European Journal of Pharmacology, 167 (1989) 211-220 211 Elsevier

EJP 50925

Beneficial hemodynamic effects of milrinone and enalapril in conscious rats with healed myocardial infarction

Albert DeFelice *, Alex Harris, Richard Frering and Patrick Horan Department of Pharmacology, Cardiopulmonary Division, Sterling Research Group, Rensselaer, N Y 12144, U.S.A.

Received 19 December 1988, revised MS received 10 May 1989, accepted 30 May 1989

Milrinone and enalapril, which inhibit PDE-III and ACE, respectively, are able to prolong survival of myocardially infarcted (MI) rats. This study sought to identify oral hemodynamic effects of these agents which could underlie such efficacy in this heart failure model. Four weeks after ligation of the left main coronary artery, basal left ventricular (LV) systolic pressure and dP/dtmax, heart rate and mean blood pressure of the MI rats were significantly less than that of sham-operated controls, and LV end-diastolic pressure (LVEDP) was markedly elevated. Milrinone, at 2.0 mg/kg, reduced LVEDP and renal blood flow of these 4-week MI rats by an average of 39 and 18%, respectively (P < 0.05) within 1 h. At 4.0 mg/kg, it reduced LVEDP by 46% and raised heart rate by 16% (P < 0.05). Enalapril (1.0 mg/kg) increased small intestine blood flow of these compromised rats by 16% (P < 0.05), and tended to reduce LVEDP ( -28%) within 1.5 h. Treatment with milrinone (2.0 mg/kg) plus enalapril (1.0 mg/kg) promoted LV dP/dtm~ x, coronary blood flow, and heart rate by 48, 40 and 13%, and reduced LVEDP by 40% (P < 0.05 for all effects). Thus these agents can reduce LVEDP and redistribute cardiac output of MI rats. Furthermore, the combination of enalapril and milrinone can restore LVEDP and LV dP/dtm~ x of MI rats to near normal and promote coronary blood flow without compromising cardiac output or renal blood flow. Such effects, it timely or sustained, may prolong survival.

Hemodynamics; Myocardially infarcted rat; Milrinone; Enalapril; Cardiomegaly (compensated)

1. Introduction

Rats with hea led severe myoca rd ia l infarc t ion (MI) have been s tud ied as an exper imenta l mode l of hear t failure. Decreased left vent r icular (LV) peak pressure and dP/dtm~x, hypo tens ion and e levated LV end-d ias to l ic fi l l ing pressure are pre- sent 3-12 weeks af ter co rona ry ar te ry l igat ion (F le t che r et al., 1981). LV impa i rmen t ranges f rom slight dysfunc t ion to overt fai lure depend ing on infarc t size and LV d i l a ta t ion (Drexler et al., 1985a). DeFe l i ce et al. in press), who mon i to red

* To whom all correspondence should be addressed: Dept. of Pharmacology, Sterling Research Group, Rensselaer, NY 12144, U.S.A.

the h e m o d y n a m i c t ra jec tory of conscious M I rats, observed progress ive changes in total per iphera l vascular resistance, ca rd iac ou tpu t index and re- g ional b lood flow in the 4-35 week pos t -occ lus ion interval . Survival of rats which escape the 50% per i -opera t ive mor t a l i t y is also m a r k e d l y reduced. One year survival for ra ts with large infarcts is on ly app rox ima te ly 10% (Pfeffer et al., 1985b; Sweet et al., 1988).

Previous s tudies pro jec t the po ten t ia l ut i l i ty of the M I ra t model for evaluat ing therapeut ic in ter - ventions. Mil r inone, a phosphod ies t e ra se - I I I in- h ib i to r which is being evalua ted cl inical ly for t rea t ing congest ive hear t failure, has vasodi la t ing and, possibly, slight posi t ive ino t rop ic effects when admin i s t e red i.v. to anes thet ized M I rats (Emmer t

0014-2999/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)

212

et al., 1987). Captopril, an angiotensin converting enzyme (ACE) inhibitor, reduces both preload and afterload in this model and improves LV perfor- mance when administered in drinking water for three months (Pfeffer et al., 1985b). Sweet et al. (1988) have reported beneficial effects of milrinone and enalapril, administered alone or combined, on long-term survival of MI rats. In view of these reports, we assessed the hemodynamic activity of oral milrinone, enalapril or a combination of these agents in conscious rats four weeks after coronary artery occlusion and sought to identify effects which might underlie the beneficial influence on survival.

