Captopril potentiates the myocardial infarct size-limiting effect of ischemic preconditioning through bradykinin B2 receptor activation

JACC Vol. 2X. No. h Novcmhcr IS. iYYh:lhlh-22

1616

‘/

Capt,opril ;Potenti@tes &e Myodardial Infarct Size-Limiting Effect of Ischemic Precqndiiioning,Throtigh Bradygnin Bi Receptor Activation TAKAYUKI TAKASHI OSAMU

MIKI, OGAWA,

IIMURA,

MD, TETSUJI MD, KATSUO

MIURA,

MD, PHD, NOBUYUKI

S1JZUKI,

MD, KAZUAKI

URA, MD, PHD,

SHIMAMQTO,

MD, PIED,

MD, PHD

.Sapporo. Japan

Objectives. To investigatethe role of kinin in preconditioning against infarction, the present study assessedthe effect of captopril, a kininase II inhibitor, on preconditioning and arterial plasma kinin levels. Eac&gmund.Recent studies suggesta possiblecontribution of kinin to preconditioning against infarction. However,its role and the site of kinin production remain uncharacterized. Methods. Six groups of rabbits (n = 6 to 13) underwent 30-min coronary occlusionand 3-h reperfusion. The infarct sizeand area at risk were determined by tetramlium staining and fluorescent particles, respectively.Arterial blood was sampled under baseline conditions, before the 30.min ischemia and after repefisioo for ranioimnmnoassayof the kinin level. ResuILF. Infarct size expressedas a percentageof area at risk (%lS/AR)was 42.9 + 2.m (mean f SEM) in the control group, 34.5 f 33% in the group preconditioned with 2 min of ischemia/Smin of reperlusion and 41.7 f 5.1%in the group given captopril (1 mg/kg bodyweight) alonebefore the 30-min ischemia. These%IS/ARvalueswere not significantly diferent between the three groups. However, a combination of captopril and subse-

quent preconditioning with 2 min of ischemia markedly limited %IS/AR to 21.2 f 2.4%. This potentiation of 2 min of preeonditioning by captopril wasnot observedwhen 2 &kg body weight of Hoe 140,a specificbradykinin 8, receptor antagonist, wasadministered before preconditioning (IISIAR = 41.2 f 5.7%),whereas Hoe 146 alone did not modify infarct size (%&/AR = 38.5 + 5.1%).Arterial plasma kinin levelswere comparable betweenthe control rabbits, the group given captopril alone and the group that receivedcaptoprll plus 2 min of preconditioning at baseline(38 f 1.0,6.3 f 1.9 and 5.2 f 1.7 pg/ml, respectively),and there was no significant change in kiain levelsafter the captopril iwtion or the combination of captopril plus 2 min of preconditioning. Conclusions.The present results indicate that captopril is capable of potentiating preconditioning without increasing the arterial klnln leveland that the beneflclalerect of captopril can be inhibited by Hoe 140. Thesefindings support the hypothesisthat kinin produced locally in the heart during pmconditioning may contribute to the cardiopmtective mechanismthrough bradykinin receptor activation. (J Am Cail Cdiol1996;28:1616-22)

-

Myocardial tolerance to 5chemic injury is markedly enhanced by exposing the myocaraium to a brief episode of &hernia followed by a period of reperfusion. This phenomenon, called precondirioning, has been widely confirmed in various animal models (1,2), and its presenceis implicated in the human heart by circumstantial evidence, including attenuation of ST segment elevation after repetitive coronary balloon inflation during percutaneous transluminal coronary angioplasty (3), and deceleration of ischemia-induced adenosine triphosphate depletion in the myocardium after preconditioning (4). Although the mediator in the preconditioning-like phenomenon

From the Second Department of Internal Medicink. Sapwro Medical Univenir; School of Medicine. Sapporo, Japan. This study was s@ported in part hy Grant-in-Aid for Scientific Research 04670547 (Dr. Miura) from the Ministry of Education, Science and Culture, Tokyo, Japan. Hoc MU was provided by Bemwald A. Scholkens MD, Hoechst AG, Frankfurt, Germany. Manuscript received September 5. 1995; revised manuscript received May 21,1996, accepted August 9,X96. Address for correstxmdence: Dr. Tetsuji Miura. Second Department of Internal Medicine, Sapporo Medical University, S-l, W-16, Chuo-ku, Sappom, 060 Japan. 81996 by the American Col!egc of Cardiology Published by Elscvier Science Inc.

