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(Circulation. 1996;93:178-186.)
© 1996 American Heart Association, Inc.

Articles

Rapid Ventricular Pacing Produces Myocardial Protection by Nonischemic Activation of K_ATP⁺ Channels

Monique M.G. Koning, MD; Ben C.G. Gho, MD; Erik van Klaarwater, BS; René L.J. Opstal, BS; Dirk J. Duncker, MD, PhD; Pieter D. Verdouw, PhD

From the Laboratory for Experimental Cardiology, Thoraxcenter, Erasmus University Rotterdam, Rotterdam, the Netherlands.

Correspondence to Pieter D. Verdouw, PhD, Laboratory for Experimental Cardiology, Thoraxcenter, Erasmus University Rotterdam, PO Box 1738, 3000 DR Rotterdam, the Netherlands. E-mail Verdouw@tch.fgg.eur.nl.

	Abstract

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Background Rapid ventricular pacing reduces the incidence of ventricular arrhythmias during a subsequent sustained period of ischemia and reperfusion. We investigated whether rapid ventricular pacing also limits myocardial infarction and determined the role of K_ATP⁺ channels in the protection afforded by ventricular pacing.

Methods and Results Myocardial infarction was produced by a 60-minute coronary artery occlusion in open chest pigs. Infarct size of pigs subjected to 10 minutes of ventricular pacing at 200 beats per minute followed by 15 minutes of normal sinus rhythm before the occlusion (79±3% of the area at risk, mean±SEM) was not different from control infarct size (84±2%). Thirty-minute pacing followed by 15-minute sinus rhythm resulted in modest reductions in infarct size (71±2%, P<.05 versus control). Thirty minutes of pacing immediately preceding the occlusion without intervening sinus rhythm resulted in considerable limitation of infarct size (63±4%, P<.05), which was abolished by pretreatment with the K_ATP⁺ channel blocker glibenclamide (78±4%, P=NS). K_ATP⁺ channel activation did not appear to involve ischemia: (1) myocardial endocardial/epicardial blood flow ratio was 1.07±0.08, (2) phosphocreatine and ATP levels and arterial-coronary venous differences in pH and PCO₂ were unchanged, (3) end-systolic segment length did not increase and postsystolic shortening was not observed during pacing, and (4) systolic shortening recovered immediately to baseline levels and coronary reactive hyperemia was absent after cessation of pacing. Administration of glibenclamide after 30 minutes of pacing at the onset of 15 minutes of normal sinus rhythm did not attenuate the protection (73±3%, P<.05 versus control), suggesting that K_ATP⁺ channels did not contribute to the moderate degree of protection that was still present 15 minutes after cessation of pacing.

Conclusions Rapid ventricular pacing protects the myocardium against infarction via nonischemic K_ATP⁺ channel activation. Continued activation of K_ATP⁺ channels does not appear mandatory for the protection that is still present 15 minutes after cessation of pacing.

Key Words: blood flow • tachycardia • myocardial infarction

	Introduction

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Myocardial preconditioning can be induced by a variety of ischemic stimuli. Thus, one or more brief total¹ or partial^{2 3 4} coronary artery occlusions can limit infarct size produced by a sustained ischemic period. Moreover, infarct size can be limited by transient ischemia in adjacent myocardium⁵ or even different organs.^{6 7} In all these studies, a temporary interruption or restriction of oxygen supply either within or outside the myocardial region of interest was required to produce protection. Recent studies suggest that nonischemic stimuli also may protect the myocardium. Thus, Ovize et al⁸ reported that an increase in left ventricular wall stretch produced by acute volume overload protected the myocardium against infarction during a subsequent 60-minute coronary artery occlusion. Also, two consecutive 2-minute periods of RVP in open chest dogs reduced the incidence of ventricular arrhythmias during and immediately after subsequent 25-minute coronary artery occlusion.⁹ In contrast, Marber et al¹⁰ failed to show a protective effect of a single 5-minute period of rapid atrial pacing against myocardial infarction in the rabbit heart. To date, no study has addressed the effect of RVP on infarct size development produced by a sustained coronary artery occlusion.

