J Mel

Cell Cardiol 23, 187-197 (1991)

Delayed but not Immediate Captopril Therapy Improves Cardiac Function in Conscious Rats, Following Myocardial Idarction Regim

G. Sckoemaker*, Department

Jacques J. M. Debets, Harry and Jos F. M. Smitst

of Pharmacology,

University of Limburg,

A.

J. Struyker-Randier

Maastricht,

The Netherlands

(Received 27 June 1990, accepted in revised form 17 September 1990) R. G. SCHOEMAKER, J. J. M. DEBETS, H. A. J. STRUYKER-BOUDIER AND J. F. M. SMITS. Delayed but not Immediate Captopril Therapy Improves Cardiac Function in Conscious Rats, Following Myocardial Infarction. foumal of A4olccular and Cellular Cardiology (1991) ‘23, 187-197. After myocardial infarction, the reninangiotensin system is found to be activated. While this response may be beneficial in acute failure, it could be detrimental in chronic stages. Therefore effects of captopril therapy were investigated during early and later phases after myocardial infarction in conscious rats, chronically instrumented for hemodynamic measurements. Hemodynamics were measured at baseline and after stimulating the heart by a volume load (cardiac function curve). Myocardial infarction decreased baseline cardiac output and impaired cardiac function, without effects on baseline mean arterial pressure, central venous pressure and heart rate. Captopril given 3 to 5 weeks after infarction improved cardiac function in a dose-dependent manner by increasing stroke volume, whereas stroke work was not affected. In contrast, captopril given from 1 to 21 days after infarction did not lead to improved cardiac function; instead, tachycardia together with a decreased stroke volume suggested deterioration, rather than improvement, of cardiac function. These data indicate that captopril therapy in chronically infarcted conscious rats improved cardiac function when treatment was started after completion of the healing process, but that early treatment not only failed to improve ventricular function, but may have a deleterious effect of the heart. KEY

WORDS:

Captopril;

Rat;

Heart

failure;

Cardiac

function;

Angiotensin

converting

enzyme

inhibitors.

IlXtldUCtiOlh

therapy is started several weeksafter myocardial infarction, but there is a tendency to start earlier [30, 421. Chronic ACE inhibition results in hemodynamic improvement [32] and increased survival [35, 4fYj in rats with heart failure. An apparent lack of differences [32] when therapy was started in the early phase of healing processes,2 days after infarction, or when these processesare considered to be completed, 21 days after infarction [IO, .%I, suggests that captopril has only minor effects when given early during the healing period. However, the beneficial effects after a long period of treatment could mask the effects of therapy during the healing period. bition is considered to improve the clinical The aim of the present study was to compare hemodynamic effects of short-term capstate [4, 5,32, 471 and to increase survival [q in congestive heart failure. In most studies topril therapy initiated at different time

Myocardial infarction activates compensatory mechanisms that act to maintain systemic perfusion. These mechanisms include enhancement of sympathetic tone and activation of the renin-angiotensin-aldosteron system resulting in vasoconstriction and volume expansion [5,13,19,29,37,43]. While these responsesmay be beneficial in acute heart failure to help maintain perfusion pressure, they could be detrimental in chronic stages of the disease [5, 13, 29, 371. Both vasoconstriction and volume expansion may be due to increased angiotensin II levels. Indeed, reduction of angiotensin II levels by angiotensin I converting enzyme (ACE) inhi-

*Please address all correspondence to: R. G. Schoemaker, Hypertension Institute, 1053 Carling Avenue, Ottawa, Ontario, Canada KlY 4E9. 'fJ, F. M.

Smits is an Established

0022-2828/91/020187

+ I1 $03.00/O

Investigator

for the Netherlands

Heart

Unit

H347,

University

of Ottawa

Heart

Foundation. Q 1991 Academic

Press Limited

188

IL G. schamrlrer

points following myocardial infarction. Rats were treated with captopril during the healing period, from 1 to 21 days following infarction, and after the healing period, from 21 to 35 days after infarction. Acute effects of captopril were also studied after the repair period. All measurementswere performed in consciousunrestrained rats to avoid effects of anesthesia to depress compensatory reflex mechanisms[7, 541.

et al.

application of a negative pressure of 1Ocm H,O.