in human hearts is still poorly understood, the key role of an adenosine (probably A,!AJ receptor has been demonstrated in the mechanism of preconditioning against infarction in most of the animal species examined to date (1,2,5-7). However, there are a number of other subst;lncesthat are thought tC be reieased during preconditioning ischemia/reperfusion that may contribute to the cardioprotectian agail:;t infarction. One such substanceis kinin, which is known to be produced in a severely ischemic myocardium (8,9), but its physiologic significance is still not clear. Recently, Wall et al. (10) reported that infa:cl size limitation by preconditioning in the rabbit was blocked by a bradykinin B, receptor antagonist, Hoe 140 (ll), and was mimicked by kinin administration. These results suggest that kinin may also be an important mediator, aswell as adenoFne, in the mechanism of preconditioning. However;it has not been demonstrated whether k-an is indeed prdduced in the myocardium during a brief period of preconditioning &hernia and contributes to cardioprotection. To gain insi&t into’ this issue, the present :‘~tIy.a~~ssed the effect of captopril, a kininase II (i.e., angiotensin-converting enzyme [ACE]) inhibitor, on the infarct size limiting effect of 0735-1097i9t2s15.00 PII 50735~1097(%)flO371-3

JACC Vol. 2X. No. 6 Ncwcmher IS. lYY6zl6lf1-?2

CAPTOPRIL

POTENTIATES

MlKi ET AL. PRECONDITIONING

: Abbreviations ACE ANGVA Pi.IS/.4R

and Acronyms

= angiolen,inconvcrling = analysis of uriance = infarct cizc ;xprcsscd

cncmc a, percemage

of arta al risk

preconditioning in a rabbit model of myocardial infarction. If production of kinin during preconditioning is indeed a crucial mechanism, inhibition of kinin degradation by captopril (12) can ,be expected to enhance the cardioprotective effect of preconditioning, and that effect should be abolished by a kinin antagonist. To easily detect the porsibie potentiation of the cardioprotection by captopril, the duration of preconditioning ischemia was selected as 2 min, which has been shown (7) to very modestly protect the heart against infarction. We also assessedthe effect of captopril on arterial plasma kinin levels to test whether captopril modifies preconditioning through increasing circulating plasma kinin levels.

Methods Surgical preparation. The study conformed to the Guidelines of Sapporo Medical University on Research Animal Use. The surgical preparation in the present study was essentially the same as tha! in our previous studies (1,6,7). Male albino rabbits (Japanese White) weighing 1.8 to 2.9 kg were intravenously anesthetized by pentobarhital (30 mgkg body weight), and additional anesthesia was given during the experiment as needed. A tracheostomy was performed, and mechanical ventilation was provided by a Halvard respirator (model 683) using room air and oxygen supplement. Tidal volume, respiratory rate and oxygen supplement were adjusted to maintain arterial blood gas within the normal physiologic range. A fluid-filled catheter was inserted into the carotid artery and connected to a Nihon-Kohden SCK-580 transducer to monitor biood pressure. Another catheter was placed in the jugular vein for infusing drugs. Electrocardiographic electrodes were placed on the chest wall, corresponding to CC5 leads. The heart was exposed by left lateral thoracotomy. and 4-O silk thread was passed around a marginal branch of the left circumflex coronary artery with a tapered needle. The ends of the silk thread were passedthrough a small vinyl &be to make a snare. The rabbits were subjected to pretreatment (see Experimental protocol), and the coronary artery was occluded by the coronary snare. Myocardial ischemia was confirmed by regional cyanosisand ST segment change on the electrocardiogram. After 30 min of coronary occlusion. the coronary artery was reperfused by releasing the snare. Reperfusion was confirmed by the restoration of color to the iyhemic region. Three hours after reperfusion; the rabbit underwent heparinizatioo with 2.ooO U of heparin and was killed by a pentobarbital overdose. The heart was quickly removed and p-d for postmortem analysis. Posbnortem adysls. The heart was mounted on a Langendorlf apparatus and perfnsed with salii to wash out the

