Pharmacological Evidence for the Persistent Activation of ATP-Sensitive K+ Channels in Early Phase of Reperfusion and Its Protective Role Against Myocardial Stunning
Background The activation of cardiac ATP-sensitive potassium channels is reported to protect myocardium during ischemia. However, the behavior and role of this channel during reperfusion remain uncertain.
Methods and Results Guinea pig right ventricular walls were studied by use of microelectrodes and a force transducer. Each preparation was perfused via the coronary artery at a constant flow rate and was stimulated at 3 Hz. In the first protocol, the preparation was subjected to 10 minutes of no-flow ischemia, which was followed by 60 minutes of reperfusion. Introduction of ischemia shortened the action potential duration (APD) to 58.7±3.1% of the preischemic values, in association with a decrease in the resting membrane potential (by 12±0.8 mV) and action potential amplitude (by 34.6±1.8 mV). On reperfusion, although the APD was restored, it remained shortened for up to approximately 30 minutes of reperfusion. In the presence of glibenclamide (10 μmol/L), the shortening of the APD during ischemia was significantly attenuated and the restoration of APD after reperfusion was significantly facilitated. When glibenclamide was applied from the onset of reperfusion, the persistent APD shortening was significantly suppressed. The developed tension decreased during ischemia and recovered after 60 minutes of reperfusion (up to 92.0±6.4% of preischemic values) in the untreated preparations. The application of glibenclamide that was started before ischemia or from the onset of reperfusion significantly suppressed the recovery of contractility (P<.05 versus untreated preparations). In the second series of experiments, 20 minutes of no-flow ischemia and 60 minutes of reperfusion were applied. This protocol produced a sustained contractile dysfunction after reperfusion (to 34.0±3.2% of preischemic values). In the presence of cromakalim (2 μmol/L), the APD shortening was enhanced during both ischemia and the early reperfusion period. Cromakalim significantly improved the contractile recovery (to 79.3±4.1% of preischemic values, P<.05 versus untreated preparations). The application of cromakalim that was started from the onset of reperfusion also improved the contractile recovery during this phase and this effect was associated with enhanced APD shortening. However, the cromakalim-treated preparations demonstrated a higher incidence of ventricular fibrillation during reperfusion.
Conclusions Cardiac ATP-sensitive potassium channels are activated by ischemia, and a fraction of these channels remains activated during the early reperfusion phase. The resulting shortening of the APD prevents the heart from developing myocardial stunning.
Cardiac K+ATP channels1 are activated as a result of a decrease in the level of intracellular ATP and shorten the duration of the action potential.2 3 Opening of the channel is also enhanced by increases in intracellular ADP4 and proton concentrations.5 6 All of these changes in the intracellular milieu occur in the setting of ischemia; the physiological role of K+ATP channels under conditions of ischemia is considered important.
Cole et al7 have shown that the activation of this channel plays a role in preserving cardiac function during ischemia. This observation was based on the finding that blockade of K+ATP channels by glibenclamide worsened the recovery of contractility after ischemia, whereas opening of this channel with pinacidil facilitated recovery.7 These findings suggest that the shortening of APD during ischemia, because of the activation of K+ATP channels, is cardioprotective. In line with these results, aprikalim, a new K+ATP channel–opening drug, prevented in vivo myocardial stunning.8 However, little is known about the behavior and pathophysiological role of this channel after reperfusion.
Recently an interesting phenomenon termed ischemic preconditioning has been reported9 in which a brief episode or episodes of ischemia result in an increased tolerance of the myocardium to a subsequent severe ischemia. However, this protective effect is lost when the interval between the initial brief ischemia and the subsequent severe ischemia is longer than 1 hour.10 Gross and Auchampach11 have suggested that activation of the K+ATP channels contributes to this phenomenon. All of these findings suggest that a “short-term memory effect” accounts for ischemic preconditioning. This effect may relate to persistent activation of K+ATP channels that is caused by the preceding brief ischemia.
We investigated the behavior and pathophysiological roles of K+ATP channels and focused our attention particularly on the early phase of reperfusion. We demonstrate here that the activation of K+ATP channels, elicited by preceding ischemia, persists after reperfusion and that this opening of K+ATP channels during the early reperfusion phase makes a major contribution to postischemic contractile recovery.