In the present study, we investigated whether RVP preceding a 60-minute TCO altered infarct size development in open chest pigs. In two groups of animals we studied the effects of either 10 or 30 minutes of RVP followed by 15 minutes of NSR on infarct size produced by 60-minute TCO, analogous to the classic preconditioning model of a brief ischemic stimulus followed by reperfusion. In view of our earlier findings that ischemia produced by a partial coronary artery occlusion can precondition the myocardium without the need of intervening reperfusion,³ we also studied a third group of animals in which the 30-minute RVP period preceded the 60-minute TCO without NSR. If RVP produces protection by causing ischemia,^{9 11 12} protection is likely to be distributed heterogeneously across the left ventricular wall because ischemia would occur predominantly in the inner layers. Consequently, infarct size also was determined for the outer and inner halves of the left ventricle. To explore the mechanism of myocardial protection produced by RVP, we investigated whether ischemia occurred in animals subjected to 30 minutes of RVP followed by 180 minutes of NSR without 60-minute TCO. Also, in view of evidence that ventricular pacing can activate ventricular K⁺ channels^{13 14} and that activation of K_ATP⁺ channels is cardioprotective in pigs,^{15 16} we also studied the role of K_ATP⁺ channels in protection produced by RVP. For this purpose, one group of animals that was subjected to 30 minutes of RVP immediately followed by 60-minute TCO were pretreated with the K_ATP⁺ channel blocker glibenclamide before the pacing period. In another group of animals that was subjected to 30 minutes of RVP followed by 15 minutes of NSR before the 60-minute TCO, glibenclamide was administered at the onset of the intervening NSR period to study the importance of continued K_ATP⁺ channel activation for the protection that was present 15 minutes after cessation of pacing.

	Methods

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All experiments were performed in accordance with the Guiding Principles in the Care and Use of Animals as approved by the Council of the American Physiological Society and under the regulations of the Animal Care Committee of the Erasmus University Rotterdam.

Experimental Groups
Studies were performed in a total of 59 pigs assigned to seven experimental groups (Fig 1). In six groups, animals underwent a 60-minute TCO followed by 120 minutes of reperfusion. Twelve animals served as controls and underwent only a single 60-minute TCO (group 1). Twenty animals underwent a 60-minute TCO preceded by either 10 minutes (n=6, group 2) or 30 minutes (n=14, groups 3 and 4) of RVP at 200 beats per minute (bpm) followed by 15 minutes of NSR. Seven of the animals subjected to 30-minute RVP received 1 mg/kg IV glibenclamide at the onset of the 15 minutes of NSR period after the 30-minute RVP (group 4). In 20 animals, the 60-minute TCO was preceded by 30-minute RVP at 200 bpm without an intermittent period of NSR (groups 5 and 6); 8 of these animals were pretreated with glibenclamide (1 mg/kg IV) 10 minutes before the start of RVP (group 6). The dose of glibenclamide was chosen because it was previously shown to block preconditioning by a single 10-minute coronary occlusion in pigs without extending infarct size produced by 60-minute TCO in control animals.¹⁷ In these two groups of animals ventricular pacing was terminated immediately (<10 seconds) after the start of the 60-minute LADCA ligation. To evaluate whether RVP produced detectable myocardial ischemia, we determined wall function in the distribution area of the LADCA, high-energy phosphates, oxygen consumption, and regional myocardial blood flow in 7 animals throughout a 30-minute period of RVP at 200 bpm followed by 180 minutes of NSR (group 7).

View larger version (22K): [in this window] [in a new window]   Figure 1. Chart shows experimental groups used in the study of the effect of rapid ventricular pacing (RVP) on infarct size produced by a 60-minute total coronary artery occlusion (TCO). Filled bars indicate 60 minutes of TCO; hatched bars, RVP. NSR indicates normal sinus rhythm; Rep, reperfusion; and Glib, glibenclamide (1 mg/kg IV).

Surgical Procedure
Domestic Yorkshire-Landrace pigs (weight, 25 to 35 kg; HVC, Hedel, the Netherlands) were sedated with ketamine (20 mg/kg IM), anesthetized with pentobarbital (25 mg/kg IV), and instrumented for measurement of arterial and left ventricular pressure and control of arterial blood gases.¹⁸ After administration of pancuronium bromide (4 mg IV, Organon Teknika BV) and a midline thoracotomy, an electromagnetic flow probe (Skalar) was placed around the ascending aorta to measure cardiac output (Fig 2). The LADCA was dissected free from the surrounding tissue to allow placement of a microvascular clamp (groups 1 through 6) and a Doppler flow probe (Crystal Biotech Inc) (groups 2 through 7). In the animals that underwent RVP, an electrode was attached to the anteriolateral left ventricular wall in the vicinity of the apex for stimulation of the myocardium by electrical monophasic stimuli with an amplitude of 2 mA and a frequency of 3.33 Hz. A small cannula was inserted into the vein accompanying the LADCA for the withdrawal of local venous blood for the determination of blood gases.

View larger version (29K): [in this window] [in a new window]   Figure 2. Schematic presentation of the experimental model. The pacing electrode was inserted into the superficial layers of the anteriolateral wall of the left ventricle (LV). Pairs of ultrasonic crystals were inserted into the mesocardial layers of the left anterior descending coronary artery (LADCA) area and the left circumflex coronary artery (LCXCA) area. The LADCA was occluded at the site of the clamp, causing the shaded area to become ischemic. EM indicates electromagnetic; SL, segment length.