1617

:

remaining blood. The coronary branch was reoccluded and fluorescent particles (3 to 30 pm in dia?eter, Duke Sc$ntific Co.) were infused into the perfusate to negativkly mark the area,at risk (the territory of the occluded &ionary artery). The heart was then froien and sectioned inro Lbmm transverse slices. The heart slices w&e incubated .tiith 1% triphenyl tetrazolium in 100 mmoliliter sodium phosphate buffer (pH 7.4) for 20 min to visualize infarcts (13). The sliceswere mounted on a glasspress and compressed to exactly 2 mm. A clear acetate sheet wasoverlaid on the glasspress,and then the infarcts (i.e., regions unstained by tetrazolium) and areas at risk (i.e., regions deficient of fluorescent particles) were traced on the acetate sheet under room light and under ultraviolet light, respectively. The traces were enlarged by 20% using a Xerox copy machine. The areas of the infarct and region at risk were measured by PIAS II, a computer-assisted image analysis system (PIAS Co., Osaka, Japan), and each volume was calculated by multiplying the area by the thicknessof the heart slice. Extdmental protocol. After hemodynamic variables had been stabilized for 10 min, baseline hemodynamic variables were measured, and the rabbits then undetwent pretreatment and 30 min of coronary occlusionl3 h of reperfusion. In the first series of experiments, 40 rabbits were randomly divided into four groups (n = 10 for each group): Control group, 2’PC group, Cap group and CapR’PC group. The Control group was subjected to the 30 min of &hernia without any pretreatment. In the 2’PC group, the heart was preconditioned with 2 min of ischemia and 5 min of reperfusion before the 30-min coronary occlusion. The Cap group received an intravenous injection of 1 mg/kg of captopril22 min before the coronary occlusion.The CapLZ’PCgroup received the same dose of captopril and was preconditioned with 2 min of ixhemid min of reperfusion. Because the mean of the size of area at risk in the Cap/2’PC group was slightly smaller than those. in the other groups (although the differences were not statistically significant), an additional five rabbits were added to this group. In the Control. Cap and Cap/2’PC groups arterial blood for kinin assaywas sampled immediately after the baseline hemodynamic measurements; the second sample was taken 2 min before the 30-min coronary artery occlusion (20 min after captopril injection in the Cap and CapFPC groups); the third sample was taken 5 min after reperfusion. The blood sample was drawn from the arterial line directly into a syringe containing a mixture of aprotinine, polybrene, soybean trypsin inhibitor, phenanthroline and EDTA to inhibit both kinin production and degradation (14). In the second series of experiments. 14 rabbits were randomly assignedto the Hoe (n = 7) and Cap/HoeL?‘PC groups (n. = 7). In addition. for every two rabbits assigned to these groups, #we,prepared one contemporary control; thus, three rabbits were subjected to no pretreatment, and four were preconditioned with 2 min of ischemia. We did not completely randomize the 21 rabbits into the four groups in this series becausetheControland2’PCgroupsinthefirstseriaofthe experiment had already provided good infarct size data for

MlKl ET AL. CAPTOPRIL POTENTIATES

1618

Table

1. Summary Group

of Kemodyamic n

JACC Vol. 2X. No. 6 Nowmbcr IS. IY9h:lbl6-22

PRECONDITIONING

Variables Basclinc

HR (hcats’min) Contrql 13 2.56 5 I I 2’PC 13 272 2. I I cap 0 283 i_ 7 Cap.2’PC 13 --‘6’ ?Y .5 Hoc 6 25.3 2 I? Cap~Hoc:3’PC 7 h,Y’.I, -+ 1.3 MAP (mm Hg) I3 s7 lr 4 Control ?‘PC 13 SWc .; Cap Y x3 i 2 Cap 2’PC I3 91) z 3 HW 6 x11 + 6 Cap;Hoc:2’PC 7 ,s6 c 5 RPPilNl Control 13 2.3 t 211 2’PC 13 272 2 15 Cap Y m-- + IO 77’ Cap,‘Z’PC 13 270t II Hoc 6 235 ? 37 CapiHoeZPC 7 2xn k 23 ? < 0.05 versus hascline values in each study group (see Enpcrimenlal 1 min bL%re corrmary wxlurion; HR = heart rak: lschemia = 3 min after after repe~.ksion. rcspeckly: RPP = rare-pressure product.