Preparation of Hearts
Male guinea pigs (300 to 400 g) were killed by cervical dislocation, and after tracheotomy each animal’s chest was opened during artificial respiration. A polyethylene cannula (OD 1.7 mm, ID 1.3 mm) was inserted in a retrograde manner into the ascending aorta. After ligation of the distal part of the aorta, the heart was perfused with oxygenated Tyrode’s solution under positive pressure (approximately 100 mm H2O) via the cannula, and the heart was excised en bloc with the proximal aorta. The proximal left coronary artery was ligated; both atria, the left ventricular free wall, and the ventricular septum were discarded, leaving only the right ventricular free wall and a small basal portion of the left ventricle surrounding the aortic root. All of these procedures were performed without interruption of coronary perfusion. The isolated right ventricular free wall preparation, the coronary artery of which was connected to a cannula by the aortic root, was mounted in the recording chamber and pinned at the base of the ventricle; the coronary perfusate was then delivered through a roller pump (MP-3, Tokyo Rikakikai Co). The flow rate was monitored by a custom-made flowmeter consisting of a droplet sensor (PG-602, PG-610, Keyence) and a pulse counter (AT-600G, Nihon Khoden) that was mounted between polyethylene tubes connecting the roller pump and the coronary artery. The flow rate was maintained at 1.0±0.2 mL · min−1 · g wet wt−1, with an intra-aortic perfusion pressure ranging from 40 to 50 mm H2O. To avoid leaks of the perfusate from the aortic valve, the ascending aorta was slightly stretched (2 to 3 mN) to expand the aortic valve leaflets. The preparation was superfused with substrate-free hypoxic Tyrode’s solution (10 mL/min) gassed with nitrogen to minimize direct oxygen diffusion into the muscles from the epicardial and endocardial surfaces. The temperatures of these solutions were maintained at 37±0.5°C. These procedures ensured a sufficient supply of oxygen and substrate to the tissue, because normal action potentials could be recorded even from very marginal portions of the preparation, and a constant amplitude of twitch contractions was recorded for more than 2 hours after an equilibration period of 90 minutes. At the end of the experiments, 10 mL of indocyanine green was injected into the coronary artery via the cannula to confirm that all preparations had been well perfused without leaks from the aortic valve. All procedures met the guidelines stipulated by the Physiological Society of Japan and the Ethical Committee of Oita Medical University for Animal Experiments.
The heart was stimulated at 3 Hz throughout the experiment by use of a pair of platinum electrodes. They were attached to the basal portion of the preparation and connected to the isolation unit of an electric stimulator (SEN-3201, SS-302J, Nihon Kohden). Square pulses of 5 milliseconds’ duration, with a pulse strength 1.5-fold greater than the threshold, were used to drive the preparation. Action potentials were recorded from a cell that was located deep (usually six to eight cell layers) in the subepicardial surface by use of a flexibly mounted microelectrode that was suspended with a fine silver wire (200 μm OD). Microelectrodes (tip resistance, 20 to 30 MΩ) were made by pulling of filamented capillary tubes (1.5 mm OD, Narishige) with a pipette puller (PE-2, Narishige), and were filled with 3 mol/L KCl. A direct current preamplifier (MEZ-7101, Nihon Kohden) with capacitance compensation was used to record the transmembrane potential. Contractile tension was recorded with a force transducer (TB-612T, Nihon Kohden) connected to the apical end of the preparation. rTension was adjusted to obtain the optimal developed tension (usually 1.0 to 1.5 mN). The membrane potential and contractile tension were monitored on a multibeam oscilloscope (VC-9A, Nihon Kohden) and recorded on a thermal arraycorder (WT-645G, Nihon Kohden). All data were stored on magnetic tapes by use of a PCM data recording system (PCM-501ES, Sony).
The composition of oxygenated Tyrode’s solution was (in mmol/L) NaCl 136.7, NaHCO3 11.9, KCl 5.4, NaH2PO4 0.42, MgCl2 0.5, CaCl2 1.8, and glucose 11 with a pH of 7.35 to 7.40 when gassed with 97% O2 and 3% CO2. Po2 of the solution was measured by an O2 monitor (POG-200BA, Unique Medical) and found to be >400 mm Hg. Hypoxic Tyrode’s solution (Po2<10 mm Hg) had the same composition as above, except it contained no glucose (pH 7.35 to 7.40 gassed with 97% N2 and 3% CO2). Stock solutions of glibenclamide (1 mmol/L; a kind gift from Hoechst Japan Co) and cromakalim (1 mmol/L; Sigma Chemical Co) were made by dissolving these drugs in 5% dimethyl sulfoxide (Sigma Chemical Co). E-4031 (a kind gift from Eizai Pharmaceutical Co) was dissolved in distilled water and kept as a stock solution (10 mmol/L). An appropriate volume of each stock solution was added to the oxygenated Tyrode’s solution immediately before use to make the various final concentrations of each drug described below.