Regional Myocardial Function
In all seven groups, pairs of ultrasonic crystals (Sonotek Corp) were positioned into the midmyocardial layers of the left ventricle in the distribution areas of the LADCA and the LCXCA for the measurement of regional segment shortening by sonomicrometry^{17 18} (Triton Technology Inc) (Fig 2). From the segment length tracings, segment length at the end of diastole (EDL, onset of positive ascending aortic flow) and the length at the end of systole (ESL, end of positive aortic flow) were determined and regional systolic segment shortening (SS) was computed as

and postsystolic segment shortening (PSS) was calculated as

Regional Myocardial Blood Flows
In the animals of group 7 (Fig 1), we investigated the effects of RVP on the distribution of transmural myocardial blood flow. For this purpose, the left atrial appendage was cannulated for injection of 1 to 2 · 10⁶ microspheres, 15±1 µm in diameter (NEN Co), labeled with either ⁹⁵Nb, ¹⁰³Ru, ¹¹³Sn, ⁴⁶Sc, or ¹⁴¹Ce. Processing of myocardial tissue samples and computation of blood flow data have been described earlier.¹⁸

High-Energy Phosphate Metabolism
High-energy phosphates were measured in transmural myocardial biopsies, taken with a Tru-Cut needle (Travenol Laboratories Inc) from the area perfused by the LCXCA at baseline and immediately before the 60-minute TCO. This procedure allowed assessment of the effects of ventricular pacing on high-energy phosphate metabolism without interfering with the infarct size determination in the area perfused by the LADCA. Biopsies were immediately dipped into 0.9% NaCl at 0°C to remove adherant blood, frozen in liquid nitrogen (within 10 seconds) and stored until analysis at -80°C. Adenine nucleotides (ATP, ADP, AMP), creatine (Cr), and creatine phosphate (CrP) were measured by isocratic ion pairing high-perfomance liquid chromatography as previously described.¹⁸ From these measurements, CrP/Cr and CrP/ATP ratios were calculated to estimate changes in oxidative phosphorylation potential. Energy charge was calculated as (ATP+0.5 ADP)/(ATP+ADP+AMP).¹⁹

Experimental Protocols
After completion of the instrumentation, 5000 IU of heparin was administered intravenously, and a stabilization period of at least 30 minutes was allowed before baseline data were obtained of systemic hemodynamic variables, coronary blood flow, and regional segment length changes. The animals were then subjected to one of the seven study groups (Fig 1). In case of ventricular fibrillation, defibrillation was started within 10 seconds using DC countershocks (15 to 30 W). If defibrillation could not be accomplished within 2 minutes, animals were excluded from further study. Throughout the experimental protocol, body core temperature was rigorously controlled with a heating pad to maintain temperature within a narrow range (37°C to 38°C) to minimize temperature-induced infarct size variability.^{20 21}

In the animals of groups 3 and 7, arterial and coronary venous blood samples for the determination of oxygen content and pH were withdrawn at baseline, at 10-minute, and 30-minute RVP, and at 2, 5, and 15 minutes of NSR. In the animals of group 7, measurements were also made at 60, 120, and 180 minutes of NSR. Myocardial biopsies for the measurement of high-energy phosphate levels in the LCXCA perfused area were obtained at baseline and at 30-minute RVP. The effects of RVP on the distribution of myocardial blood flow were determined in group 7 by injection of radioactive microspheres at baseline and at 30-minute RVP.

Area at Risk and Infarct Size
The methods to determine the AR and IA have been described^{16 17} and validated extensively.^{22 23 24 25 26} Briefly, after reocclusion of the LADCA, the area at risk was identified by an injection of 30 mL of a 5% (wt/wt) solution of fluorescein sodium (Sigma Chemical Co) into the left atrium. Ventricular fibrillation then was induced with a 9-V battery, and the heart was excised. Both atria, the right ventricular free wall, and the left ventricular epicardial fat were removed. The left ventricle was cooled in crushed ice, filled with alginate impression material (CAVEX Holland BV), and sliced parallel to the atrioventricular groove into five segments of approximately 1.5 cm in thickness. The cut surface(s) of each segment and the demarcated AR then were traced onto an acetate sheet under UV light. The viable myocardium then was stained dark purple by incubating the segments for 20 minutes in 0.125 g para-nitroblue tetrazolium (Sigma Chemical Co) per liter of phosphate buffer (pH 7.1) at 37°C. The nonstained pale infarcted tissue was traced onto the acetate sheet. The surface of each ring was subdivided into an endocardial (inner) half and an epicardial (outer) half by drawing a line that divided the myocardial wall into only two layers of equal thickness. Division into two layers was done because it provides information on the transmural distribution of infarct size yet preserves sufficient accuracy of infarct size determination in the two halves. Surface areas of the subendocardial and subepicardial halves and of the subendocardial and subepicardial AR and IA were determined and averaged for the apical and basal sides of each individual ring. The fractions of the ring that was infarcted and at risk then were multiplied by the weight of the ring to yield the weights of the AR and IA for that ring. Subsequently, the weights of the subendocardial and subepicardial halves and the total weight of each ring were summed to yield the LV_endo, LV_epi, and total LV mass. The weights of the endocardial, epicardial, and total areas at risk of each ring were summed to yield AR_endo, AR_epi, and total AR mass; the weights of the endocardial, epicardial, and total infarct areas of each ring were summed to yield IA_endo, IA_epi, and total IA mass. Endocardial, epicardial, and total AR and IA data were expressed as a percentage of LV_endo, LV_epi, and total LV mass, respectively.