2. Materials and methods

2.1. Experimental infarction and instrumentation

Male Sprague-Dawley rats (Taconic Farms; 75- 80 days old; 320-350 g) were anesthetized with sodium pentobarbital (40 m g / k g kp.), intubated with PE 240 tubing, and ventilated with a positive pressure respirator (86 strokes/min). The heart was exposed via a left-sided thoracotomy and exteriorized by squeezing the chest walls. Four drops of 2% carbocaine were then applied to the LV surface, and the left coronary artery was ligated with 5-0 silk approximately 2 mm from this origin. The heart was then replaced in the thorax and the incision was rapidly closed with a purse string suture after expelling air from the cavity. Sham- operated rats were prepared similarly except for arterial ligation. Mortality within the first 48 h was approximately 50%.

Four weeks after infarction or sham operation, rats were anesthetized with 25% ethrane-75% 02, intubated, and maintained on 3% ethrane-97% 02. A LV catheter was inserted and used for monitor- ing LV pressure as well as for injecting radio- labelled microspheres. The fight femoral artery was cannulated for measuring systemic blood pressure and for withdrawal of a reference blood sample during microsphere injection (see below). Both cannulas were exteriorized at the neck and filled with heparinized saline (1000 uni ts /ml) and

gaseous anesthesia was terminated. Rats were al- lowed to recover for 3-4 h.

2.2. Hemodynamic monitoring

The LV catheter was attached to a Millar 3F Micro-tip transducer (Millar Instruments, Hous- ton) to record heart rate, peak LV developed pressure and LV end-diastolic pressure on a Grass Polygraph (model 7). LV dP/dtm~ x was derived from LV pressure using a Grass differentiator (Model 7P). Systemic blood pressure was recorded from the femoral artery catheter, Immediately after LV and systemic parameters were recorded, mi- crospheres were injected and cardiac output was measured as described by Heymann et al. (1977). Total systemic vascular resistance was determined as the ratio of mean arterial blood pressure to cardiac output.

Microspheres (16.5 + 0.1 ~tM in diameter; New England Nuclear, Boston, MA) labelled with 153Gd or l°3Ru (10-12 mCi /g ) were used to measure regional blood fl0w and cardiac output as de- scribed by Wagner et al. (1969) and Heymann et al. (1977), and adapted for rat use (Flaim et al., 1979). The spheres were suspended in normal saline with 0.01% polysorbate 80 (to retard sphere aggregation) to a specific activity of 0.025 m C i / m l of vehicle and a concentration of approximately 1 000000 spheres/ml. The suspension was soni- cated for 15 min, and 0.1 ml i.e. approximately 100000 microspheres was drawn into a reservoir and counted on an LKB Compugamma gamma counter (model 1282). After hemodynamic param- eters were recorded, the spheres were again soni- cated and injected into the LV chamber over a 20 s period using a 1.0 ml flUsh of lactated Ringer solution (37 o C). Twenty seconds before injection, blood withdrawal from the femoral artery was begun at a rate of 0.23 m l / m i n using a Harvard Pump (model 975). The total withdrawal period was 3 rain for each injection. Blood sample weight and withdrawal time were carefully measured and, based on rat blood ~pecific gravity of 1.06 (Donaldson, 1924), were used to obtain an accu- rate withdrawal rate expressed as ml b lood /min . 153Gd-labelled spheres were injected prior to medication, and l°3Ru-labelled spheres were given

post-treatment (see below). At the end of the study rats were killed and tissues were harvested.

