +/- 0.07 and 1.21 +/- 0.18 ppm, relative to the cesium signal from the perfusate. The observation of two distinct intracellular cesium signals suggests slow exchange on an NMR chemical shift time-scale (k exchange > 0.02 s). The area of the high-frequency component represented 62 +/- 10% (N = 12) of the total intracellular cesium signal. Manipulation of the intracellular environment using anoxia with aglycemia or digitonin produced changes in the distribution between the two intracellular compartments, showing their dynamic nature. Changes measured in association with metabolic manipulation suggest cytoplasm and mitochondria as the origin of the high and low-frequency intracellular peaks, respectively.

The role of myocardial KATP-channel blockade in the protective effects of glibenclamide against ischaemia in the rat heart

Glibenclamide preserves postischaemic myocardial function in the isolated, erythrocyte perfused, working rat heart model. This study addresses the possible involvement of KATP channels in this beneficial action of glibenclamide. We hypothesized that if glibenclamide improved postischaemic cardiac function by blocking of KATP channels, opening of these KATP channels should result in the opposite, namely detrimental effects on postischaemic heart function. Postischaemic functional loss and coronary blood flow were recorded during treatment with glibenclamide (4 micromol x l(-1); n = 5), the KATP channel openers pinacidil (1 micromol x (l-1); n = 5) and diazoxide (30 micromol x l(-1); n = 5), the combination of glibenclamide with pinacidil (n = 5) and glibenclamide with diazoxide (n = 5), and vehicle (n = 8). Both pinacidil and diazoxide significantly increased coronary blood flow 2-3 times, which was abolished by glibenclamide pre- and postischaemically. This confirms that under both flow conditions glibenclamide significantly blocks KATP channels in the coronary vasculature. The 12 min. global ischaemic incident resulted in a cardiac functional loss of 22.2 +/- 2.9% during vehicle. Glibenclamide reduced the cardiac functional loss to 4.3 +/- 1.2% (P < 0.01). Interestingly, both pinacidil and diazoxide reduced the cardiac functional loss to 4.0 +/- 1.5% (P < 0.01) and 2.9 +/- 1.4% (P < 0.001), respectively. The combination pinacidil+glibenclamide resulted in additional protection compared with the individual components (0.6 +/- 0.1 versus 4.0 +/- 1.5%, P < 0.05). Thus, in contrast to its effect on coronary vascular tone, the glibenclamide-induced improvement of postischaemic cardiac function may not be mediated through blockade of the KATP channel. Alternative mechanisms may be operative, such as uncoupling of the mitochondrial respiratory chain, thereby preconditioning the hearts against stunning.

Glibenclamide attenuates ischemia-induced acidosis and loss of cardiac function in rats

Previous research has shown that the sulfonylurea derivative glibenclamide may improve post-ischemic cardiac functional recovery. Although K(ATP) channel blockade is a possible explanation for this observation, alternative mechanisms exist. Therefore, we simultaneously recorded cardiac function and the intracellular concentration of ATP, phosphocreatine, Pi and pH before and after ischemia in the presence of glibenclamide or vehicle. (31)Phosphorus magnetic resonance (MS) spectroscopy on erythrocyte-perfused, isolated working rat hearts was performed. Glibenclamide 4 micromol l(-1) or vehicle alone was tested (both n=5). The following protocol was used: 8 min performance assessment, 10 min drug treatment, 12 min global ischemia, 20 min reperfusion with drug treatment and 8 min functional recovery assessment. Compared with vehicle, glibenclamide significantly decreased coronary blood flow (59.5+/-7.0% vs. 94.3+/-1.3%, P=0.008), ischemia-induced cardiac functional loss (7.4+/-1.3% vs. 18.8+/-3.3%; P=0.019) as well as the ischemia-induced intracellular acidosis (6.75+/-0.01 vs. 6.43+/-0.03 for vehicle, P=0.03). In conclusion, glibenclamide is able to reduce the myocardial functional loss after ischemia while preserving pH but not ATP levels during ischemia. This suggests that the beneficial response to glibenclamide is probably not the result of myocardial K(ATP) channel blockade, but may be explained by inhibition of glycolysis.