___. Afwr

TX

khcmia

2in 2 273? 287 T 266 Y 250 Y 3s f

11 II s : 1-l s

262 z 275 :: 374 i 26.7 z 36 i 2X’) 5

II II s 7 I4 x

$4 ss 71 76 86 79

2 I 5 t 2 t

s 3 3 3” 5 4’

751 * ----264I ‘19 z 24nk x1 -t 2h3 +

7, 1-l 9 I4 2.5 17

SY I 4 X6 2 3 7-J x 2 80 f 4 Xl 2 5 78 Lc 4 251 266 258 253 236 262

x 2 2 2 + +

I4 1s 9 11 23 14

Rcpcrfukn

5’

26X Y IO m ‘c Y xi1 : 9

-. -. 7 759 2% z 9 x7 .: IO 6’) I 77 2 58 z f-36z 74 I h9?3220 237 ml 210 219 237

i : + -c + 2

5:’ 3” 3* 1’ 8

2-l 13 13‘ 13’ 2x I5

Rqxrfusicrn 25s ? 2.57 f 2x2 f 263 I 271 .‘I 3Y3 z

I fill’ 7 12 13 II 19 13

SY 24’ 6: + 4‘ 53 z 4% 63 + 4* 57 ? x* 64 f: .I* IX5 203 198 21’ 191 0-w 7-1

). z 2 + z +-

14‘ 1I * 16’ is= 27 ,5

protocol for cxpianatiw of study groups). Dal2 pruscnted are mean valur 2 SD. After TX = coronary occlusion: MAP = mean arterial prcssurc: Repcrfurion 5’ and 180’ = 5 and 1x0 min

nonpreconditionld and preconditioned hearts. In the Hoe group, 2 ccg/kgof t!oe 140 was injected intravenously 12 min before coronary occlu-ion. The CaplHoelZ’PC group received captopril and Hoe 140 ,.efore preconditioning with 2 min of ischemiaB min of reperfusim. In this group, the timing of the injection and the dose of capi?pril and Hoe 140 were the same as that in the Cap and the ;qoe group, respectively. The 2-&kg dose of Hoe 140 was selerZd becausehalf of this dose has been shown (10) to abolish preLqnditioning with 5 min of ischemia. We also confirmed in pilot cvperiments that 2 Fg/kg of Hoe 140 almost completely blocked ;qr at least 60 min the hypotensive responseto an injection of 10~ nglkg of bradykinin into the leti atrium, which was a decrease in Qod pressure of -30 mm Hg in untreated rabbits. Exclusion criteria. We excluded rabbits if th;y developed ventricular fibrillation that did not terminate spor$aneously within 1 min or if diastolic blood pressure after recovel:r from ventricular fibrillation was ~40 mm Hg. Our previous study (6) showed that hypotension below this level may cause an extension of infarction during the reperfusion period in the present model. No pharmacologic agents were used for defibrillation or to maintain blood pressure. PIasma kinio assay. After plasma was separated by centrifugation of the blood sample at 4”C, kinin was extracted with ethanol and assayedby a highly sensitive radioimmunoassay method, as previously reported (14). : Chemicals. Captopril was purchased from Sigma Co., and Hoe 140 was provided by Hoechst AG, Germany. Statistical analysis. Hemodynamic variables and alterations in plasma kinin levelswere compared between groups by

two-way repeated measures analysisof variance with a multivariate linear model (SigmaStat, Jandel Scientific) to correct for the differing group sizes (15). Differences in infarct size data between groups Nere tested by one-way ANOVA. When the overall difference was significant, a multiple comparison was performed by the Student-Newman-Keuls post hoc test. Results are expressedas mean value 5 SEM.

Results Becausebaseline hemodynamic variables, heart weight and the size of area at risk were comparable between the study groups in the first seriesof experiments and those in the second series,the results of both serieswere combined and presented together as follows. Exclusion of rabbits. According to the exclusion criteria, we excluded five rabbits from the following analysis; two rabbits in the Capl2’PC groups, one in the 2’PC group, one in the Cap group and one in the Hoe group. The two rabbits in the Cap/2’PC were not defibrillated, and the other three failed to maintain diastolic blood pressure >4O mm Hg after recovery from ventricular fibrillation. Hemodynamic variables. Hemodynamic variables in the six study groups are summarized in Table 1. Heart rate, mean blood pressure and rate-pressure product were comparable in all groups at baseline. Two-way repeated measures ANOVA indicated no significant changes in heart rate but a significant time-related reduction in mean blood pressure and ratepressure product during the experiments. No group-related effects were detected for either mean blood pressure or

JACC Vol. 28. No. b Nnvemlwr IS. 1996:lh16-L!