2.3. Experimental protocol

After 3-4 h recovery from the gaseous anesthe- sia, baseline pressure tracings were obtained from the LV and femoral artery catheters, and 153Gd- labelled microspheres were then injected into the ventricular chamber of sham-operated and in-" farcted rats (Study A). To determine acute oral effects of milrinone, enalapril and milrinone, and enalapril alone in infarcted rats (Study B), base- line hemodynamic status and cardiac o u t p u t / distribution were measured as described above. Rats were then pretreated with enalapril maleate (Merck; 1.0 mg base /kg) or vehicle alone (1% gum tragacanth; 1 ml /kg) . One-half hour later, rats were treated with either milrinone (2.0 or 4.0 mg base /kg) or vehicle alone (1% gum tragacanth; 1 m l /kg ) creating five treatment cohorts: vehicle alone, enalapril alone, enalapril with 2.0 m g / k g of milrinone, and milrinone per se at 2.0 or 4.0 mg/kg . One hour after the second treatment, he- modynamic status was again determined followed by injection of l°3Ru-labelled microspheres and killing of animals. Treatment effects were thus assessed at 1 and 0.5 or 1 h after administration of enalapril or milrinone, respectively. These times correspond to the time to peak enalapril and milrinone levels in the infarcted rat model when these drugs are given alone or in succession (Dr. J. Baker et al., unpublished observations). Enalapril was administered at 1.0 m g / k g p.o. since this dose completely inhibits ACE in the rat within 1 and 0.5 h, of administration (Gross et al., 1981). O n e hour after an oral dose of 2.0 g m g / k g to 4 weeks MI rats, blood levels of milrinone (1.04 ___ 0.36 eq. t tg /ml : Dr. J. Baker et al., unpublished observa- tions) exceeds those seen clinically in milrinone- treated congestive heart failure patients (Stroshane et al., 1984). Furthermore, 2.0 m g / k g p.o. per day of milrinone significantly prolongs survival of rats with healed myocardial infarcts (Sweet et al., 1988). After killing, tissues were harvested, weighed and radioactivity of samples was determined as de- scribed above. After radioactivity of the lung tis-

213

sue was determined, lung dry weight was obtained by incubating the tissue at 90 ° C for 2 days.

2.4. Histology

After the rats were killed, the hearts were weighed, fixed in 10% formalin for 24 h, and cut into seven transverse slices from base to apex. From the projected silhouette of each section, the lengths of scar and total left ventricle on both endocardial a n d epicardial surfaces of each sec- tion were measured with a planimeter (Numonics Corp., Model 1240-1). Infarct size was calculated by dividing the sum of the endocardial and epicar- dial circumferences occupied by the infarct by the sum of the total epicardial and endocardial cir- cumferences of the left ventricle. This ratio × 100 yielded infarct size expressed as % of LV circum- ference. Cross sectional area measurements of in- farct s ize were not made because these under- estimate size owing to resorption of necrotic tis- sue, subsequent wall thinning (Fishbein et al., 1978), and late development of compensatory myocardial hypertrophy (Rubin et al., 1983).

2.5. Statistical analysis

ANOVA followed by Dunnett ' s test was used to identify any significant baseline differences be- tween the infarcted groups and the sham-operated control group at 4 weeks post-infarction, and to determine the hemodynamic effects, i.e. changes from baseline, induced by milrinone, enalapril a n d combination therapy in infarcted rats. Synergy between milrinone and enalapril was tested using the null hypothesis H0: [(milrinone effect + enalapril effect) - combination effect - placebo] = 0. All analyses were made using a P < 0.05 significance level.

2.6. Drugs

Enalapril maleate was obtained from Merck, Sharp and Dohme Research Labs. (West Point, PA) and milrinone was supplied by the Sterling Research Group (Rensselaer, NY). Carbocaine and ethrane were provided by Winthrop-Breon Labs (NYC, NY) and Anaquest (Madison, WI), respec- tively,

214

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3 . R e s u l t s

3.1. Baseline parameters of conscious sham-operated (SO) vs. M1 rats (Study A)

Basa l h e m o d y n a m i c p ro f i l e s o f c o n s c i o u s ra t s 4

w e e k s a f t e r surgery , a n d p r i o r to a n y t r e a t m e n t ,

a re s h o w n in t ab l e 1. T h e M I rats , w h i c h h a d

4 3 _ 1% o f t he lef t v e n t r i c u l a r c i r c u m f e r e n c e re-

p l a c e d wi th sca r t issue, h a d s ign i f i can t ly i n c r e a s e d

L V e n d - d i a s t o l i c p r e s s u r e ( a p p r o x i m a t e l y 23 vs. 6