Effects of sulfonylurea derivatives on ischemia-induced loss of function in the isolated rat heart

This study determined whether sulfonylurea derivatives affect cardiac function prior to and after a mild ischemic incident (stunning). This was investigated using an isolated, erythrocyte-perfused, working rat heart model. In total, 11 groups were studied: five increasing (clinically relevant) concentrations of the classical glibenclamide (range 0.005-4 micromol l(-1)), five increasing concentrations of the newly developed glimepiride (range 0.005-0.8 micromol l(-1)), and one control group. Pre-ischemically, glibenclamide and glimepiride reduced coronary blood flow concentration dependently to 55.2+/-4.5% and 58.5+/-5.5%, respectively (P<0.001). Twenty minutes after a 12-min ischemic incident, these reductions of flow were even more pronounced (to 38.3+/-6.7% and 45.8+/-5.8%, P<0.001). This shows that both sulfonylureas reduce coronary blood flow at concentrations slightly higher than therapeutic ones. In the control group, the ischemic incident significantly lowered cardiac function by 22.2+/-2.9%. In the therapeutic range, glimepiride, but not glibenclamide, significantly reduced this ischemia-induced cardiac functional loss to 4.9+/-1.2% (P<0.01). Therefore, we suggest that both sulfonylureas and in particular glimepiride can be used safely in patients with type 2 diabetes mellitus, as long as the coronary vascular system is not compromised. Because of the obvious vasocontrictor response to sulfonylurea derivatives, these drugs must be used with caution in patients with a reduced coronary reserve.

Activation of ATP-sensitive potassium channels in hypoxic cardiac failure is not mediated by adenosine-1 receptors in the isolated rat heart

BACKGROUND

Hypoxic cardiac failure is accompanied by action potential shortening, which in part might be a consequence of opening of cardiac ATP-sensitive potassium channels (K(ATP) channels). Coupling of the adenosine-1 receptor (A-1 receptor) to these channels has been described; however, the interaction of A-1-receptors and K(ATP) channels in different models of ischemia is still under debate. The hypothesis as to whether A-1 receptors are involved in hypoxic K(ATP) channel-activation in the saline-perfused rat heart was tested.

METHODS AND RESULTS

Pharmacologic modulation of the K(ATP) channel by Glibenclamide (inhibitor) and Rimalkalim (activator) and of the A-1 receptor by R(-)-N6-(1-methyl-2-phenylethyl)-adenosine (R(-)-PIA, agonist) and 1,3-diethyl-3,7-dihydro-8-phenyl-purine-2,6-dione (DPX, antagonist) at different oxygen tensions (95% O2 and 20% O2) was performed in isolated Langendorff-rat hearts. Peak systolic pressure (PSP, intraventricular balloon), duration of monophasic action potential (epicardial suction electrode, time to 67% of repolarization: MAP(67%)), coronary flow, and heart rate (HR) were registered. Hypoxic perfusion resulted in a significant reduction of PSP (from 106 +/-11 to 56 +/-8 mmHg, P < 0.005) and shortening of MAP(67%) (from 37 +/-3 to 25 +/-4 ms, P < 0.005). With application of 1 microM Glibenclamide, MAP(67%) returned to normoxic values and PSP increased to 78 +/-9 mmHg (P < 0.005 vs hypoxia). In normoxia, 2 microM Rimalkalin resulted in reduction of MAP(67%) and PSP, which was reversed by Glibenclamide. Application of 0.1 microM R(-)-PIA in normoxia resulted in a decrease of HR (from 235 +/-36/min to 75 +/-41/min, P < 0.005), which was accompanied by an increase of PSP from 96 +/-7 to 126 +/-9 mmHg (P < 0.05) without changes in MAP(67%). These effects were reversible by 1 microM DPX and remained unaffected by application of 1 microM Glibenclamide. Application of 1 microM DPX in hypoxia had no effect on the measured parameters.

CONCLUSION

In isolated rat hearts, the K(ATP) channel-system is activated in hypoxic cardiac failure and contributes to action potential shortening and reduced contractile performance. These effects seem to be independent of the A-1 receptor in this model.

Anti-arrhythmic effects of levcromakalim in the ischaemic rat heart: a dual mechanism of action?

The action of pharmacological openers of K(ATP) channels depends on the availability and levels of various intracellular nucleotides. Since these are subject to change during myocardial ischaemia, K(ATP) channel openers may affect ischaemic and non-ischaemic tissue differentially. Using a recently developed dual coronary perfusion method, we investigated the effects on arrhythmias of the prototypical K(ATP) channel opener levcromakalim when applied selectively to ischaemic and/or non-ischaemic tissue. A novel perfusion cannula was used to independently perfuse the left and right coronary beds of hearts isolated from rats. Selective infusion of levcromakalim (3, 10 or 30 microM) into the left coronary bed in the absence of ischaemia did not induce ventricular arrhythmias. Regional zero-flow ischaemia was induced by cessation of flow to the left coronary bed and hearts received levcromakalim selectively into either the left, right, or both coronary beds. When applied selectively to the ischaemic left coronary bed, levcromakalim (3, 10 or 30 microM; n=10/group) delayed the onset of ventricular tachycardia in a dose-dependent manner (by 21*, 43* and 112%* at 3, 10 and 30 microM; *P<0.05 vs. control). When applied only to the non-ischaemic right coronary bed, levcromakalim reduced the incidence of ventricular tachycardia during later phases of ischaemia (from 100% in controls to 30%*). When present in both coronary beds, levcromakalim had a striking anti-arrhythmic effect--the overall incidence of ventricular tachycardia being reduced from 100% in controls to 20%*. We conclude that levcromakalim may have an anti-arrhythmic effect when applied either to ischaemic or non-ischaemic tissue but that the mechanisms may differ depending on the metabolic state of the heart.