YIKI

ET AL

CAPTOPRlL PCTTENTlAlXS PRECTNWITIONING

1619

Table 2. Summary of Infarct Size Data Hcan

COlllrOl Cap Cap’Z’PC Hoe Cap/Hne!?PC ‘See Experimental

protocol

Arca

;II Risk

(cm’)

6.3 z 11.3

0.7)i e !).I!+

6.8 ‘c 0.3 i.c + n.3

n.7’) : 0.w 0.7s z o.ux

13

6.4 2 ‘I.? I-

6 7

6 .-’ -: Il.4 6.6 _C(I.3

I).fl5 I 0.06 II.: ? 0.1 I

-

13 1: 9

2’PC

Weight @I

n

Group’

for explanation

0.73 2 0.11

of stud!, group>. tp i 0.05 ~crsus Control

rate-pressure product (p = NS). These results suggest that the effects of preconditioning, captopril and Hoe 140, and a combination of these, did not significantly Jlter the time course of these hemodynamic variables in the present rabbit model of infarction. Infaret sizedata. Infarct sizedata are summarized in Table 2. Heart weight and size of the area at risk were similar in all groups. Infarct size as a percentage of the area at risk (%IS/AR) was 42.9 + 2.9% in the Control group and 41.7 2 5.1% in the Cap group (p = NS). The %lS/AR was smaller in the 2’PC group (34.5 ? 3.3%) than the Control group. although the difference did not reach statistical significance. In contrast, %IS/AR in the CapEPC group was 21.2 2 2.4%, which was significantly smaller than that in the Control and Cap groups. Before three rabbits were added (see Experimental protocol) to this group to make the size of area at risk more comparable (thus, the area at risk was 0.60 -)_0.07 cm3. n = 10). %IS/AR was 20.2 2 3.0% (p < 0.05 vs. Control group). These findings indicate that pretreatment with captopril potentiated the infarct size-limiting effect of 2 min of precotidiu tioning. In the Hoe group and Cap/HoeEPC group, %IS/AR was 41.2 ? 5.7% and 38.5 2 5.1%, respectively,both of which were similar to the control infarct size, and these values were significantly larger than the %IS/AR in the Capl2’PC group. Because %IS/AR is an index that can be influenced by the risk area size in the rabbit model of infarction (16), the effect of captopril on preconditioning was also assessedin light of the infarct size-risk area size relation. As shown in Figure 1, the regression line between the infarct size and risk area size shifted slightly toward smaller infarcts in the 2’PC group, but this shift was not statistically sign&ant (panel A). Although captopril Jone did not shift the regression line between infarct size and risk area size, the regression line in the CapL!‘PC group was significantly less steep than that of the Control group (Fig. 1, panel B). Hoe 140 alone did not shift the regression line, but this agent blocked the dovvnward shift of the regression line by eaptopril plus 2 mm of preconditioning (Fig. 1, panel C). This analysisalso showsthe significant infarct size limitation by the combination ofcaptopril and 2 min of preconditioning, which was inhiiitable by Hoe 140. PIasma kInIn Ieds. Plasma kinin levels at baseline’were 3.8+ 1.0, 63 t 1.9 and 5.2 +- 1.7 in the Control, Cap and

group. Dats prcunrcd

;+re mean value z SD.

Figure 1. 5catterplots of the relation hetwecn infarct size and size of area at risk. Paw1 A. Control group versus ?‘PC group. The wgfrsrioa line of the “PC group (y = 0.27~ + 0.06. r = 0.36) was not different from that of the Control group (y = 0.55x -0.09. r = 0.87). Panel 8. Control group versus Cap and Cap/2’PC groups. The rrgmsion liw of the CaplZ’PC group (y = 0.20x + 0.01. r = 0.70) was significantly less steep than that of the Control group. whereas there was no significant difference between the regression line of the Cap group (y = 0.76x 0.24. r = 0.91) and that of the Control group. Panel C, Control group versus Hoe group and Cap/Hoe/I!‘PC group. The qressian lines of the Hoe (y = 0.65x - 0.16, r = 0.85) and CaplHoel2’PC groups (y = 0.61x - 0.13. r = 0.94) did not differ from that of the Control group. See Experimental protocol for explanation of studysoups. 0.80 Comrol 0.6’

l

2’K

l A

cap C~PC

0.4.