m m H g for SO ra t s ) a n d s ign i f i can t ly d e p r e s s e d

L V d p / d t m a x ( - 4 0 % ) re la t ive to SO rats . T h e L V

p e a k d e v e l o p e d a n d s y s t e m i c b l o o d p r e s s u r e s were

215

s ign i f i c an t l y d e p r e s s e d b y a p p r o x i m a t e l y 20 m m

Hg, i.e. 15%. M e a n h e a r t r a t e w a s d e p r e s s e d b y

13% (P < 0.05) w h e r e a s c a r d i a c o u t p u t i n d e x a n d

to t a l p e r i p h e r a l v a s cu l a r r e s i s t a n c e of t hese c o m -

p e n s a t e d M I ra t s w e r e s imi la r to t h o s e for SO rats .

T h e c a r d i a c o u t p u t d i s t r i b u t i o n w h i c h we m e a s -

u r e d in the c o n t r o l r a t s ( t ab le 2) w a s s imi la r to

t h a t o f i n t ac t ( K a n d a a n d F l a i m , 1983) or SO

( D r e x l e r et al. 1985a; D r e x l e r e t al., 1985b) ra t s

r e p o r t e d b y o t h e r i nves t iga to r s . M e a n b l o o d f low

to t he t i s sues s a m p l e d was n o t s ign i f i can t ly a l t e r ed

in t he i n f a r c t e d ra t s ( t ab le 2).

O r g a n w e i g h t s a n d L V i n f a r c t size a t a u t o p s y

a re s h o w n in t ab l e 3. N o r m a l i z e d h e a r t a n d lung

TABLE 2

Basal regional blood flow of conscious rats prior to oral treatment with milrinone, enalapril or combination therapy a

Parameter a Sham Myocardial infarction: treatment b

operation Vehicle MIL-2 MIL-4 ENAL MIL-2 + ENAL (N = 14) (N = 21) (N = 20) (N = 15) (N = 18) (N = 18)

Kidney 5.5 +0.2 5.1 _+0.2 5.8 +0.3 6.0_+0.3 5.5 _+0.3 5.4 +0.3 Heart 2.7 _+0.2 2.6 +0.2 2.5 _+0.1 2.5_+0.2 2.9 _+0.2 2.5 _+0.2 Small intestine 1.6 _+0.1 1.7 _+0.1 1.6 _+0.2 1.9_+0.2 1.7 _+0.1 2.0 _+0.1 Skeletal muscle

(gastrocnemius, biceps brachialis) 0.08 _+ 0.01 0.09 _+ 0.02 0.09 _+ 0.01 0.1 _+ 0.01 0.07 _+ 0.01 0.08 _+ 0.01

Brain 0.9 _+0.1 1.0 +0.1 0.9 +0.04 1.0+0.1 1.3 _+0.1 1.0 _+0.1 Stomach 0.7 _+0.1 0.6 +0.1 0.6 -+0.1 0.8_+0.1 0.5 +0.03 0.6 _+0.01

a Determined using 153Gd-labelled microspheres infused into the left ventricle. Values (ml/min) are mean_ S.E. b MI rats which were subsequently treated orally with 1% gun tragacanth (vehicle), milrinone at 2 or 4 mg/kg (MIL-2; MIL-4), enalapril at 1.0 mg/kg (ENAL) or both agents (MIL-2 + ENAL).

TABLE 3

Myocardial infarct size and organ weights in 4 weeks post-infarcted (MI) rats a

Parameter a Sham Myocardial infarction: treatment b

operation Vehicle MIL-2 MIL-4 ENAL MIL-2 + ENAL (N = 14) (N = 21) (N = 20) (N = 15) (N = 18) (N = 18)

Infarct size (% LV) 0 +0 42.3 +1.2 a 42.0 +2.3 a 44.8 +1.8 a 42.6 +1.4 a 42.2 +1.5 a Heartwt.(g/kg) 3.4 +0.1 4.2 +0.1 d 4.2 _+0.1 a 4.1 _0.1 a 4.6 _+0.1 't 4.4 __+0.1 a Lung

Wetwt.(g/kg) 5.3 +0.4 12.7 +0.7 a 12.7 +1.1 a 11.5 +1.1 a 17.0 +1.1 a's 14.5 +1.1 d Dry wt./wet wt. c 0.21 +0.004 0.21 + 0.01 0.21 _0.01 0.21 +0.004 0.20+0.01 0.19+0.003