Electrophysiological effects of dofetilide in an in vitro model of "border zone" between normal and ischemic/reperfused myocardium

BACKGROUND

To evaluate both class III activity and antiarrhythmic action of dofetilide at the level of the "border zone," we investigated its electrophysiological effects on guinea pig ventricular strips submitted partly to normoxia (normal zone, NZ) and partly to simulated severe ischemia, then reperfusion (altered zone, AZ).

METHODS AND RESULTS

Because of the differential class III effects of dofetilide in normal and ischemic regions, the dispersion of the action potential duration at 90% repolarization (APD(90)) between NZ and AZ was reduced by 5 nmol/L of drug during early ischemia (at 10 minutes, APD(90) NZ/APD(90) AZ was 1.68+/-0.22 versus 2.82+/-0.17 in control, P<0.05), whereas 50 nmol/L dofetilide worsened it during late ischemia (at 30 minutes, APD(90) NZ/APD(90) AZ was 4.62+/-0.76 versus 2.57+/-0.21 in control, P<0.05). Concomitantly, dofetilide at 5, 10, and 50 nmol/L abolished the early extrastimulus (ES)-induced arrhythmias, and at 10 and 50 nmol/L, it significantly enhanced the incidence of late spontaneous repetitive responses (in 86% and 75% of preparations treated with 10 and 50 nmol/L, respectively, versus 25% in control, P<0.05). During reperfusion, dofetilide at 5, 10, and 50 nmol/L exhibited concentration-dependent class III effects, as it did in the NZ, and did not modify the incidence of spontaneous arrhythmias.

CONCLUSIONS

Dofetilide 5 nmol/L decreased APD(90) dispersion between NZ and AZ and reduced the early ES-induced arrhythmias. However, dofetilide 50 nmol/L increased APD(90) dispersion, and at 10 and 50 nmol/L, it increased the late spontaneous arrhythmias.

Cellular and molecular therapeutic targets for treatment of contractile dysfunction after cardioplegic arrest

Transient left ventricular (LV) dysfunction can occur after hypothermic hyperkalemic cardioplegic arrest. This laboratory has developed an isolated LV myocyte system of simulated cardioplegic arrest and rewarming in order to examine cellular and molecular events that may contribute to the LV dysfunction after cardioplegic arrest. Contractile function was examined using high-speed video microscopy after reperfusion and rewarming. After cardioplegic arrest and reperfusion, indices of myocyte contractility were reduced by over 40% from normothermic control values. The capacity of the myocyte to respond to an inotropic stimulus was examined through beta-adrenergic receptor stimulation with isoproterenol. After cardioplegic arrest, the contractile response to isoproterenol was reduced by over 50% from normothermic values. The next series of studies focused upon preventing these changes in myocyte contractile processes after cardioplegic arrest. First, the cardioplegic solutions were augmented with adenosine or an ATP-sensitive potassium channel opener, aprikalim. Both adenosine and aprikalim augmentation significantly improved myocyte function compared with cardioplegia alone values. A potential intracellular mechanism for the protective effects of either adenosine or the ATP-sensitive potassium channel is the activation of protein kinase C (PKC). A brief period of PKC activation before cardioplegic arrest provided protective effects on myocyte contractility with subsequent reperfusion and rewarming. In another set of studies, the potential protective effects of the active form of thyroid hormone (T3) were examined. In myocytes pretreated with T3, myocyte contractile function and beta-adrenergic responsiveness were significantly improved after hypothermic cardioplegic arrest and rewarming. Thus, endogenous means of providing improved myocardial protection during prolonged cardioplegic arrest can be achieved through a brief period of PKC activation or pretreatment with T3. Future studies, which more carefully deduce the basis for these pretreatment effects, will likely yield novel methods by which to protect myocyte contractile processes during cardioplegic arrest.