Data Analysis
Infarct size data have been presented by plotting the IA against AR for the endocardial and epicardial halves and for the whole left ventricular wall. Linear regression analysis was performed to determine the relation between endocardial and epicardial IA and AR in the control group. For all experimental groups, individual infarct size data points are presented. Intergroup differences between IA_endo, IA_epi, or total IA were analyzed by ANCOVA, with AR_endo, AR_epi, or total AR as the respective covariate. When a significant effect was observed, comparisons between individual groups were made with ANCOVA followed by a modified Bonferroni procedure to correct for multiple comparisons. Intragroup differences between IA_endo and IA_epi were analyzed with the use of ANCOVA for repeated measurements, with AR_endo and AR_epi as covariates. The effect of RVP on the incidence of ventricular fibrillation during 60-minute TCO was analyzed by Fisher's exact test.

Hemodynamic and regional myocardial function data were analyzed by two-way ANOVA followed by either paired t test (intragroup) or unpaired t test (intergroup) with modified Bonferroni procedure to correct for multiple comparisons. A value of P<.05 was considered statistically significant (two-tailed). Data are presented as mean±SEM.

	Results

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Ventricular Fibrillation and Mortality
In the control group, one animal was excluded because of unsuccessful defibrillation during the 60-minute TCO. All other animals that fibrillated were defibrillated successfully. Table 1 shows that ventricular fibrillation occurred in 8 of the 13 control animals during the 60-minute TCO. The incidence of ventricular fibrillation in all groups that were subjected to RVP before the 60-minute TCO (groups 2 through 6) was not significantly different from the control group, indicating that in this model RVP did not protect against ventricular fibrillation during the subsequent coronary artery occlusion. Ventricular fibrillation was rare when reperfusion was reinstated, which is in agreement with previous observations that in pigs, ventricular fibrillation during reperfusion occurs predominantly after 10- and 30-minute coronary artery occlusions.²⁷

View this table: [in this window] [in a new window]   Table 1. Ventricular Fibrillation in All Pigs

IA–AR Relation
Mean areas at risk (expressed as percentage of left ventricular mass) for the five experimental groups of animals that underwent the 60-minute TCO were not different from each other (34±2%, 33±2%, 36±2%, 33±2%, 32±2%, and 33±3% for groups 1, 2, 3, 4, 5, and 6, respectively; F=.40, P=.85).

In the 12 control animals, transmural infarct area was linearly related with the area at risk (r=.92, P<.01; Fig 3). Separation of the left ventricular wall into two layers of equal thickness revealed a highly linear relation in both endocardial half (r=.92, P<.01) and epicardial half (r=.86, P<.01) of the left ventricle. Ten minutes of RVP, separated from the 60-minute TCO by a 15-minute period of NSR, failed to reduce transmural, epicardial, and endocardial infarct size compared with the control group (Fig 3). When the period of RVP was extended to 30 minutes, infarct size in both the endocardial half (F=6.0, P<.05) and epicardial half (F=12.5, P<.01) was slightly but by ANCOVA significantly reduced (Fig 4); the degree of protection was not significantly different between the two layers. The transmural IA/AR ratio was also significantly lower than that in the control group (71±2% versus 84±2%, P<.01).

View larger version (14K): [in this window] [in a new window]   Figure 3. Scatterplots show effects of rapid ventricular pacing (RVP) on the transmural, endocardial, and epicardial relations between infarct area (IA) and area at risk (AR). Shown are the relations in the control group (60 minutes of total coronary artery occlusion, TCO) and animals subjected to 10 minutes of RVP separated from the 60-minute TCO by 15 minutes of normal sinus rhythm (NSR). Note that 10-minute RVP had no effect on infarct size.

View larger version (27K): [in this window] [in a new window]   Figure 4. Scatterplots show effects of rapid ventricular pacing (RVP) on the transmural, endocardial, and epicardial relations between infarct area (IA) and area at risk (AR). Shown are the relations in the control group (60 minutes of total coronary artery occlusion, TCO) and animals subjected to 30 minutes of RVP separated from the 60-minute TCO by 15 minutes of normal sinus rhythm (NSR). Note that 30-minute RVP produced small but statistically significant decreases in transmural infarct size. Lower plots illustrate that treatment with glibenclamide (Glib, I mg/kg IV) at the onset of the intervening 15-minute period of NSR had no effect on the infarct size reduction produced by the pacing protocol.

