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Effect of intracoronary captopril on coronary blood flow and regional myocardial function in dogs
European Journal of Pharmacology, 110 (1985) 11-19
11
Elsevier
EFFECT OF I N T R A C O R O N A R Y C A P T O P R I L O N C O R O N A R Y B L O O D F L O W AND R E G I O N A L M Y O C A R D I A L F U N C T I O N IN D O G S KATSUHIKO NOGUCH1 *, TAKAYUKI KATO, HIROSUM1 ITO, YOKO ANIYA ** and MATAO SAKANASHI Department of Pharmacology, School of Medicine, Faculty of Medicine, University of the Ryukyus, and ** Laboratory of Pharmacology and Toxicology, School of Health Sciences, Faculty of Medicine, University of the Ryukyus, Nishihara, Okinawa 903-01, Japan
Received 15 October 1984, revised MS received 27 November 1984, accepted 18 December 1984
K. NOGUCHI, T. KATO, H. 1TO, Y. ANIYA and M. SAKANASHI, Effect of intracoronary captopril on coronary blood flow and regional myocardial function in dogs, European J. Pharmacol. 110 (1985 ) 11 - 19. To evaluate the cardiac effect of an inhibitor of angiotensin-converting enzyme, the effect of intracoronary (i.c.) captopril on coronary blood flow and regional myocardial function was examined in the anesthetized open-chest dog. Blood flow of the left circumflex coronary artery (LCX), left ventricular pressure (LVP), aortic pressure (AoP) and regional myocardial segment length were measured continuously. Captopril i.v. (0.3 mg/kg) produced an immediate reduction in AoP and an increase in percent shortening of myocardial segments followed by a decrease in coronary vascular resistance and increases in heart rate and LVdP/dt. Reductions in LCX flow induced by i.c. angiotensin were attenuated and i.c. bradykinin-induced increases in LCX flow were augmented after captopril. On the contrary, i.c. infusion of captopril (0.01 mg/min) into the LCX caused no change in hemodynamic variables and myocardial shortening although responses to angiotensin I and bradykinin were markedly modified. These results suggest that captopril may have no direct cardiac effect. Angiotensin-converting enzyme Captopril
Regional myocardial function
1. Introduction Although captopril, an angiotensin-converting enzyme inhibitor, has consistently been shown to improve the left ventricular performance of patients with severe congestive heart failure (Antonaccio, 1982), the mechanism(s) responsible for the salutary effect of captopril is not yet fully understood (Cohn and Levine, 1982). Angiotensin-converting enzyme is thought to be localized on the endothelium of various blood vessels (Ryan et al., 1975; Caldwell et al., 1976; Miyazaki et al., 1984), and might serve to maintain the tone of the respective vascular beds through
* To whom all correspondence should be addressed. 0014-2999/85/$03,30 © 1985 Elsevier Science Publishers B.V.
Intracoronary infusion
Coronary blood flow
locally formed angiotensin II (Britton and Di Salvo, 1972; Cohen and Kurz, 1982). However, it is not established whether local inhibition of angiotensin-converting enzyme in the coronary vasculature influences coronary vascular tone a n d / o r myocardial contraction. To understand the mechanism by which captopril exerts beneficial effects in congestive heart failure, it is important to know to what extent the direct effect of captopril on coronary circulation and myocardial contraction participates in the hemodynamic response to systemically given captopril. The present study aimed to examine the direct effect of an angiotensin-converting enzyme inhibitor on the heart. To this end we evaluated the effects of i.v. and i.c. captopril on coronary blood flow and regional myocardial function in anesthetized dogs.
12
2. Materials and methods
Experiments were conducted on 24 mongrel dogs of either sex weighing 10 to 24 kg (mean = 16.0 kg). The dogs were anesthetized with pentobarbital sodium (25 m g / k g i.v.) and ventilated with room air by a Harvard respirator (model 607). Left thoracotomy was performed through the fifth intercostal space. The pericardium was opened and the heart was suspended in a pericardial cradle. A catheter tip manometer (Millar Instrument Co., Houston, TX) was introduced through the right femoral artery into the left ventricle to measure left ventricular pressure. A cannula filled with heparinized saline (0.9% NaCI) was inserted through the left carotid artery into the aortic root and connected to a Statham pressure transducer (P23ID) to measure the aortic pressure. An appropriately sized electromagnetic flow probe (Statham, SP7515) was placed around the left circumflex coronary artery (LCX) and connected to an electromagnetic flowmeter (Statham, SP2204) for the measurement of coronary blood flow. Zero flow was determined by temporary occlusions of the artery just distal to the flow probe with a snare occluder.