Cardioselective antiischemic ATP-sensitive potassium channel (K(ATP)) openers. 6. Effect of modifications at C6 of benzopyranyl cyanoguanidines

The effect on potency and selectivity of modifications at the C6 position of the cardioprotective K(ATP) opener BMS-180448 (2) is described. Structure-activity studies show that a variety of electron-withdrawing groups (ketone, sulfone, sulfonamide, etc.) are tolerated for cardioprotective activity as measured by EC(25) values for an increase in time to the onset of contracture in globally ischemic rat hearts. Changes made to the sulfonamido substituent indicate that compounds derived from secondary lipophilic amines are preferred for good cardioprotective potency and selectivity. The diisobutyl analogue 27 (EC(25) = 0.04 microM) is the most potent compound of this series. The cardiac selectivity of 27 results from a combination of reduced vasorelaxant potency and enhanced cardioprotective potency relative to the potent vasodilating K(ATP) openers (e.g., cromakalim). The diisobutylsulfonamide analogue 27 is over 4 orders of magnitude more cardiac selective than cromakalim (1). These results support the hypothesis that the cardioprotective and vasorelaxant properties of K(ATP) openers follow distinct structure-activity relationships. The mechanism of action of 27 appears to involve opening of the cardiac K(ATP) as its cardioprotective effects are abolished by the K(ATP) blocker glyburide.

KATP channels and 'border zone' arrhythmias: role of the repolarization dispersion between normal and ischaemic ventricular regions

1. In order to investigate the role of KATP channel activation and repolarization dispersion on the 'border zone' arrhythmias induced by ischaemia-reperfusion, the effects of glibenclamide and bimakalim, agents modifying action potential (AP) duration, were studied in an in vitro model of myocardial 'border zone'. 2. The electrophysiological effects of 10 microM glibenclamide and 1 microM bimakalim (n=8 each), respectively KATP channel blocker and activator, were investigated on guinea-pig ventricular strips submitted partly to normal conditions (normal zone, NZ) and partly to simulated ischaemic then reperfused conditions (altered zone, AZ). 3. By preventing the ischaemia-induced AP shortening (P<0.0001), glibenclamide reduced the dispersion of AP duration 90% (APD90) between NZ and AZ (P<0.0001), and concomitantly inhibited the 'border zone' arrhythmias induced by an extrastimulus (ES), their absence being significantly related to the lessened APD90 dispersion (chi2=8.28, P<0.01). 4. Bimakalim, which also reduced the APD90 dispersion (P<0.005) due to differential AP shortening in normal and ischaemic tissues, decreased the incidence of myocardial conduction blocks (25% of preparations versus 83% in control, n=12, P<0.05) and favoured 'border zone' spontaneous arrhythmias (75% of preparations versus 25% in control, P<0.05). 5. During reperfusion, unlike bimakalim, glibenclamide inhibited the ES-induced arrhythmias and reduced the incidence of the spontaneous ones (12% of preparations versus 92% in control, P<0.05), this latter effect being significantly related (chi2=6.13, P<0.02) to the lessened ischaemia-induced AP shortening in the presence of glibenclamide (P<0.0001). 6. These results suggest that KATP blockade may protect the ischaemic-reperfused myocardium from 'border zone' arrhythmias concomitantly with a reduction of APD90 dispersion between normal and ischaemic regions. Conversely, KATP channel activation may modify the incidence of conduction blocks and exacerbate the ischaemia-induced 'border zone' arrhythmias.

Effects of bimakalim on human cardiac action potentials: comparison with guinea pig and nicorandil and use-dependent study

Electrophysiologic effects of K(ATP) channel openers (KCOs) are rarely studied for tissue and species specificity, and use-dependent investigations in human tissues are lacking. We therefore investigated in vitro the concentration-dependent effects of the KCO bimakalim [from 10 nM to 10 microM, at 1,000 ms of cycle length (CL) and 37 degrees C] on human (atrium, n = 4, and ventricle, n = 6) and guinea pig (atrium, n = 7, and ventricle, n = 6) transmembrane action potential (AP). The frequency relation (from CL 1,600 to 300 ms, 31 degrees C) of human atrial AP duration 90% (APD90) shortening (10 microM vs. baseline, n = 7) also was determined. A parallel study was performed with the KCO nicorandil (from 10 nM to 1 mM, n = 3) in human atrial APs, at 31 degrees C. Resting membrane potential and maximal upstroke velocity of AP were not modified by bimakalim at maximal concentration, whereas AP amplitude was decreased in both guinea pig preparations (p < 0.05); APD90 was shortened in all tissues (p < 0.01). Median effective concentration (EC50) for APD90 shortening at 37 degrees C was 0.54 and 2.74 microM in atrial and ventricular human tissue, respectively, and 8.55 and 0.89 microM in atrial and ventricular guinea pig tissue, respectively. In human atrial tissue at 31 degrees C, EC50 with bimakalim was 0.39 microM; a much higher value was seen with nicorandil (210 microM). Bimakalim (10 microM)-induced APD90 shortening as a function of stimulation rate was greatest at longest CL. Evidence is provided for (a) species (human vs. guinea pig) and tissue (atrium vs. ventricle) differential AP sensitivity to bimakalim; (b) an approximately 500-fold higher efficacy of bimakalim versus nicorandil to shorten human atrial APD90; and (c) normal use-dependence of human atrial APD90 shortening with bimakalim at 10 microM.