Experimental Protocol and Data Analysis
After equilibration (>90 minutes) the coronary flow was completely stopped by closing of an electromagnetic valve that was placed at the very end of the tubing, resulting in no-flow ischemia of the entire preparation (global ischemia).
In protocol 1, the preparations were subjected to 10 minutes of no-flow ischemia that was followed by 60 minutes of reperfusion, in the absence or presence of glibenclamide, a K+ATP channel blocker. Glibenclamide (10 μmol/L) was applied (1) from 20 minutes before the introduction of no-flow ischemia until the end of the observation period (preischemic treatment, or the pretreatment group) or (2) from the beginning of reperfusion until the end of the observation period (postischemic treatment, or the posttreatment group). In some experiments, E-4031 (1 μmol/L) was used instead of glibenclamide (posttreatment only).
In protocol 2, the preparations were subjected to 20 minutes of no-flow ischemia that was followed by 60 minutes of reperfusion, in the absence or presence of cromakalim, a K+ATP channel opener. Cromakalim (2 μmol/L) was applied (1) from 20 minutes before the introduction of no-flow ischemia until the end of the observation period or (2) from the beginning of reperfusion until the end of the observation period. In some experiments, different concentrations of cromakalim (1 and 5 μmol/L) were applied from 20 minutes before ischemia until the end of the observation period.
All electrical and mechanical parameters that were stored on magnetic tapes were replayed and processed by use of a personal computer (PC-9801, NEC) equipped with an analog-to-digital converter (ADX-98, Canopus). The preparations that developed severe arrhythmias (sustained VT or Vf persisting for more than 30 seconds) were excluded from the electrical and mechanical analyses. Statistical significance was evaluated by two-way ANOVA and paired or unpaired t tests. A P value of less than .05 was regarded as statistically significant.
Under conditions of a normal perfusion, characteristics of the action potentials that were recorded from the right ventricular muscle preparation were comparable to those that have been reported by other investigators.7 12 13 14 After the period of equilibration (>90 minutes), the preparation maintained stable electrical and contractile activities with no evidence of arrhythmias, for example, VPC, VT, or Vf, during a 3-hour period of perfusion (n=3). However, when the perfusion of the preparation was suddenly halted, several important time-dependent changes occurred in both electrical and contractile activities. Representative recordings of electrical and contractile activities before, during, and after 10 minutes of no-flow ischemia are shown in Fig 1⇓. Table 1⇓ shows statistical analyses of the difference in the mean values of APD90, APA, RMP, dTension, and rTension.
Ten minutes of no-flow ischemia produced fully reversible changes in electrical and contractile activities, and no severe arrhythmias (VT or Vf) developed during the ischemia or reperfusion period. The APD90 decreased after the onset of ischemia and reached 59% of preischemic values in 10 minutes. On reperfusion, the APD90 was rapidly restored, although it remained shortened until about 10 minutes of reperfusion (Fig 1⇑ and Table 1⇑). The RMP depolarized by 12 mV during 10 minutes of ischemia, and this depolarization persisted for approximately 10 minutes after the initiation of reperfusion. The APA was also reduced by ischemia, to 71% of its preischemic value, and this reduction persisted for 30 minutes after reperfusion (Table 1⇑).
To determine the role of K+ATP channels in the shortening of APD during ischemia and the early reperfusion period, which was associated with decreases in the RMP and APA, we examined the effect of glibenclamide, a potent blocker of K+ATP channels, on these action potential changes.
Effect of Glibenclamide on Action Potential
Fig 2⇓ shows the effects of glibenclamide on changes in APD90 before, during, and after 10 minutes of no-flow ischemia. Application of glibenclamide (10 μmol/L) was started either (1) 20 minutes before introduction of ischemia (pretreatment group, Fig 2A⇓) or (2) at the onset of reperfusion (posttreatment group, Fig 2B⇓). Statistical analyses of the change in electrical and contractile parameters are summarized in Table 1⇑.
Glibenclamide had no effect on electrical activity (APD90, RMP, or APA) or mechanical activity (rTension or dTension) under normal (preischemic) conditions. However, in the presence of glibenclamide (pretreatment group), the extent of APD shortening caused by ischemia was markedly attenuated, and the recovery of APD90 after reperfusion was faster than in the untreated group (Fig 2A⇑). The APD90 shortened only by 28.1±2.8% during 10 minutes of ischemia in treated preparations versus 41.3±3.1% in untreated preparations (P<.05). The APD90 was restored to up to 98.6±2.6% of the preischemic values within 5 minutes of reperfusion in treated preparations versus 90.5±2.7% in untreated preparations (P<.05). Pretreatment with glibenclamide slightly facilitated the depolarization of RMP after 10 minutes of ischemia but had no effect on APA. After reperfusion the drug slightly accelerated the rate of recovery of APA but diminished the recovery of RMP.