Y
SEGMENT LAD
IIM.:NXIT:
Fig. 1. Schematic representation of the experimental preparation. The left circumflex coronary artery (LCX) w a s d i s s e c t e d free near its origin. An electromagnetic flow (EMF) probe w a s placed around LCX, and a 27-gauge hypodermic needle connected to a fine polyethylene tube was inserted distal to the flow probe for constant infusion and injections of drugs. Two pairs of piezoelectric crystals were implanted within the LCXperfused area and the left anterior descending coronary artery (LAD)-perfused area, respectively.
As shown in fig. 1, a siliconized 27-gauge hypodermic needle connected to a fine polyethylene tube (3 cm long) was inserted into the LCX 0.5 to 1 cm distal to the flow probe for the i.c. administration of drugs. The distal end of the tube was connected to two other polyethylene tubes for constant infusion of either saline or captopril and for the injection of drugs, respectively. To prevent coagulation within the tube, saline was infused by means of a Harvard infusion pump (model 975) at a constant rate of 0.1 m l / m i n throughout the experiments. Regional myocardial segment length was measured by sonomicrometry (Rushmer et al., 1956; Theroux et al., 1974) which provides continuous quantification of regional myocardial function (Theroux et al., 1974). Two pairs of 5 MHz piezoelectric crystals (2 mm in diameter) were implanted in the left ventricular wall close to the endocardium in a circumferential plane. One pair was placed in the area perfused by the LCX to estimate the effect of drugs on myocardial segment length, and another pair was placed in the area perfused by the anterior descending coronary artery (LAD) to serve as a control segment (fig. 1). The leads of the crystals were connected to a four-channel ultrasonic amplifier (Mecc, UDM-5B) which generated voltages linearly proportional to the transit time of acoustic impulses travelling between each pair of crystals, and thus giving electrical signals proportional to the distance between the paired crystals. The system was precalibrated in vitro at 37°C and was repeatedly calibrated by substituting signals of known time duration from a calibrated pulse generator to eliminate any drift. Mean coronary blood flow was measured by using an electronic resistance-capacitance filter with a 2 s time constant. Left ventricular d P / d t ( L V d P / d t ) was derived from differentiating the left ventricular pressure signal using an electronic differentiator (Nihon Kohden, ED-601G). The heart rate was counted continuously with a cardiotachometer (Nihon Kohden, AT-600G) triggered by the pressure pulse. Data were recorded continuously on a multichannel pen recorder (Nihon Kohden, WT-685G) and stored on tape
13
using an FM data recorder (TEAC, R-81) for subsequent data analysis. Experiments were started at least 30 min after instrumentation. The dogs were divided into four groups. In the first group (n = 6), the animals received i.v. captopril (0.3 mg/kg), which has been shown to cause significant hypotension and more than 70% inhibition of pressor response to i.v. angiotensin I (Noguchi et al., 1983). To evaluate the extent of inhibition of angiotensin-converting enzyme within the coronary vasculature, 200, 400 and 800 ng of angiotensin I (Peptide Institute Protein Research Foundation) and 50, 100 and 200 ng of angiotensin II (Peptide Institute Protein Research Foundation) were injected into the LCX before and 10 min after captopril administration. In the second (n = 6) and third (n = 6) groups, responses to i.c. angiotensin I and angiotensin II, or bradykinin (Peptide Institute Protein Research Foundation) and acetylcholine chloride (Daiichi Seiyaku Co. Ltd.) were examined respectively before and during i.c. infusion of captopril (0.01 mg/min) to estimate the efficacy of inhibition of converting enzyme in the heart, since blockade of this enzyme is expected to result in attenuation of vasoconstrictor response to angiotensin I or augmentation of vasodilatory response to bradykinin. The percent inhibition of coronary vasoconstrictor and systemic pressor response to angiotensin I was calculated as the difference between values before and after captopril administration × 100 divided by the respective control value. In the fourth group (n = 6), the effect of i.c. infusion of captopril (0.01 m g / m i n ) into the LCX on coronary blood flow and regional myocardial function was observed for 30 min. The reason why 0.01 m g / m i n of captopril was employed in this study was that it was the maximal dosage which had not affected the systemic blood pressure in pilot studies. All drugs were dissolved in physiological saline. A final volume of 0.1 to 0.4 ml angiotensin I, angiotensin II, bradykinin or acetylcholine was injected into the LCX for 10 to 15 s. The i.c. injection of the corresponding volume of physiological saline did not cause any significant changes in the parameters measured. Percent regional myocardial shortening was assessed as the percent ratio of end-diastolic length
minus end-systolic length to end-diastolic length. Coronary vascular resistance was calculated as the quotient of mean aortic pressure and mean coronary blood flow. Time sequence data were analyzed by two-way analysis of variance and paired data were analyzed with the paired t-test. The level for statistical significance was P < 0.05. All results were expressed as means 5- standard error.