0.6’

0.4’

Area at Risk (cm3)

that arterial kinin ievqls were not elesatcd by captopril dots not n&cssafily contradict the success&linhibition of kiniriase II hv captopril in the heart because the effect of kininasc I! inhibition on kinip levels depends on the rate of kinin production. which. presumably, was high in the ischemic myocardium. Several earlix sturlie , demonstrated that kinit production in the myocardium is -,ccelerated h) ischemia both in vivo (8.9) and in vitro (19.2’;). Baumgarten et al. (19) showed thal the kinin relcase after IS min of coronar?; occlusion was enhanced I-,y pretreatment with an ACE inhibitor. ramiprilat. in isolated

o--

-17 Baselint:

After TX

Reperfu~ion

Figure 2. Time courw of change in plasma kinin Icwls in arterial hid. Basclins plasma kinin levels were comparable in the Control (open circles). Cap (solid circlrrl and Cap/?PC groups (triangles). and there was no significant alteration in the plasma kinin levels before and after the cor,mary occlusion in these three groups. After TX = immediately before cnronaq occlusion: Reperfusion = 5 min after corona9 reperfusion. See Experimental protocol for explanation nf study groups.

rat hearts. In contrast. the s!ight blood pressure decrease after captopril (Table I) can he explained by the suppression of angiotrnsin II production by inhibiting ACE. Taken together, the present findings suggest that the kinin that contributed to the cardioprotection of preconditioning was produced locally in the rabbit heart. Effect of captopril in nonpreconditioned hearts. In contrast to the marked cardioprotectivr effect of a combination of captopril and preconditioning, captopril alone failed to limit infarct size This result is consistent

with the observations

in

our prclious study (21) as well in others (22-24). However. there are also reports of infarct size limitation by the same agents (25-28). and the reason for the discrepancy remains unclear.

Cap/2’PC

groups, respectively

(p = NS). In all three groupa

there was no significant alteration in arterial plasma kinin I<.:els during the time course of prsischemic and reperfusiun periods, as shown by Figure 2.

Pntentiatioo of preconditioning: captopril versus nucleuside transport. Like captopril in the present study, nucleoside

transport inhibitors were shown to potentiate preconditioning with 2 min of ischemia against infarction. In our previous studies

Discussion In the present study, captopril potentiated the infarct size-limiting effect of preconditioning with 2 min of ischemia. and this cardioprotec!ive effect was abolished by Hoe 140, a specific bradykinin B, receptor antagonist. However, plasma kinin levels in arterial blood were not changed b; iile administration of captopril or its combination with preconditioning. These results support the hypothesis that generation of kinin in the heart during preconditioning contributes, through Bz receptor activation, to infarct size limitation by preconditioning. Kinin production in heart. A limitation of the present study is the absence of direct data of kinin production in the ischemic myocardium. It is not technically possible to collect venous blood from the territory of the left marginal artery in the rabbit heart, and blood sampling from the coronary sinus does not allow accurate assessmentof kinin metabolism in the ischemicregion of the heart. Accordingly, we took an approach that excluded involvement of systemic circulating kinin in preconditioning. Hoe 140, a specific B, receptor antagonist (1 I), complett ly abolished the cardioprotection of captopril and precondhioning, suggesting a, contribution of ‘the !$ receptor. In cc~ntrast,the dose of captopril used in the present study did not change arterial kinin levels with or wiihout the combination of preconditioning (Fig. 2), although different doses of ACE inhibitors have been shown (17,lH) to increase circulating plasma kinin levelsin humans. However, the finding

(7.2’)).

pretreatment

of rabbits

with

dipyridamole.