To gain further insight into the contribution of K+ATP channels to the persistent shortening of APD in the early reperfusion phase, we administrated glibenclamide (10 μmol/L) starting at the onset of reperfusion (Fig 2B⇑). The time course of APD shortening during ischemia was identical to that previously seen in untreated preparations; however, during reperfusion, the APD90 returned to preischemic values more rapidly than in the untreated group (98.0±1.7% of the preischemic values at 5 minutes of reperfusion versus 90.5±2.7% in untreated preparations, P<.05). The APA returned to preischemic values slightly faster in treated than in untreated preparations, although the recovery of RMP was significantly delayed in the former (Table 1⇑). These observations lend support to the notion that an outward K+ current through K+ATP channels, which is activated during ischemia, remains activated in the early reperfusion period, accounting for sustained APD shortening in this phase of reperfusion.
Effects of Glibenclamide on Contraction
Application of glibenclamide attenuated the shortening of APD not only during ischemia but also in the early phase of reperfusion. Therefore, this drug is expected to indirectly affect the contractile function in addition to APD shortening during ischemia/reperfusion. Fig 3⇓ shows representative recordings of contractile tension before, during, and after 10 minutes of no-flow ischemia with or without glibenclamide. Percent changes of dTension are shown in Fig 4⇓. In untreated preparations, dTension declined rapidly after the initiation of ischemia and was lost within 10 minutes; this occurred without significant changes in rTension (Fig 3A⇓). After reperfusion, dTension gradually recovered, reaching 92.0±6.4% of preischemic values in 60 minutes (Figs 3A⇓ and 4⇓). rTension was slightly but significantly elevated on reperfusion and eventually returned to the preischemic level (Table 1⇑).
Pretreatment with glibenclamide (10 μmol/L) caused a significant elevation of rTension during ischemia that persisted for 60 minutes of reperfusion (Fig 3B⇑ and Table 1⇑). dTension was restored quickly on reperfusion and reached a peak value within 7 to 8 minutes of reperfusion, although this dTension decreased again later in reperfusion (Fig 4⇑). The net recovery of dTension estimated at 30 and 60 minutes of reperfusion was significantly lower in the glibenclamide pretreatment preparations than in untreated preparations (Table 1⇑).
In contrast, when drug was applied from the start of reperfusion, ie, posttreatment, a rapid and transient increase of dTension appeared on reperfusion (Figs 3C⇑ and 4⇑) and was followed by a subsequent decrease, identical to the findings in the pretreatment group. The dTension measured at 10, 30, and 60 minutes of reperfusion was significantly lower than in untreated preparations (Table 1⇑). The rTension of posttreated preparations, measured at 30 and 60 minutes after reperfusion, was significantly greater than that in untreated preparations (Table 1⇑).
To further examine the effect of rapid prolongation of APD on the recovery process of contraction, we applied a new class III antiarrhythmic agent, E-4031,15 instead of glibenclamide. E-4031 (1 μmol/L) significantly prolonged the APD (from 156±2 to 180±3 milliseconds) without affecting contractility under control (preischemic) perfusion. Application of this agent starting from the onset of reperfusion that followed 10 minutes of ischemia significantly suppressed the recovery of developed tension measured at 60 minutes after reperfusion (72.6±1.8% of preischemic values in treated preparation (n=5) versus 92.0±6.4% in untreated preparation (n=7); P<.01). This effect was associated with a faster recovery of the APD after reperfusion.
Effects of Glibenclamide on Reperfusion Arrhythmias
There was no evidence of severe arrhythmias before or during ischemia in the presence or absence of glibenclamide. In untreated preparations, during reperfusion, we observed sporadic VPCs in four of seven preparations and short runs of VT (persisting for less than 3 seconds) in one preparation. In the presence of glibenclamide (in both pretreatment and posttreatment groups), VPCs were observed in all preparations tested (n=5 for both), and short runs of VT developed in three of five pretreatment preparations and in two of five posttreatment preparations. Sustained VT or Vf was not seen in either group. Thus, glibenclamide increased the incidence of VPCs and short runs of VT in the reperfusion phase but did not produce prolonged VT or Vf.