The transmural IA in 10 of the 12 animals that underwent 30-minute RVP immediately followed by 60-minute TCO was located well below the regression line describing the relation between IA and AR in the control group (Fig 5). The IA/AR of this group of animals was 63±4% (P<.01 versus control group). ANCOVA showed that 30-minute RVP immediately followed by 60-minute TCO significantly reduced the infarcted area for a given AR compared with the control group (F=21.7, P<.01). Further analysis indicated that the infarct size reduction was not significantly different between subepicardium and subendocardium (Fig 5).

View larger version (27K): [in this window] [in a new window]   Figure 5. Scatterplots show effects of rapid ventricular pacing (RVP) on the transmural, endocardial, and epicardial relations between infarct area (IA) and area at risk (AR). In the top plots are shown the control animals (60 minutes of total coronary artery occlusion, TCO) and the animals subjected to 30 minutes of RVP immediately preceding the 60-minute TCO; 30-minute RVP produced significant reductions in transmural, epicardial, and endocardial infarct size. Lower plots illustrate that pretreatment with glibenclamide (Glib, 1 mg/kg IV) prevented the protective effects of RVP.

Mechanism of Protection by RVP
Pretreatment with glibenclamide prevented the protective effect of 30-minute RVP in both the endocardial and epicardial halves (Fig 5). This is also illustrated by the IA/AR ratio, which was 78±4% (P=NS versus control group and P<.05 versus 30-minute RVP+60-minute TCO). In contrast, administration of glibenclamide at the onset of a 15-minute intervening period of sinus rhythm failed to inhibit the modest protection afforded by 30-minute RVP followed by 15 minutes of NSR (Fig 4), suggesting that continued K_ATP⁺ channel activation is not mandatory for the moderate degree of protection that was still present 15 minutes after cessation of pacing.

Hemodynamic Responses to Rapid Ventricular Pacing and Total Coronary Artery Occlusion (Groups 1 through 6)
In the six groups that underwent 60-minute TCO (groups 1 through 6, n=52), there were no significant differences between baseline values of heart rate (110±2 bpm), mean aortic pressure (88±1 mm Hg), cardiac output (2.9±0.1 L/min), left ventricular dP/dt_max (1880±80 mm Hg/s), or left ventricular end-diastolic pressure (8±1 mm Hg). RVP in groups 2, 3, 4, and 5 (n=32) was associated with immediate decreases in mean arterial blood pressure (39±2%), cardiac output (44±2%), and stroke volume (70±1%), while systemic vascular resistance, left ventricular end-diastolic pressure, and LVdP/dt_max remained unchanged. This hemodynamic profile was maintained during the remainder of the ventricular pacing period (groups 3, 4, and 5, n=26) and was not different between the three groups. In the animals of groups 2 and 3 (n=13), in which pacing was terminated without an immediate occlusion of the LADCA, all variables returned to baseline within 1 minute of NSR except for heart rate, which remained slightly (15 bpm) elevated during the 15 minutes after cessation of ventricular pacing. In groups 1 through 5 (n=44), TCO resulted in decreases in cardiac output (16±3%) and mean aortic pressure (6±2%) and increments in heart rate (12±3%) and left ventricular end-diastolic pressure (38±6%) compared with baseline values (all P<.05); these responses were not different between the five groups. None of the hemodynamic variables recovered significantly toward baseline levels during 120 minutes of reperfusion.

Glibenclamide (group 6) produced modest increments in left ventricular end-diastolic pressure from 8±1 to 12±1 mm Hg (P<.05) and mean aortic pressure from 86±3 to 98±5 mm Hg (P<.05). The latter was due to systemic vasoconstriction as cardiac output was not altered by the K_ATP⁺ channel blocker (2.9±0.3 L/min at baseline and 3.0±0.3 L/min after administration of glibenclamide). Glibenclamide had no effect on LADCA blood flow (1.15±0.15 mL · min^-1 · g^-1 at baseline and 1.19±0.14 mL · min^-1 · g^-1 after administration of glibenclamide), LADCA vascular resistance (88±16 mm Hg · mL^-1 ·min^-1 · g^-1 at baseline and 93±17 mm Hg · mL^-1 ·min^-1 · g^-1 after glibenclamide) or systolic shortening in the LADCA perfused segment (16±2% at baseline and 15±2% after glibenclamide). Pretreatment with glibenclamide enhanced the pacing-induced decreases in mean aortic pressure (49±5%), cardiac output (61±3%), and coronary blood flow (36±6%) but had no effect on hemodynamic changes produced by 60-minute TCO.