Liver(g/kg) 34.7 _1.0 35.1 +1.0 35.8 +0.8 35.8 +1.3 37.7 +0.9 40.1 +1.9 a's Bodywt.(g) 392.5 _+5.7 394.5 _+8.5 396.8 _+8.7 391.0 +8.5 393.0 +7.8 387.5 _+6.0

a Mean+S.E. Rats were killed 1 h after treatment with milrinone and 1.5 h after treatment with enalapril, b MIL-2, MIL-4: milrinone, 2 or 5 mg/kg p.o., respectively. ENAL: enalapril, 1 mg/kg, p.o. c Lung dry wt./wet wt. ratio was obtained by weighing a portion of lung prior to and after 48 h dessication at 90 o C. d p < 0.05 vs. values for sham-operation using Dunnett's test. s p < 0.05 vs. vehicle-treated MI rats using Dunnett's test.

216

wet weights of MI rats were significantly greater (P < 0.05) than values for SO control rats, whereas mean body weights were comparable. However, the lung d ry /we t weight ratio was similar in both groups. Furthermore, the lung weight of enalapril- treated rats and the liver weight of rats given combination therapy were elevated.

3.2. Acute oral hemodynamic effects of enalapril and milrinone (Study B)

LV parameters, heart rate, systemic blood pres- sure and total peripheral vascular resistance prior to treatment were similar for the five groups of MI rats (tables 1, 2) as was mean infarct size (42-45% of LV circumference: table 3). The predominant effects of MIL-2 and MIL-4 treatment were to markedly reduce LVEDP ( - 3 9 % ; -46%) and to slightly increase heart rate ( + 16% at the higher dose), whereas increases in mean LV dP/dtm~ x (32%; 23%) were not statistically significant (fig.

1). After enalapril, mean LV dP/d tma ~ was in- creased and LVEDP was also decreased but changes were not statistically significant. The combination of milrinone and enalapril reduced LV filling pressure as effectively as milrinone per se and, furthermore, significantly promoted LV d P / d t ~ × . Combined therapy also raised heart rate by 13%.

Treatment effects on regional blood flow are shown in fig. 2. Milrinone significantly decreased mean renal blood flow at the lower dose ( - 1 8 % , P < 0.05) and tended to elevate coronary blood flow by approximately 25%. Whereas splanchnic blood flow fell by 13-20% after treatment with vehicle alone, small intestine and stomach blood flow were maintained at the higher milrinone dose. Enalapril augmented small intestinal blood flow modestly (16%) and tended to promote renal blood flow slightly. The combination therapy substan- tially increased coronary blood flow (40%), and maintained normal renal blood flow.

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TOTAL PERIPHERAL VASCULAR RESISTANCE &,l,I ~.1 LV END-DIASTOLIC PRESSURE CARDIAC OUTPUT INDEX

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TREATMENT GROUP TREATMENT GROUP TREATMENT GROUP

Fig. 1. Oral hemodynamic effects of milrinone at 2 or 4 m g / k g (MIL-2, MIL-4), enalapril at 1 m g / k g (ENAL) or vehicle in conscious myocardial infarcted rats at 1 h (MIL) or 1.5 h (ENAL) after dosing. * P < 0.05 vs. change in vehicle group.

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Fig. 2. Oral effect of milrinone, enalapril and vehicle on blood flow (radiolabelled microsphere technique) in conscious myocardial infarcted rats at 1 h (MIL) or 1.5 h (ENAL) after dosing. MIL-2, MIL-4: milrinone at 2 or 4 mg/kg ; ENAL: enalapril at 1 mg/kg .

* P < 0.05 vs. change in vehicle group.

Statistical tests for drug interaction, i.e. positive or negative synergy were not significant. Accord- ingly, the complementary effects of combination therapy appear to reflect additive effects of en- alapril and milrinone.

Hemodynamic effects of milrinone at 2 or 4 m g / k g p.o. were indistinguishable using Student's t-test. When data at the 2 and 4 m g / k g dosages were combined and compared to data from both vehicle groups, the predominant significant changes in milrinone-treated rats were alterations in LVEDP and coronary blood flow averaging - 42 and + 27%, respectively.