Effects of Cromakalim on Electrical and Mechanical Activities During Ischemia and Reperfusion
In the second series of experiments, we examined the effect of cromakalim, a K+ATP channel opener, by use of a protocol entailing 20 minutes of no-flow ischemia that was followed by 60 minutes of reperfusion. We used a 20-minute ischemic period (instead of the 10-minute ischemia used with glibenclamide) because this ischemic period produced a sustained contractile depression after 60 minutes of reperfusion.
The alterations of electrical and contractile parameters during 20 minutes of no-flow ischemia and 60 minutes of reperfusion in the presence or absence of cromakalim are summarized in Table 2⇓, and the representative changes in action potential configuration and the time course of changes in APD90 are shown in Fig 5⇓. In untreated preparations ischemia markedly shortened the APD90, and in two of four preparations the electrical excitability was completely lost in 17 minutes. In these preparations, action potentials could not be elicited even when the intensity of stimulation was increased 10-fold. This electrical inexcitability was probably not the result of a decrease in the RMP, ie, by means of inactivation of the inward sodium current, because the RMP, at −69.5±0.2 mV, remained more negative than −60 mV at the end of 20 minutes of ischemia (n=4). Consequently, this electrical inexcitability may be due to a large increase in the outward current mediated by K+ATP channels. The amplitude of the action potential declined progressively during the ischemic period. The ischemia significantly depolarized the RMP (Table 2⇓). On reperfusion, all electrical parameters (APD90, APA, and RMP) returned essentially to preischemic levels within 60 minutes; however, contractile parameters (rTension and dTension) were not fully restored. The contractile tension remained depressed (≈34% of preischemic contraction) even at 60 minutes of reperfusion, and resting tension remained elevated (Table 2⇓). These impaired contractile parameters did not return to normal even when the reperfusion period was extended to 3 hours (n=2), indicating that 20 minutes of no-flow ischemia produced myocardial stunning, a sustained postischemic depression of contractile function.
Under control (preischemic) conditions, cromakalim (2 μmol/L) slightly but significantly shortened the APD90 (by 8.8±0.7%) with no apparent effects on APA, RMP, or contractile parameters (Table 2⇑). During ischemia and reperfusion, however, this agent markedly modified the action potentials and contractility (Figs 5⇑ and 6⇓, Table 2⇑). Cromakalim shortened the APD90 more quickly during ischemia and led to electrical inexcitability in all preparations tested (n=5) at a much earlier time (≈15 minutes) than in untreated preparations (Fig 5⇑). The shortening of APD90 in the early reperfusion phase was greater and was sustained much longer in the presence of cromakalim. However, changes in RMP during ischemia were not altered by cromakalim, and the recovery of RMP during reperfusion was facilitated (Table 2⇑). The dTension in cromakalim-treated preparations tended to decline slightly faster during ischemia, and exhibited much greater recovery after reperfusion than in untreated preparations (Fig 6⇓). At 60 minutes after reperfusion, dTension was restored to 79.3±4.1% of the preischemic values in the cromakalim-treated preparations versus only 34.0±3.2% in untreated preparations (P<.05). The elevation of rTension during ischemia was not affected by the presence of cromakalim (Table 2⇑), although the reperfusion-induced increase of rTension was significantly suppressed in the treated preparations.
We examined the effect of cromakalim at higher (5 μmol/L) or lower (1 μmol/L) concentrations. Cromakalim at 5 μmol/L also improved contractile functions during reperfusion; however, this concentration exerted a significant negative inotropic effect (by ≈30%) in preischemic conditions (data not shown). In contrast, 1 μmol/L cromakalim hardly improved the recovery of contraction during reperfusion (n=3, data not shown).
Because the application of cromakalim before ischemia remarkably improved the contractile recovery, we examined the effect of cromakalim applied from the onset of reperfusion. The application of cromakalim (2 μmol/L) enhanced the shortening of APD during reperfusion compared with untreated preparations; ie, the APD90 was 112.0±2.4 milliseconds at 10 minutes and 136.0±1.8 milliseconds at 30 minutes of reperfusion (n=5, P<.05 versus preparations). In contrast, the dTension measured at 60 minutes of reperfusion was significantly greater (48.7±3.2% of preischemic values, n=7; P<.05) than that in untreated preparations (34.0±3.2% of preischemic values, n=4).