Effect of Rapid Ventricular Pacing on Myocardial Performance (Groups 3 and 7)
RVP in groups 3 and 7 was associated with immediate decreases in mean arterial blood pressure and cardiac output and hence, myocardial work, whereas left ventricular dP/dt_max, systemic vascular resistance, and left ventricular end diastolic pressure were maintained (Table 2). Thirty minutes of RVP decreased coronary blood flow by 19±5% (n=14), accompanied by a small increase in myocardial oxygen extraction from 72±2% to 77±2% (both P<.05, Table 3). Oxygen consumption per gram of myocardium tended to decrease during RVP, but this failed to reach levels of statistical significance. Microsphere data revealed that the subendocardial to subepicardial blood flow ratio at 30-min RVP was maintained well above unity (1.07±0.08, n=6), although absolute levels were slightly lower than at baseline (1.23±0.07) (P<.05). Coronary vascular resistance (calculated as mean arterial pressure divided by coronary blood flow per gram of myocardium) was also maintained during pacing. Systolic shortening decreased markedly in both the LADCA and the LCXCA perfused segments. However, this was due to a marked decrease in end-diastolic length of both the LADCA (17±1%) and the LCXCA (15±2%) perfused segments, as end-systolic length of both LADCA (4±1%) and LCXCA (5±1%) segments decreased slightly (Table 3). Furthermore, the decrease in systolic shortening during RVP was not accompanied by the appearance of postsystolic shortening. Throughout the pacing protocol, the arterial-coronary venous differences in pH (0.04±0.01 and 0.04±0.01 at baseline and 30-minute RVP, respectively) and in PCO₂ (11.4±0.5 and 10.2±0.9 mm Hg at baseline and 30-minute RVP, respectively) were maintained. In further support of aerobic metabolism, we also did not observe decreases in ATP levels (36.3±1.4 µmol/g protein at baseline versus 36.5±1.4 µmol/g protein at 30-minute RVP), CrP/Cr ratio (1.24±0.12 versus 1.36±0.12), CrP/ATP ratio (1.52±0.26 versus 1.65±0.29), or energy charge (0.922±0.003 versus 0.924±0.003) at 30-minute RVP versus baseline, respectively.

View this table: [in this window] [in a new window]   Table 2. Systemic Hemodynamics at Baseline, During 30 Minutes of Rapid Ventricular Pacing, and During Subsequent 15 Minutes of Normal Sinus Rhythm

View this table: [in this window] [in a new window]   Table 3. Regional Myocardial Performance at Baseline, During, and After 30 Minutes of Rapid Ventricular Pacing and During Subsequent 15 Minutes of Normal Sinus Rhythm

Immediately after RVP was stopped, systemic hemodynamic variables recovered to baseline values except for heart rate, which remained slightly elevated after restoration to normal sinus rhythm. Mean aortic pressure increased to levels slightly higher than baseline during the first minute but had recovered to baseline levels at 15 minutes after cessation of RVP. During the first minute of postpacing, systolic shortening in both the LADCA and LCXCA perfused segments recovered to baseline values, although this was followed by a slight decrease in systolic shortening in the LCXCA area during the remainder of the protocol. Because reactive hyperemia was also absent, these findings indicate that 30-minute RVP was not associated with detectable myocardial ischemia.

	Discussion

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The present study has yielded several important findings: (1) Infarct size after 60-minute coronary artery occlusion is limited when the occlusion is immediately preceded by a period of RVP. (2) Pretreatment was glibenclamide prevented the protective effect of RVP, suggesting the involvement of activation of K_ATP⁺ channels in producing the protection. (3) The pacing-induced activation K_ATP⁺ channels was not due to myocardial ischemia. (4) When the period of RVP was separated from the 60-minute TCO period by 15 minutes of NSR, the protective effect of RVP was markedly less, indicating that the duration of the pacing-induced protected state is much shorter than that for ischemic preconditioning. (5) However, the protection that was still present 15 minutes after cessation of pacing did not involve continued activation of K_ATP⁺ channels.

In the present study, we observed that 10 minutes of RVP followed by 15 minutes of NSR was ineffective in producing protection against myocardial infarction produced by a 60-minute coronary artery occlusion. In contrast, we previously reported that 10 minutes of coronary artery occlusion followed by 15 minutes of reperfusion produced considerable myocardial protection. Similarly, whereas a 5-minute coronary artery occlusion followed by 10 minutes of reperfusion in the rabbit heart is sufficient to precondition the myocardium,²⁸ 5 minutes of atrial pacing followed by 10 minutes of NSR failed to exert a protective effect on myocardial infarct size in rabbit hearts.¹⁰ Our finding that 30 minutes of ventricular pacing followed by 15 minutes of NSR did result in a moderate degree of protection suggests that the duration of stimulation by pacing needs to be longer than that required with a coronary artery occlusion. The duration of protection produced by RVP appears relatively short, since 15 minutes after cessation of pacing most of the protective effect had disappeared. In contrast, myocardial protection by ischemic preconditioning lasts up to at least 1 hour after a 10-minute TCO in pigs and can last up to 5 hours in individual animals.²⁹ These findings suggest that RVP at 200 bpm represents a weaker stimulus than an equivalent period of coronary artery occlusion in pigs, requiring a longer period of stimulation and resulting in a shorter duration of the protected state.