Preparationfor hemodFmic measurements One week before hemodynamic measurements an electromagnetic flow probe (2.6 to 2.7 mm, Skalar, Delft, The Netherlands) was implanted on the ascending aorta through a right thoractomy as described in detail elsewhere [#I. The connector was exteriorized in the neck and fixed to the skin. Negative presMaterials and Methods sure was again restored through a drain. AniMale Wistar rats (Broekman, Borchem, mals were allowed to recover 5 to 6 days from FRG), weighing 321 & 38 g (mean + SD), this surgery. Following the recovery period were used throughout this study. Animals animals were re-anesthetized with ether and were housed separately in macrolon cageson a catheter (PE-10 heat-sealed to PE-50) was sawdust in climatized rooms with a 12 hour inserted into the abdominal aorta through a light-dark cycle. Before and during all experi- femoral artery for measurement of arterial ments, animals had free accessto standard blood pressure. Two catheters were introlab food and tap water (Hope Farms, Woer- duced into the vena cava through a femoral den, The Netherlands). vein. The first one (PE-IO heat-sealed to All experiments were performed in adher- PE-50) was inserted for approximately 4 cm, enece with the ethical codes of the University locating its tip in the abdominal vena cava. of Limburg, The Netherlands. The second venous catheter (Silastic 602175) was advanced into the femoral vein for about 9cm, which locates its tip in the thoratic vena cava near the right atrium. The Coronaryartery ligation former catheter was used for infusion, wherRats submitted to ligation of the left anterior eas the latter was used to measure central descending coronary artery, as described in venous pressure.All catheters were filled with detail elsewhere [ZO, 341, were anesthetized normal saline, closed with metal plugs and with pentobarbital (60 mg/kg i.p.). After the exteriorized in the neck. Following this surtrachea was intubated, skin and muscles gery, animals were allowed one day for overlying the 4th left intercostal space were recovery. separated and cut. Prophylactically, 2 mg/kg lidocaine was given i.m. [3, 231. The thorax was opened after positive pressurerespiration Treatment was started and the heart was carefully pushed to the left or exteriorized by applying Acute captoptil was administered 5 weeks pressureto the right side of the thorax. A 6-O after myocardial infarction asa bolus injection silk suture was looped under the left coronary of 10 mg/kg in 50~1 intravenously. Shortartery near the origin of the pulmonary term captopril was given, using Alzet model artery. In sham operated rats, the suture was 2001 osmotic minipumps, which pump for 8 looped through the myocardium next to the to 9 days. They were implanted subcutacoronary artery. After the heart was returned neously between the shoulder blades, under to its normal position, the suture was tied. light ether anesthesia. Pumps were replaced Ribs were pulled together with 3-O silk, every 7 days. Early captopril therapy (500 musclesand skin were sutured separately and pg/kg*h) was started one day after myocarsubatmospheric pressure was restored by dial infarction and continued for 3 weeks. inserting a silastic drain through a stab The later therapy, with either 100 pg/kg-h or wound between the 6th and 7th rib and 500 pg/kg*h was started at 21 days after

cmptopd

Folhdg

Myoauditd

189

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infarction and continued for 2 weeks. Control infarcted rats received no treatment.