Effects of Cromakalim on Reperfusion Arrhythmias and Modification by Glibenclamide
In untreated preparations, reperfusion of ventricular muscles after 20 minutes of no-flow ischemia resulted in arrhythmias more frequent and more severe than those seen with reperfusion after 10 minutes of ischemia. VPCs and/or short runs of VT were observed in all preparations tested (n=6). Sustained VT (with a duration of 108 seconds) and Vf (not spontaneously defibrillated) were each observed in one preparation. Treatment with cromakalim (2 μmol/L) before ischemia did not increase the number of VPCs or short runs of VT on reperfusion; however, sustained VT (with an average duration of 53 seconds) occurred in two (20%) and Vf in four (40%) of the preparations tested (n=10). Application of cromakalim (2 μmol/L) from the beginning of reperfusion also increased the incidence of VT (29%) and Vf (57%); however, these arrhythmias disappeared spontaneously within 3 minutes in all cases (n=7). These findings suggested that the presence of cromakalim tends to increase the risk of VT and Vf.
Fig 7⇓ shows a typical example of reperfusion-induced arrhythmias observed in the presence of a relatively high concentration of cromakalim (5 μmol/L). Reperfusion after 20 minutes of no-flow ischemia immediately produced sustained VT (Fig 7E⇓), which deteriorated to Vf (Fig 7F⇓). Because Vf persisted for more than 5 minutes, glibenclamide (10 μmol/L) was applied in the continued presence of cromakalim to examine the contribution of K+ATP current to this arrhythmia. With the introduction of glibenclamide, Vf terminated; this termination was preceded by a rapid prolongation of APD (Fig 7G⇓). The same effect of glibenclamide was seen in two of three cases of Vf after treatment with 2 μmol/L cromakalim. In our experience, Vf with a duration of more than 3 minutes never terminated spontaneously in the absence of glibenclamide. Glibenclamide might have terminated Vf by lengthening the APD or the refractory period by blockade of K+ATP channels. It must be noted here that the recovery of contraction in cromakalim-treated tissues was fairly good despite the development of Vf (Fig 7H⇓). This finding implies that the development of Vf did not result from the ischemia/reperfusion-induced calcium overload (which usually leads to triggered activity arising from delayed afterdepolarization).
Recently the cardioprotective effect of K+ATP channel activation during ischemia has attracted much attention. Numerous attempts have been made to clarify the mechanisms of cardioprotection of this channel. However, there has been no report describing the role of this channel after reperfusion. We first demonstrated the possibility that the activation of K+ATP channels elicited by preceding ischemia persists for a fairly long time (≈30 minutes) after reperfusion and contributes to the contractile recovery during this period.
The shortening of the APD produced by no-flow ischemia persists during the early phase of reperfusion, and this APD shortening is preventable by glibenclamide applied from the onset of reperfusion. Therefore, the persistent APD shortening was mostly attributed to the residual activation of K+ATP channels. This persistent APD shortening, although relatively small compared with the APD shortening observed during ischemia, is nonetheless important for the recovery of contractile function and the evolution of arrhythmias during reperfusion. In theory, shorter APDs would lead to a reduction in the time for Ca2+ influx via voltage-gated Ca2+ channels and to an increase in the time during which the Na+-Ca2+ exchanger may operate to extrude Ca2+.16 17 Resultant decreases in transsarcolemmal Ca2+ influx would help maintain [Ca2+]i at physiological levels when other Ca2+ extrusion mechanisms such as the Ca2+ pump are impaired because of a reduction of [ATP]i.18 Accordingly, in the very early reperfusion phase when the Ca2+ extrusion mechanisms have not been fully restored, the application of glibenclamide, started even at the onset of reperfusion, might increase the [Ca2+]i compared with that seen in the absence of glibenclamide. The rise in [Ca2+]i and the concomitant increase in ATP consumption (due to enhanced contraction) might have inhibited the subsequent recovery of contractility (Fig 4⇑). This notion is supported by the finding that glibenclamide produced a rapid but only transient increase in contractions in the very early phase of reperfusion, which led to a secondary sustained decline of contractility later during the reperfusion phase (Figs 3⇑ and 4⇑). In preliminary experiments, we applied glibenclamide (10 μmol/L) at 15 minutes after reperfusion that followed 10 minutes of no-flow ischemia. On application, the action potential quickly prolonged, and this was associated with an increase in contraction. However, this increase persisted only for less than 2 minutes and was followed by a severe decline of the contraction in the later phase of reperfusion.19 We propose that the rapid prolongation of APD caused by glibenclamide applied during the reperfusion period increases [Ca2+]i and produces a transient increase in contraction. This is, however, followed by a subsequent severe decline in contractility, probably because of the occurrence of a calcium overload.