K(ATP)-channel activation: effects on myocardial recovery from ischaemia and role in the cardioprotective response to adenosine A1-receptor stimulation

1. Optimization of myocardial energy substrate metabolism improves the recovery of mechanical function of the post-ischaemic heart. This study investigated the role of K(ATP)-channels in the regulation of the metabolic and mechanical function of the aerobic and post-ischaemic heart by measuring the effects of the selective K(ATP)-channel activator, cromakalim, and the effects of the K(ATP)-channel antagonist, glibenclamide, in rat fatty acid perfused, working hearts in vitro. The role of K(ATP) channels in the cardioprotective actions of the adenosine A1-receptor agonist, N6-cyclohexyladenosine (CHA) was also investigated. 2. Myocardial glucose metabolism, mechanical function and efficiency were measured simultaneously in hearts perfused with modified Krebs-Henseleit solution containing 2.5 mM Ca2+, 11 mM glucose, 1.2 mM palmitate and 100 mu l(-1) insulin, and paced at 300 beats min(-1). Rates of glycolysis and glucose oxidation were measured from the quantitative production of 3H20 and 14CO2, respectively, from [5-3H/ U-14C]-glucose. 3. In hearts perfused under aerobic conditions, cromakalim (10 microM), CHA (0.5 microM) or glibenclamide (30 microM) had no effect on mechanical function. Cromakalim did not affect glycolysis or glucose oxidation, whereas glibenclamide significantly increased rates of glycolysis and proton production. CHA significantly reduced rates of glycolysis and proton production but had no effect on glucose oxidation. Glibenclamide did not alter CHA-induced inhibition of glycolysis and proton production. 4. In hearts reperfused for 30 min following 30 min of ischaemia, left ventricular minute work (LV work) recovered to 24% of aerobic baseline values. Cromakalim (10 microM), administered 5 min before ischaemia, had no significant effect on mechanical recovery or glucose metabolism. CHA (0.5 microM) significantly increased the recovery of LV work to 67% of aerobic baseline values and also significantly inhibited rates of glycolysis and proton production. Glibenclamide (30 microM) significantly depressed the recovery of mechanical function to < 1% of aerobic baseline values and stimulated glycolysis and proton production. 5. Despite the deleterious actions of glibenclamide per se in post-ischaemic hearts, the beneficial effects of CHA (0.5 microM) on the recovery of mechanical function and proton production were not affected by glibenclamide. 6. The data indicate that the cardioprotective mechanism of adenosine A1-receptor stimulation does not involve the activation of K(ATP)-channels. Furthermore, in rat fatty acid perfused, working hearts, stimulation of K(ATP)-channels is not cardioprotective and has no significant effects on myocardial glucose metabolism.

Proarrhythmic effects of DL- and D-sotalol on the "border zone" between normal and ischemic regions of isolated ventricular myocardium and antiarrhythmic effects on reperfusion

Considering the Survival With ORal D-sotalol (SWORD) study results, in which mortality was higher in patients treated by the pure class III agent D-sotalol, we tested DL- and D-sotalol (5 and 10 microM) in an in vitro model of "border zone" arrhythmias. Isolated guinea-pig ventricular strips were partly exposed to normoxia ("Normal Zone," NZ) and partly to modified Tyrode's solution ("Ischemic Zone," IZ) for 15 or 30 min ("ischemia"), followed by return to normoxia for 30 min ("reperfusion"). Resting membrane potential, action potential (AP) amplitude, and maximal upstroke velocity of AP were not significantly modified. DL- And D-sotalol, 5 and 10 microM, lengthened AP duration 90% (APD90) in NZ (p < 0.05), whereas these drugs were unable to prevent ischemia-induced APD shortening. By using the accelerated failure time Weibull's model, and a large number of reference experiments to control random variability of analyzed covariates, DL- and D-sotalol increased significantly the incidence of spontaneous arrhythmias during ischemia (chi2 = 24.79; p = 0.0367): 83 (5 microM D- and DL-sotalol), 86, and 62% (10 microM D- and DL-sotalol, respectively) versus 32% of controls. During reperfusion, 10 microM DL-sotalol prevented the occurrence of spontaneous arrhythmias (chi2 = 46.74; p = 0.0001) similar to what seen with the beta-blocking agent propranolol (10 microM). These data, providing evidence for proarrhythmic effects of DL- and D-sotalol on border-zone arrhythmias, concomitant with differential class III actions on NZ versus IZ, might be considered for understanding the SWORD study results.