The protection afforded by 30 minutes of RVP against irreversible myocardial damage produced by a sustained period of ischemia was prevented by pretreatment with glibenclamide, indicating that K_ATP⁺ channels are involved in the protective mechanism of RVP. Since we and others have shown that K_ATP⁺ channel blockade inhibits ischemic preconditioning in several species including rabbits,³⁰ dogs,³¹ and swine,^{17 32} it could be hypothesized that ventricular pacing produced preconditioning via induction of myocardial ischemia. In support of this hypothesis, Szilvassy et al¹¹ reported in rabbits that 10 minutes of RVP at 500 bpm increased left ventricular end-diastolic pressure from 4±1 to 33±3 mm Hg and produced myocardial ST-segment elevations suggestive for the occurrence of myocardial ischemia. Similarly, Vegh et al⁹ reported in open chest dogs that during 2 minutes of RVP in which heart rate was increased from 141±8 bpm at baseline to 300 bpm, left ventricular end-diastolic pressure increased from 9±2 to 23±3 mm Hg also accompanied by ST-segment elevations. In the present study, we did not observe an increase in left ventricular end-diastolic pressure when we paced the left ventricle at 200 bpm. The lower left ventricular filling pressure combined with the lower heart rate during pacing could explain why we did not observe signs of ischemia in metabolic, perfusion, and functional variables during RVP at 200 bpm: (1) transmural myocardial blood flow during RVP remained equally distributed across the inner and outer layers of the left ventricular wall, (2) the decrease in systolic shortening was entirely due to a decrease in end-diastolic length, (3) development of postsystolic shortening was not observed,³³ and (4) no changes were observed in myocardial ATP and phosphocreatine levels, energy charge, and arterial or coronary venous pH levels.³⁴ Furthermore, after restoration to NSR evidence for myocardial ischemia during the preceding period of RVP was also absent because (5) reactive hyperemia did not occur, (6) coronary venous oxygen tension was minimally affected after restoration to NSR, (7) systolic segment shortening recovered instantaneously to baseline levels, at which it was maintained throughout the subsequent 180-minute NSR period, and (8) there was no sustained postsystolic shortening during NSR, suggesting that post–ischemic myocardial stunning did not occur. Taken together, these findings fail to support the occurrence of significant myocardial ischemia in the present study. Although we cannot entirely exclude the occurrence of subtle subendocardial ischemia, this certainly would have been insufficient to produce ischemic preconditioning, as Ovize et al² have shown that a 25-minute 50% flow reduction immediately preceding a 60-minute TCO (resulting in total loss of contractile function in the area perfused by the partially occluded coronary artery) failed to limit infarct size. In addition, we recently observed that 30-minute or 90-minute periods of 30% coronary blood flow reduction, associated with a 25% decrease in systolic segment shortening (due to an increase in end-systolic length), did not protect the myocardium against infarction produced by 60 minutes of TCO immediately after the 30% flow reduction.³⁵ Therefore, if some subendocardial ischemia might have gone undetected, it is highly unlikely that this was responsible for the protective effect produced by RVP.

The exact mechanism of K_ATP⁺ channel activation by ventricular pacing cannot be determined from the present study. However, there is evidence that ventricular pacing is capable of activating transient outward K⁺ currents. Thus, Geller and Rosen¹⁴ observed that an increase in electrical activation rate of canine ventricular slabs from 90 to 130 pulses per minute shortened the action potential. The action potential shortening persisted for several minutes after the activation rate was lowered to 90 pulses per minute, indicative of myocardial "memory" for the activation stimulus, which could be antagonized by blockade of the transient outward K⁺ current. Although the specific role of K_ATP⁺ channels was not investigated in that study, our findings that 15 minutes after cessation of 30 minutes of RVP a small but statistically significant reduction in infarct size occurred could be interpreted to suggest that K_ATP⁺ channel activation by ventricular pacing might also display memory. This is supported by observations in dogs that ischemic preconditioning can induce memory for myocardial protection, which involves K_ATP⁺ channels. Thus, when glibenclamide (0.3 mg/kg) was administered intravenously in dogs immediately after a 5-minute occlusion of the LCXCA 10 minutes before a 60-minute circumflex occlusion, ischemic preconditioning was abolished,³¹ indicating that continued K_ATP⁺ channel activation is mandatory for the protective effect to occur even though the initial stimulus that provided the protection is no longer present. However, when we administered glibenclamide at the onset of 15-minute NSR immediately after the pacing period, the moderate degree of myocardial protection afforded by 30 minutes of RVP followed by 15 minutes of sinus rhythm was not attenuated. Thus, while the induction of myocardial protection by pacing does involve K_ATP⁺ channel activation, continued K_ATP⁺ channel activation did not appear to be mandatory for myocardial protection that was present at 15 minutes after cessation of pacing.