4. Discuss ion

The hemodynamic profile of the infarcted rat after 4 weeks of recovery is that of compensated cardiac hypertrophy i.e. normal cardiac output, but ejected from hearts weighing an average of

24% more than normal, and operating at markedly elevated filling pressure. Maintained pumping function and blood flow to vital organs 4 weeks after acute loss of myocardium could reflect oper- ation of several compensatory responses. The large increase in end-diastolic filling pressure and sig- nificant increase in heart weight despite extensive transmural fibrosis indicates that increased pre- load and myocardial hypertrophy act in concert to sustain forward output and organ perfusion. Al- though norepinephrine levels were not measured, the slight bradycardia, tendency toward a reduc- tion of TPVR, and unaltered renal blood flow suggest that enhanced neuronal or adrenal calecholamine release does not play a compensa- tory role at this stage. The Frank-Starling mecha- nism may, therefore, have a major role in preserv- ing cardiac output and preventing even greater reductions in LV dP/d tma x in these extensively scarred, dilated and hypertrophied ventricles.

218

The predominant effect of milrinone in these compensated rats was to markedly reduce LV filling pressure without compromising cardiac out- put. The reduction in pre-load could reflect venous vasodilator activity of milrinone observed in vitro (DeFeo and Morgan, 1985). The LV chamber compliance, which is reduced in the MI rat (Fletcher et al., 1981) could also be improved by milrinone to help reduce filling pressure in view of evidence from other studies that this agent enhances LV diastolic relaxation (Sys et al., 1986; Monrad et al., 1983; Jaski et al., 1985). Milrinone did not reduce cardiac output despite the appre- ciable fall in preload. The reason may be that the LV chamber of the MI rat operates on the steep portion of the diastolic pressure-volume curve (Fletcher et al., 1981) such that a mean reduction in LVEDP of 11 mm Hg, as afforded by milrinone, should reduce LV chamber volume, and thus myocyte length, by less than 8%. Greater reduc- tions in filhng pressure may well compromise cardiac output, however, and the importance of maintaining critical levels of preload (14-18 mm Hg) is appreciated in patients with acute MI (Bussman et al., 1977) or chronic C H F (Williams et al., 1977).

Since milrinone did not significantly restore LV d P / d t . . . . it does not appear to consistently in- crease LV fiber shortening velocity and, therefore, myocardial contractility in the 4 week post-MI rat. Emmert et al. (1987) also reported that LV dP/d tma x of 10 week post-MI rats was increased by only 9% when monitored after 7-12 days treat- ment with milrinone (2 m g / k g p.o. per day). Results are consistent with the lack of stimulatory effect of milrinone on isolated rat papillary muscle (Alousi et al., 1983; Sys et al., 1986). While the in vitro studies indicate that milrinone has no direct positive inotropic activity in the rat, any indirect effects on myocardial contractility in vivo might not be apparent when d P / d t is used as the marker. The marked reduction in pre-load (LVEDP) achieved by milrinone could offset any stimula- tory influences on LV dP/dtma X since marked reductions in normal pre-load and, therefore, di- astolic wall stress can significantly reduce LV d P / d t max (Zimpfer and Vatner, 1981).

The changes in regional blood flow may be due

to direct or indirect vasodilating activities of milrinone. The increase in blood flow to the heart may result from direct coronary artery vasodilat- ing activity since milrinone selectively relaxes coronary arteries in vitro (Harris et al., in press). Alternatively, increased coronary blood flow could be an autoregulatory response if myocardial oxygen demand had increased. However, as neither cardiac output nor systemic blood pressure were elevated by milrinone, cardiac pump work, and therefore oxygen demand, would not be expected to increase. The modest decrease in renal blood flow seen after treatment with the lower dose of milrinone may represent a 'steal ' phenomenon since cardiac output did not change. Alternatively, it may reflect angiotensin II-mediated renal vaso- constriction because renal blood flow was main- tained when enalapril was administered 30 min prior to milrinone. However, milrinone did not significantly reduce renal perfusion at the 4.0 m g / k g dose. This may be due to renal vasodila- tion at the higher dose since Harris et al. (in press) report that milrinone relaxes renal arteries, but not selectively.