Adenosine inhibition of neutrophil damage during reperfusion does not involve K(ATP)-channel activation

This study tests the hypothesis that cardioprotection exerted by adenosine A2-receptor activation and neutrophil-related events involves stimulation of ATP-sensitive potassium (K(ATP)) channels on neutrophils during reperfusion. The adenosine A2 agonist CGS-21680 (CGS) inhibited superoxide radical generation from isolated rabbit polymorphonuclear neutrophils (PMNs) in a dose-dependent manner from 17.7 +/- 2.1 to 7.4 +/- 1.3 nmol/5 x 10(6) PMNs (P < 0.05). Pinacidil, a K(ATP)-channel opener, partially inhibited superoxide radical production, which was completely reversed by glibenclamide (Glib). Incremental doses of Glib in combination with CGS (1 microM) did not alter CGS-induced inhibition of superoxide radical generation. CGS significantly reduced PMN adherence to the endothelial surface of aortic segments in a dose-dependent manner from 189 +/- 8 to 50 +/- 6 PMNs/mm2 (P < 0.05), which was also not altered by incremental doses of Glib. Infusion of CGS (0.025 mg/kg) before reperfusion reduced infarct size from 29 +/- 2% in the Vehicle group to 15 +/- 1% in rabbits undergoing 30 min of ischemia and 120 min of reperfusion (P < 0.05). Glib (0.3 mg/kg) did not change the infarct size (28 +/- 2%) vs. the Vehicle group and did not attenuate infarct size reduction by CGS (16 +/- 1%). Glib did not change blood glucose levels. Cardiac myeloperoxidase activity was decreased in the ischemic tissue of the CGS group (0.15 +/- 0.03 U/100 mg tissue) compared with the Vehicle group (0.37 +/- 0.05 U/100 mg tissue; P < 0.05). We conclude that adenosine A2 activation before reperfusion partially reduces infarct size by inhibiting neutrophil activity and that this effect does not involve K(ATP)-channel stimulation.

Does the opening of ATP-sensitive K+ channels modify ischaemia-induced ventricular arrhythmias in anaesthetised dogs?

These experiments were designed to determine whether there is any change in the severity of ventricular arrhythmias resulting from coronary artery occlusion in anaesthetised mongrel dogs if ATP-sensitive potassium channels were already open at the time of coronary occlusion. To achieve this we locally infused the K(ATP) channel opener levcromakalim, in a total dose of 3 microg/kg, and given by slow infusion over a 30 min period directly into a side branch of the left anterior descending coronary artery. This dose increased blood flow in that main artery by 30% (and by 7% in the adjacent left circumflex artery). The degree of inhomogeneity of electrical activation, measured from the left ventricular wall distal to the occlusion site, was unaffected by levcromakalim administration but there was significant epicardial ST-elevation, perhaps indicating K+ egression from cells. Following coronary artery occlusion there was no marked difference in the severity of arrhythmias between control and levcromakalim-treated dogs, except for an increased number of episodes of ventricular tachycardia due entirely to effects in two of the nine treated dogs. We conclude that opening cardiac K(ATP) channels with levcromakalim, at this one dose level, and administered directly to the left ventricular wall, does not significantly modify arrhythmia severity during ischaemia. These results cannot be extrapolated to studies in which such drugs markedly reduce coronary perfusion pressure.

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.

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.

Sulphonylurea treatment of NIDDM patients with cardiovascular disease: a mixed blessing?