Methodological Considerations
In the present study, infarct size produced by a 60-minute coronary artery occlusion was determined in myocardium reperfused for 2 hours using para-nitroblue tetrazolium sodium. In viable myocytes, para-nitroblue tetrazolium is reduced to form a dark purple diformazan precipitate by intracellular diaphorases that use NADH or NADPH as electron donors.²³ False-positive staining is minimized by allowing myocardial reperfusion to facilitate washout of NADH and NADPH from necrotic myocardium. Schaper et al²⁴ reported that only 30 minutes of reperfusion is sufficient for accurate detection of infarct size. Horneffer et al²⁶ also evaluated para-nitroblue tetrazolium staining for the determination of myocardial infarct size in pigs and reported that infarct size produced by 15, 30, or 90 minutes of coronary artery occlusion was not different when either 2 or 48 hours of reperfusion was allowed. Fujiwara et al²⁵ showed that determination of infarct size produced by a 60-minute TCO in porcine myocardium reperfused for 1 hour was identical to infarct size determined after 3 and 7 hours of reperfusion. In the latter study, histochemical analysis of infarct size correlated well with histological measurements. The available evidence clearly indicates that 2 hours of reperfusion after a 60-minute coronary artery occlusion allows accurate histochemical determination of infarct size with para-nitroblue tetrazolium staining in pigs.

In the present study, we used glibenclamide to selectively block K_ATP⁺ channels. Glibenclamide was administered intravenously in a dose of 1 mg/kg, which we have previously shown to block ischemic preconditioning in pigs without extending infarct size produced by a 60-minute coronary artery occlusion.¹⁷ Glibenclamide is known to stimulate the release of insulin and consequently decrease blood glucose levels. In view of a preliminary study in the isolated rabbit heart that reported that substitution for glucose by pyruvate in the perfusate during reperfusion abolished myocardial preconditioning,³⁶ the possibility that glibenclamide modulated the protective effect of ventricular pacing by decreasing blood glucose levels deserves consideration. Gross and Auchampach³¹ reported a 16% decrease in blood glucose levels in response to administration of 0.3 mg/kg glibenclamide in dogs. Similarly, in rabbits, glibenclamide (0.15 to 3.0 mg/kg) produced a significant fall (23% to 38%) in blood glucose levels.³⁷ In contrast, we recently observed that glibenclamide in a dose of 1 mg/kg had a minimal (<10%) effect on blood glucose in pigs.¹⁷ The reason for the minimal effect of glibenclamide on glucose in pigs is not clear but may be due to the relatively low baseline glucose levels in the overnight-fasted pigs¹⁷ or a species difference in sensitivity of the pancreatic cells for glibenclamide. Importantly, the novel cardioselective K_ATP⁺ channel blocker 5-hydroxydecanoate, which is devoid of blood glucose–lowering properties, was as effective as glibenclamide in blocking ischemic preconditioning in dogs,³⁸ indicating that the inhibitory effect of these compounds was due to blockade of cardiac K_ATP⁺ channels and was not due to the decrease in blood glucose.³¹

Conclusions
Thirty minutes of RVP decreased myocardial infarct size produced by a 60-minute coronary artery occlusion in open chest pigs. This protective effect was prevented by pretreatment with K_ATP⁺ channel blockade, indicating that K_ATP⁺ channel activation is involved in the mechanism of protection. Since we failed to observe significant myocardial ischemia during RVP, it appears that K_ATP⁺ channel activation was produced via a nonischemic mechanism. The duration of the protected state produced by 30 minutes of RVP appears relatively brief, since much of the protection was lost when 15 minutes of NSR separated the 30-minute RVP period from the sustained occlusion. When K_ATP⁺ channel blockade was produced immediately after ventricular pacing at the onset of the intervening 15-minute period of NSR, the residual protection remained unchanged. Thus, the moderate degree of protection that was still present 15 minutes after cessation of RVP did not require continued activation of K_ATP⁺ channels.

	Selected Abbreviations and Acronyms

 

AR = area(s) at risk IA = infarct area(s) LADCA = left anterior descending coronary artery LCXCA = left circumflex coronary artery NSR = normal sinus rhythm RVP = rapid left ventricular pacing TCO = total coronary artery occlusion

	Acknowledgments

 
This work was supported by grant 92.144 from the Netherlands Heart Foundation and by grant CIPA-CT-92-4009 of the European Economic Community. The research of Dr Duncker was made possible by a Fellowship of the Royal Netherlands Academy of Arts and Sciences. The authors gratefully acknowledge the expert technical assistance of R.H. van Bremen and S. Nieukoop.

Received February 9, 1995; revision received August 3, 1995; accepted August 16, 1995.

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