A similar reperfusion-induced transient increase in contraction (followed by a secondary decline) has been noted in pigs in vivo and is termed postischemic hypercontraction.20 The mechanism of this phenomenon has been attributed to a rapid and excessive increase of [Ca2+]i that occurred in the early phase of reperfusion. These findings lend support to the notion that most of the calcium overload develops during reperfusion rather than during ischemia, resulting in depressed contractile recovery associated with an elevation of resting tension. Cascio et al21 have shown that electrical cell-to-cell uncoupling occurs simultaneously with an increase in rTension. This indicates that Ca2+-induced electrical uncoupling is also a consequence of application of glibenclamide.
On the other hand, short APDs that are caused by activation of K+ATP channels may decrease the contraction of the heart muscle and consequently help preserve [ATP]i by reducing energy consumption during ischemia.1 22 However, it is unlikely that inhibition of the decline of contractions during ischemia is a prime cause of the deleterious effect of glibenclamide on the contractile recovery seen in the late reperfusion period. This is because glibenclamide impaired contractile recovery, even when it was applied from the onset of reperfusion (Fig 4⇑).
Endogenous activation of K+ATP channels during ischemia has recently been reported to improve contractile recovery; however, this cardioprotective effect did not correlate with the preservation of high-energy phosphate.23 Moreover, verapamil, a Ca2+ antagonist, produced dose-dependent protection against ischemic injury in the globally ischemic rat heart. However, this effect could not be entirely attributed to the preservation of [ATP]i and creatine phosphate.24 These findings are consistent with our speculation that the major protective effect of K+ATP channels (activated by ischemia) is inhibition of the development of calcium overload (by shortening of APD).
To support our hypothesis, we examined the effect of E-4031 on the recovery of contractions after ischemia. The application of E-4031 starting from the onset of reperfusion significantly suppressed the recovery of contractility, which was associated with longer APD compared with untreated preparations. These results are in line with our hypothesis that the rapid APD prolongation in the early reperfusion phase is deleterious to the recovery of contraction, even when the APD prolongation is made by means of blockade of K+ channels (eg, delayed-rectifier K+ channels) other than K+ATP channels.
Glibenclamide is reported to inhibit K+ATP channels by binding to a specific site on the channel protein.25 However, these sulfonylurea drugs also affect cardiac metabolism. Glibenclamide stimulated glycolytic ATP synthesis without changes in oxygen consumption and did not elicit a positive inotropic effect in rat hearts.26 Impaired postischemic recovery of contractility found in glibenclamide-treated preparations may not be attributed to this metabolic effect, because an enhancement of ATP production is more likely to improve (and not impair) the recovery of contractility.
K+ATP channels are also involved in the regulation of coronary blood flow,27 as demonstrated by the prevention by glibenclamide of hypoxia-induced coronary vasodilatation in isolated guinea pig hearts; moreover, these channels regulate the basal coronary flow in anesthetized dogs.28 However, this effect of glibenclamide cannot account for the impairment of postischemic contractile recovery, because the rate of coronary flow was constant in our experiments. Furthermore, the decrease of coronary flow by glibenclamide cannot explain the transient increase of contraction seen immediately after introduction of this drug (Fig 3C⇑).
Cromakalim improved mechanical recovery and ameliorated the development of myocardial stunning. It has been postulated that some K+ATP channel openers prevent irreversible cell injury during ischemia/reperfusion.7 29 30 Cromakalim increases coronary flow by means of its vasodilating effect.31 However, in our study the rate of perfusion was constant throughout the experiments. The protective effects of cromakalim could be attributable, at least in part, to a reduction of energy consumption due to the decline in contractility.32 Such an energy-sparing effect may facilitate contractile recovery during reperfusion. However, in our experiments this mechanism did not appear to play a prime role, because the rate of decline in contractions seen during ischemia in the presence of cromakalim did not differ statistically from that found in the absence of this drug (Table 2⇑), although it tended to occur slightly faster (Fig 6⇑). Accordingly, we attribute the protective effect of cromakalim mainly to the attenuation of the increase in [Ca2+]i during ischemia and reperfusion. This hypothesis is supported by a finding that the rise in rTension after reperfusion was significantly less in the presence of cromakalim compared with untreated preparations (Table 2⇑). The reduction of rTension may reflect a decrease in the resting level of [Ca2+]i.
It should be mentioned that the recovery of RMP was significantly faster and greater in the cromakalim-treated group (Table 2⇑). This faster and greater recovery of RMP is probably mediated by the increase of outward current via cromakalim-activated K+ATP channels and seems to be beneficial for the contractile recovery. The extrusion of cytosolic calcium via the (electrogenic) Na+-Ca2+ exchanger may be facilitated in such deeper RMPs, thereby attenuating the development of calcium overload.