Non-insulin-dependent diabetic (NIDDM) patients show a high incidence of cardiovascular disease, with greater risk of recurrent myocardial infarction and a less favourable clinical outcome than non-diabetic patients. The majority of NIDDM patients are treated with sulphonylurea (SU) derivatives. In the 1970's the University Group Diabetes Program concluded that tolbutamide treatment caused increased cardiovascular mortality; the study, which led to curtailment of oral antidiabetic treatment in the USA, was received with scepticism in Europe. Later criticism of its methodology reduced the impact of the study; however, the question of the safety of SU in NIDDM patients with cardiovascular disease has been re-opened in the face of new experimental data. The heart and vascular tissues do have prerequisites for SU action, i.e. SU receptors and ATP-dependent K+ (K+ATP) channels. These channels play an important role in the protection of the myocardium against ischaemia-reperfusion damage, and their closure by SU could lead to amplified ischaemic damage. Here we review evidence from animal and human studies for deleterious SU effects on ischaemia-induced myocardial damage, either by direct action or through diminished cardioprotective preconditioning. Closure of K+ATP channels by SU can lead to reduction of post-infarct arrhythmias; the drug has also been claimed to improve various atherosclerosis risk factors. The evidence for these beneficial effects of SU is also reviewed. We look at the major difficulties that hamper transfer of information from experimental studies to clinical decision-making: a) The affinity of SU for heart K+ATP channels is orders of magnitude lower than for beta-cell channels; is it reasonable to expect in vivo cardiac effects with therapeutic 'pancreatic' SU doses? b) Most studies utilized high doses of acutely administered SU; are effects similar in the chronic steady-state of the SU-treated diabetic patient? c) Convincing SU effects have been demonstrated in acutely induced ischaemia by acutely administering the drug; do such effects persist in the clinical situation of gradually progressive ischaemia? d) Ischaemia and modification of K+ATP channel activity induce complex events, some with opposing effects; what is the net result of SU action, and do different SU derivatives lead to different outcomes? e) In the chronic (and hence clinically relevant) situation, how can direct (deleterious or beneficial) SU effects be separated from beneficial effects mediated by the metabolic action of the drug? Only large prospective clinical studies, making use of advanced technology for assessment of cardiovascular function, can answer these questions. Millions of NIDDM patients are treated with SU derivatives; many are in the age group where cardiovascular risks are extremely high. The question of whether SU derivatives are beneficial or deleterious for these patients must finally be settle unequivocally.

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.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Ischemic myocardial cell protection conferred by the opening of ATP-sensitive potassium channels

The responses of the cardiac myocyte to a potentially injurious ischemic stress are multiple. The opening of the ATP-sensitive K+ channels may constitute one such response. These channels are present in the plasmalemma at very elevated density and have a large unitary conductance. Consequently, the opening of a small fraction (0.01-0.1%) of these channels during ischemia can help to drive the myocyte into an "emergency" state, in which its syncytial functions become rapidly downregulated and strategies appropriate to preserving cell viability are implemented. Thus, ATP-sensitive K+ channels in cardiac myocytes would appear to be an efficient and apparently redundant natural means of defense against metabolic stress. These channels can undergo physiologic modulation, as occurs during cardiac ischemic preconditioning in several species, including humans. The term cardioprotection refers to an endogenous cardioprotective strategy, whereby the myocardium slows its energy demands, produces fewer toxic glycolytic products, and exhibits reduced injury following a potentially lethal ischemic stress. Openers of cardiac ATP-sensitive K+ channels, a class of drugs that includes, in particular, aprikalim and nicorandil, also afford cardioprotection by reducing the functional and biochemical damage produced by ischemia. Hence, these compounds can improve the recovery of cardiac contractility, reduce the extracellular leakage of intracellular enzymes, delay the loss of ATP, and preserve the cell ultrastructure in isolated heart preparations subjected to transient ischemic conditions. Furthermore, when segmental contractility has been strongly depressed by a stunning insult, nicorandil and aprikalim can accelerate recovery at the reperfusion. Finally, nicorandil and aprikalim decrease substantially the size of the necrotic region that results from a prolonged ischemic insult followed by reperfusion. All of these desirable effects of K(+)-channel openers can be abolished by blockers of ATP-sensitive K+ channels, such as glibenclamide. The fundamental mechanism of the myocyte viability protection conferred by K(+)-channel openers is not yet clear. It may exploit some of the same pathways that mediate cardiac ischemic preconditioning. If this suggestion holds true, drugs opening cardiac ATP-sensitive K+ channels would mimic, exploit, or intensify those cardioprotective means that are naturally available to the cardiac myocyte for overcoming metabolic stress.

The risk of myocardial stunning is decreased concentration-dependently by KATP channel activation with nicorandil before high K+ cardioplegia

BACKGROUND

Drug-induced opening of the adenosine triphosphate-sensitive potassium channel (KATP) during hypoxia and/or ischemia, achieved significant myocardial protection in several in vitro and in vivo models. Pretreatment with KATP openers simulated preconditioning and thus enhanced recovery from ischemia. We have demonstrated that the risk of hypoxia-induced myocardial stunning is reversed by KATP activation with 1 mmol/l nicorandil before cold cardioplegic arrest. Whether lower concentrations were effective is not known.