The mechanism(s) by which enalapril reduces LV end-diastolic pressure is unclear as is the cor- responding, but more pronounced, effect of ACE inhibitors on pulmonary artery capillary pressure in patients with chronic overt C H F (Curtiss et a., 1978; Ader et al., 1980). As enalapril was used as a dosage which almost completely inhibits ACE (Gross et al., 1981), angiotensin II levels are ex- pected to decrease. This could dilate veins and reduce central venous pressure indirectly by in- hibiting norepinephrine release (Emerson, 1966). The variable effect of enalapril on LVEDP argues against a major or consistent role of the renin- angiotensin system in maintaining elevated basal LV filling pressure in 4 weeks conscious MI rats. Nor does enalaprilat, the active metabohte of en- alapril, consistently reduce filling pressure in anesthetized 10 weeks MI rats, unlike mi!rinone (Emmert et al., 1987). Rather, the elevated filling pressure is attributed to significantly decreased LV diastolic compliance (Fletcher et al., 1981). Since enalapril reduces the 15% mortality seen in the 1-4 week post-infarction interval (Sweet et al., 1988), even modest reductions in LV filling pres-

sure could be vital if achieved early in the healing phase of massive infarction (see below). The sig- nificant increases in small intestine and main- tained renal blood flow after enalapril treatment is consistent with the relatively selective vaso- constrictor activity of angiotensin on splanchnic, cutaneous and renal beds (Forsyth et al., 1971), and suggests that angiotensin-mediated vaso- constriction occurs in some vascular beds in this rat model. Bradykinin, a potent visceral and renal vasodilator (Douglas, 1968; DeFelice and Brous- seau, 1988) could also contribute to ability of enalapril, alone or combined with milrinone, to maintain intestinal and renal blood flows since the kinin is metabolized by ACE even in the rat (Vandongen et al., 1982).

Whereas relatively slight hemodynamic effects of enalapril per se in the 4 weeks MI rats implies normal or inconsistently increased PRA at this stage, response to enalapril could be more exten- sive in subjects with elevated PRA e.g. older de- compensated subjects or those receiving diuretic or vasodilator therapy. Thus captopril signifi- cantly reduces total vascular resistance of 3 months MI rats (Pfeffer et al., 1985a) and in patients with chronic congestive heart failure (Faxon et al., 1980) where PRA is often elevated (Curtiss et al., 1978). In the present study, effects of milrinone on re- gional blood flow were altered appreciably by enalapril. Enalapril obviated the effect of the lower dose of milrinone on renal blood flow, and, fur- thermore, larger and more consistent increases in coronary blood flow occurred during combination therapy. This suggests that certain of milrinone's effects may be tempered by reflex stimulation of PRA. Simultaneous use of milrinone and the ACE inhibitor captopril also produces complementary changes in the peripheral circulation of C H F pa- tients, and synergistically improves cardiac perfor- mance (LeJemtel et al., 1985). The latter interac- tion might have been predicted from the present study where cardiac performance i.e. LV d P / d t m a x

of the MI rat was consistently, and significantly, improved only during combination therapy.

As both LV dP/d tma x and LV tilting pressure of these compromised rats are grossly abnormal as early as 4 weeks, it is unclear whether factors responsible for depressed contractility or in-

219

creased cardiac load are more critical in precipitat- ing terminal heart failure in this model. Since milrinone was more effective than enalapril in restoring LV end-diastolic pressure in this study and in prolonging survival of the MI rat in other studies (Sweet et al., 1988), early relief of excessive pre-load may be vital in improving the prognosis of massive LV infarction. Combined therapy with milrinone and enalapril, which significantly pro- moted LV dP/dtma x but did not further reduce LV tilting pressure in our studies, was not more effective than milrinone per se in reducing mortal- ity in this model (Sweet et al., 198). This again indicates that mortality can be reduced by alleviat- ing preload judiciously. We conclude that the pre- dominant acute oral effect of enalapril and espe- cially milrinone in the 4 week compensated MI rat is to reduce LV filling pressure. This suggests that relief of preload may underlie the increased survival and reduced heart size reported for MI rats chronically treated with ACE inhibitors or milrinone (Pfeffer et al., 1985b; Sweet et al., 1988).

Acknowledgements

The authors thank Ms. Peggy Branch for typing the manuscript and Dr. J. Baker for determining plasma milrinone levels in pilot studies.

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