Cromakalim improved contractile recovery even when it was applied from the onset of reperfusion. This improvement was associated with an enhanced shortening of the APD during the reperfusion period. However, the degree of improvement in contractility was considerably lesser than that seen with pretreatment. This finding implies that the cell injury developing during prolonged ischemia (20 minutes) was attenuated by cromakalim in pretreated preparations.
Cole et al7 have reported that glibenclamide (10 μmol/L) increased the incidence of reperfusion-induced triggered activity, and sustained VT, originating from delayed afterdepolarization. In our observation this agent increased the incidence of VPCs and short runs of VT in the reperfusion phase but did not produce sustained VT or Vf. Differences between results obtained with similar experimental methods may be due to differences in the stimulating frequencies and ischemic periods that were applied. Cole et al examined the effects of glibenclamide by using 20 minutes of no-flow ischemia with a stimulation frequency of 2 Hz, whereas we used only 10 minutes of ischemia and a stimulation frequency of 3 Hz. Therefore, their ischemic insult might have been greater than ours.
Alternatively, the application of cromakalim increased the incidence of Vf during reperfusion in our study. We could not find any difference in action potential parameters (APD, RMP, and APA) between the groups that developed or did not develop Vf. However, because the Vf that developed in cromakalim-treated preparations was readily terminated by the application of glibenclamide, we speculate that enhanced activation of K+ATP channels by cromakalim provides a substrate for reentry, ie, shortening of the refractory period. The arrhythmogenic actions of K+ATP channel openers in acute ischemia have been reported by other investigators.33 34 35 Cole et al7 have claimed that application of pinacidil decreases the incidence of reperfusion arrhythmias, the opposite of our findings. The difference between the results may, again, arise from different lengths of the ischemic periods: they used 30 minutes of ischemia whereas we used 20 minutes. Prolonged ischemia results in much greater increases in [Ca2+]i and produces triggered arrhythmias arising from delayed afterdepolarization. Shortening of APD by a K+ATP channel opener might diminish this type of arrhythmia by decreasing [Ca2+]i. Alternatively, the tachyarrhythmias we observed after 20 minutes of ischemia (Fig 7E⇑ through 7G) might have been provoked by a reentry mechanism; these arrhythmias were readily terminated by glibenclamide, which prolonged the APD and hence the effective refractory period.
Comparison of the effects of K+ATP channel blockers and openers may shed some light on the problem of a straightforward antiarrhythmic strategy in reperfusion arrhythmias. This is because both potentiation of action potential shortening (reentrant arrhythmia) and action potential lengthening (increase in [Ca2+]i and resulting delayed afterdepolarizations) may precipitate arrhythmias at a critical time of reperfusion. Furthermore, the effects of both interventions on electrical cell-to-cell uncoupling remain to be determined.
Brief episodes of ischemia have been reported to increase the tolerance of the heart muscle to a subsequent severe ischemic insult; this effect has been termed ischemic preconditioning.9 In addition, Gross and Auchampach11 have reported that glibenclamide prevented this preconditioning effect in dogs. Furthermore, the ischemic preconditioning effect was lost when the interval between the foregoing short-term ischemia and the subsequent long and severe ischemia was prolonged to more than 1 hour.10 The mechanism of this short-term memory effect in preconditioning is uncertain. In the present study we observed that a substantial number of K+ATP channels remained activated over the early reperfusion phase. From these observations, it is tempting to speculate that persistent activation of K+ATP channels in early reperfusion is responsible for the memory effect of ischemic preconditioning. Under such circumstances, the activation threshold of the K+ATP channels to subsequent ischemia may be lowered such that these channels could be more readily activated by the subsequent ischemia, resulting in better tolerance of the myocardium to this second ischemic insult.
Selected Abbreviations and Acronyms
|APA||=||action potential amplitude|
|APD||=||action potential duration|
|APD90||=||APD at 90% repolarization|
|dTension||=||amplitude of developed tension|
|K+ATP channels||=||ATP-sensitive potassium channels|
|RMP||=||resting membrane potential|
|VPC||=||ventricular premature contraction|
The authors gratefully acknowledge Dr A.G. Kléber, Department of Physiology, University of Bern, Switzerland, for his critical readings and valuable suggestions on the manuscript and M. Ohara for comments on the manuscript. We thank K. Moriyama for secretarial assistance.
- Received October 6, 1994.
- Revision received March 9, 1995.
- Accepted May 4, 1995.
- Copyright © 1995 by American Heart Association
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