METHODS

In guinea pig papillary muscle preparations contracting isometrically (driven at 1600 ms cycle), nicorandil was superfused (15 min) either 1 mumol/l (n = 4), 30 mumol/l (n = 4), 100 mumol/l (n = 4), or 1 mmol/l (n = 8) in Tyrode's solution (oxygen content 16 ml/l, 37 degrees C, 5 ml/min). Controls were superfused with saline (Tyrode's solution: n = 8). A group containing vehicle (DMSO 1%, n = 8) was also studied. In four preparations the KATP channel blocker glibenclamide 1 mumol/l was given before nicorandil 1 mmol/l. Then, long-lasting (120 min) but moderately hypoxic (oxygen content 5 ml/l: 31% of Tyrode's solution) superfusion with hypothermic (20 degrees C) high K+ (16 mmol/l) cardioplegic solution (5 ml/min) was performed. Recovery of contractility was evaluated after further 60 min of reoxygenation with Tyrode's solution based on DT/TPT (developed tension divided by time to peak tension) as percent of prehypoxia basal values (%DT/TPT60). DT/TPT was also studied following 15 min of inotropic stimulation with dobutamine 10 mumol/l (%DT/TPT75). To assess the risk of stunning, we used a multivariate linear model by all possible subsets analysis (BMDP-9R) aimed at predicting both %DT/TPT60 and %DT/TPT75 (as continuous dependent variables).

RESULTS

During cardioplegia induction, time to arrest (TTA) was (mean +/- S.D.) 103 +/- 48s in control preparations which had poor recovery of contractility (stunning) after reoxygenation (%DT/TPT60: 71 +/- 20%; %DT/TPT75: 443 +/- 272%). Nicorandil (1 mumol/l-1 mmol/l) abbreviated TTA concentration-dependently (163 +/- 74, 149 +/- 103, 82 +/- 20, and 56 +/- 27s) and improved both %DT/TPT60 (63 +/- 9, 78 +/- 17, 87 +/- 13, and 98 +/- 11%) and %DT/TPT75 (587 +/- 333, 619 +/- 107, 971 +/- 301, and 666 +/- 400%). Glibenclamide reversed the effects of nicorandil 1 mmol/l (TTA: 165 +/- 30 s, P < 0.01; %DT/TPT60: 43 +/- 12, P < 0.01; %DT/TPT75: 272 +/- 147, P < 0.05). Multivariate prediction of myocardial stunning at both 60 and 75 min reoxygenation showed that nicorandil (30 mumol/l-1 mmol/l) was a significant (P < 0.001) protectant whereas glibenclamide was a significant risk factor (P = 0.009). It is unclear whether negative inotropic effects of nicorandil (%DT/TPT at the end of pretreatment) was mechanistically related to reduced risk of stunning since contribution was seen only to predict %DT/TPT75 (t = 3.24, P = 0.003) whereas a positive association was observed with %DT/TPT60 (t = 1.89, P = 0.068).

CONCLUSION

Pretreatment with nicorandil concentration-dependently enhanced the cardioprotective effect of hypothermic high K+ cardioplegia. The risk of myocardial stunning was decreased by KATP opening with nicorandil and increased by KATP block with glibenclamide. Inotropic stimulation with dobutamine might unravel the role of negative inotropic effect of KATP opening as a contributory factor to explain the efficacy of nicorandil in our model.

Cromakalim and cicletanine against pacing-induced myocardial ischemia in conscious rabbits

Myocardial ischemia assessed by intracavital ST-segment elevation, shortening of ventricular effective refractory period (VERP), and increase in left ventricular end-diastolic pressure (LVEDP) was provoked by ventricular overdrive pacing (VOP) in conscious rabbits. Cromakalim (10 micrograms/kg), an ATP-sensitive K+ channel opener, and cicletanine (30 mg/kg), a cGMP-phosphodiesterase inhibitor, reduced VOP-induced ST-segment elevation and LVEDP-increase. Under resting conditions, cromakalim lowered blood pressure, increased heart rate (HR), and shortened VERP, whereas cicletanine decreased HR, prolonged VERP without changing blood pressure. Co-administration of cromakalim and cicletanine additively reduced VOP-induced ST-segment elevation, shortening of VERP, and LVEDP-increase. Cicletanine did not change cromakalim-induced hypotension but abolished reflexogenic tachycardia. This suggests that VERP shortening is not a prerequisite for the anti-ischemic effect of cromakalim, and the combination of these drugs may afford a potent and safe anti-ischemic effect without affecting hypotension induced cromakalim.