dilazcp and R75231 before preconditioning with 2 min of ischemia limited infarct to -30% of the infarct size in the untreated control rabbits, whereas any of the nucleoside transport inhibitors alone failed to modify infarct size.Furthermore, the effect of nucleoside transport inhibitors was inhibited by H-phenyl-theophylline (7), suggesting an adenosinemediated mechanism. In contrast, the captopril-induced cardioprotection of 2 min of preconditioning was abolished by Hoe 140, which is a specific antagonist of the B, receptor and has no known antagonistic action on an adenosine receptor. The extent of infarct size limitation by the combination of dipyridamole or captopril with 2 min of preconditioning was comparable to that by preconditioning with 5 min of ischemia. A possible explanation of these findings may be that accumulation of either kinin or adenosine in the cardiac interstitium up to a certain threshold could trigger the cardioprotective’ mechanism, although the levels of both substancesare below the threshold during 2 min of preconditioning without captopril or nucleo$de transport inhibitors. Mechanism of precunditiuniag downstream to B, rercptor activation. The present study cannot specify the cardioprotec-

tive mechanism downstream to activation of the B, receptor. The & receptor iscoupled with phospholipasesAZ and C through G. proteins (13,30), and stimulation of this receptor leads to the production of two important vasoactivesubstances-prostacyclin and nitric oxide. However, neither cyclooxygenaseinhibitors (i.e., aspirin and meclofenamate) (31) nor the nitric ox& synthase

JhCC Vol 3. No h Novemhcr 15. lWfxlh16-22

inhibitor, nitro-L-arginine methyl ester (32.33) attenuated infarct siielimitition by preconditioning in the rabbit model of infarction. ln~rontrast. p$ymyxin B. a blocker of protein kinase C. reportedly abolished.infarct sizelimitation by pretreatment with kinin in a study by Goto et al. (32). Although Goto et al. (32) did not try other protein kinaseC inhibitors on kinin-induced cardioprotection in that particular study. they demonstrated that polymyxin B. staurosporinc(34) and cheletythrine ;35) are capableof bkx+tg preconditioning against infarction in the in vitro and in vivo rabbit model of infarction. Furthermore. a recent study using isolated cardiac myocytes shmed that B, receptor stimulation caused significant inositol triphosphatr production (36) suggesting that simultaneously generated diacylglycerolcould activate protein kinase C. Taken together, this circumstantial evidence suggeststhat protein kinase C activation may play a key role in cardioprotection after I+, receptor activation. The important role of kinin in preconditioning against ischemia-induced arrhythmia has also been pointed out. Vegh et al. (37.38) found that the antiarrhythmic effect of preconditioning was Mocked by Hoe 140 and mimicked by an intracoronary infusion of kinin in canine hearts. However. in contrast with the antiinfarct effect of preconditioning (32,33). the antiarrhythmic effect of preconditioning was blocked by No-methyl-t.-arginine, suggesting an involvement of nitric oxide (39). I~rthermore, a recent study (40) showed a proarrhythmic effect of the protein kinase C activator during myocardial hypoxia/reoxygenation. Thus. B, receptor activation before ischemic insult might enhance myocardial tolerance against both infarction and ischemic arrhythmia. but the underlying subcellular mechanism may be different (i.e.. protein kinase C activation and nitric oxide production). depending on the end points. Rok of an&ten&r II? The present study used captopril to decelerate degradation of kinin by kininase II in the myocardium. A methodologic disadvantage of this aptxoach is the simultaneous suppression of angiotensin II production. However, it is more likely that the effect of captopril on angiotensin II levels would cause an underestimation of the role of endogenous kinin in preconditioning. A preliminary study (41) suggested that angiotensin II production may be responsible for a part of cardioprotection by preconditioning. Furthermore. a recent study (42) showed that angiotensin II receptor activation can mimic preconditioning. Thus, a part of cardioprotection by the combination of captopril and preconditioning might have been canceled by removing the angiotensin II-mediated component. If a specific kininase inhibitor without ACE inhibitory action is available, it might be able to potentiate preconditioning more than captopril and other ACE inhibitors. ConcIuaions. The present study provided evidence that kinin endogenously produced in the heart during preconditioning contributes to the cardioprotective mechanism of preconditioning through Bz receptor activation. The relative’importance of plasma and the cardiac kahikrein-kinin system and the subcellular rneehanism subsequent to Bz receptor activation warrant further investigation.

CAPTOPfUL

POTENTlATH

WC thank Ryhei Soraw.~ ~tali+lical anal!w3.

MIKI ET AL. PRECONDITIONI~C

PhU and Jamcc 51. Dowry.

References

PhD for a*\isIing

1621

with

.’

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