days 1 and 3 of the protocol, non-treated infarcted rats were randomly subjected to control volume loading as described above, Hemodpamic measuwtnents or to volume loading after acute captopril Hemodynamic measurements were per- administration. For the latter experiment, formed at 3 or 5 weeks after myocardial after stabilization for 45 to 60 min, 10mg/kg infarction. The total protocol for measure- captopril was given i.v. in 50~1. After 30 min, ments encompassedthree days. On the first ACE inhibition was verified by measuring day animals were connected to the measure- TPR responsesto 150 rig/kg ANG I. When ment equipment: a sine-wave electromag- hemodynamics had returned to pre-ANG I netic flowmeter (Skalar, Delft, the values, or maximally 5 min after the ANG INetherlands), miniature low volume dis- injection, CO was stimulated by volume placement pressure transducers (CP 01, Cen- loading. After the conclusion of the experiments, tury Technology Company, Inglewood, CA, USA) and an ECG amplifier. Signals were all animals were anesthetized with pentobarfed into a 68B09-based preprocessor and an bital, the hearts were arrested in diastole with AT compatible microcomputer for on-line 1 M KC1 i.v. and excised. After the hearts derivation of hemodynamic parameters. were washed in saline, vesselsand atria were From the flow signal cardiac output, heart removed and the ventricles weighed. Hearts rate, aortic flow acceleration, peak flow and were prepared for infarct size measurements. stroke volume were derived. CO* corrected for body weight, cardiac index was calculated. Mean arterial pressure and central venous pressure were recorded and resting Measurements of infarct sky respiration rate was determined. Stroke work was computed as SV (MAP- CVP). Total After weighing, the hearts were quickly froperipheral resistance was computed as zen ( -80°C) and cut into slicesof 1mm from apex to base. The slices were stained with (MAP - CVP) /CO. After equilibration for 45 to 60 min, base- nitro blue tetrazolium (NBT), as described line values for all variables were obtained elsewhere [23, 401. NBT, which stains all tisduring 15 min. Then 12 ml of warm (37°C) suethat was viable at the time of death, gives Ringer’s solution were infused over 1 min. no information of the area at risk. From each This volume loading increased CO to a pla- slice the free wall of the right ventricle was teau after about 45s, in spite of further removed. Color pictures were taken from the increasesin CVP [40, II]. During this inter- remaining left ventricle slices including the vention all hemodynamics were monitored septum. The lengths of the infarction and of continuously and recorded till approximately the viable muscle for both the endocardial 30 min after infusion. Twenty-four hours and epicardial circumference of each slice later, animals were reconnected to the equip- were determined by planimetry, as described ment, baseline values were obtained and a by Pfeffer et al. [32]. Final infarct size was dose-responsecurve for ANG I was made to expressedin per cent of left ventricular cirverify ACE inhibition. Doses of 25, 75, 150 cumference, calculated as the average of and 300 rig/kg ANG I were administered infarct size of endocardial and epicardial surintravenously in a volume of 50 ~1 and the faces of all slices. The mean endocardial cireffects of TPR were determined. cumference was used as an estimate of left Acute effects of captopril were studied in ventricular dilatation. parallel at 5 weeks after myocardial infarction. The protocol encompassed3 days. On *CO: cardiacoutput; HR: heart rate; dF/dt: aortic flowacceleration; PF: peakflow;SV: stroke volume; CI: cardiacindex; MAP: mean arterial pressure; CVP: central venous pressure; ipheral resistance.

SW: stroke

work;

TPR:

total per-

Data analysis

Results comprise data from 7 to 11 animals per group. The sham operated animals

R

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G. schoemaker

underwent the same surgery and protocol, received no treatment and had measured infarct sizesof 0%. Data from three rats that were treated for 2 weeks, but had no infarcts, were excluded from analysis. Overall comparisons of the experimental groups per time period were made by one-way ANOVA and a modified (Bonferroni) t-test [53l. Effects of infarction or short-term captopril therapy were compared by one-way ANOVA and Dunnett’s test. Effects of timing of short-term therapy were analyzed, using ANOVA and Bonferroni test. Correlations between hemodynamics and infarct sizes were determined by linear regressionanalysis. Baseline hemodynamics before and after acute captopril treatment were compared by a Student’s t-test for paired observations, whereas control cardiac function and cardiac function after acute captopril treatment were compared, using an unpaired t-test. Data are expressed as means & S.E.M., unlessindicated otherwise. Differences or correlations were regarded as statistically significant if P