3. Results
3.1. Effect of intravenous captopril on systemic and coronary circulation and regional myocardial function Captopril (0.3 m g / k g i.v.) produced an immediate and sustained reduction in mean aortic pressure, followed by a decline of calculated coronary vascular resistance and by increases in L V d P / d t and heart rate. Mean coronary blood flow was practically unchanged in spite of a significant reduction in mean aortic pressure (fig. 2). Left ventricular end-diastolic pressure decreased from a control value of 2.5 5- 0.2 mm Hg to 2.0 50.3 mm Hg 10 min after the injection but the change was not significant. A significant decrease in end-systolic segment length occurred within 1 min of the administration, corresponding to the change in aortic pressure. End-diastolic segment length also decreased only slightly. Thus, the resultant percent myocardial shortening calculated from the end-systolic and diastolic length was significantly increased to 22.6 5- 2.2% at 3 min after the injection, from a control value of 19.9 _ 1.5% (fig. 2). Intracoronary injections of angiotensin I 200, 400 and 800 ng decreased coronary blood flow by 13.9 + 2.2%, 22.4 + 4.5% and 34.8 5- 6.3% under control conditions. After i.v. administration of captopril (0.3 mg/kg), these responses were significantly attenuated to 5.5 ± 1.5%, 6.4 5- 2.0% and 11.5 5- 2.7%, respectively. Thus, 0.3 m g / k g i.v. of captopril inhibited the coronary vasoconstrictor response to i.c. angiotensin I by 65.0 + 5.3% (n = 18) of the control response. The maximal rise in aortic pressure induced by angiotensin I, which
14
CAPTOPRIL O,3mg/kg
i.v.
3.2. Responses to angiotensin I and l I or bradykmin and acetylcholine injected into the L C X during intracoronary infusion of captopril
l 11o
MAoP mmHg
lOO
I
go MCBF ml/min
35[ 25
e.o[
CVR
i
mm H g / s e e
MR
-~*
leo[
[
**
LVEDP
mrnHg SEGMENT LENGTH mm
II
e
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[
,
'
1"*
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T-
| 1
I 3
I 5
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I
18
I BEFORE
I 0
~ rain
I 10
Fig. 2. Time courses of mean aortic pressure (MAoP), mean coronary blood flow (MCBF), coronary vascular resistance (CVR), left ventricular dP/dt (LVdP/dt), heart rate (HR), left ventricular end-diastolic pressure (LVEDP), myocardial segment length and percent myocardial shortening after an intravenous injection of captopril (0.3 mg/kg). All values are expressed as means_+S.E. * P < 0.05; ** P < 0.01. n = 6.
was noted following the maximal decrease in coronary blood flow, was also reduced by 65.2 ± 5.5% of the control response, indicating that blockade of angiotensin conversion in the systemic circulation was comparable to that in the coronary vascular beds. On the contrary, the responses of coronary blood flow to i.c. angiotensin II (50 and 100 rig) were significantly enhanced by captopril (15.5 ± 4.0% and 21.5 ± 3.5% to 21.8 ± 3.2% and 29.7 ± 4.6%, respectively).
Under control conditions, i.c. injections of angiotensin I (200, 400 and 800 ng) elicited dose-related and transient decreases in coronary blood flow 16 to 26 s after the injection followed by increases in aortic pressure (figs. 3 and 4). During the infusion of captopril (0.01 mg/min), the percent reductions in coronary blood flow induced by 200, 400 and 800 ng of angiotensin I were significantly attenuated from the control response of 11.4 _+ 2.4%, 21.3 + 3.7% and 26.4 ± 5.6% to 1.2 ± 0.8%, 4.5 ± 1.5% and 6.6 ± 2.0%, respectively (fig. 4). Thus, captopril (0.01 rag/rain) inhibited i.c. angiotensin l-induced decreases in coronary blood flow by 81 + 4% (n = 18) of the control response; this dose of i.c. captopril blocked angiotensin-converting enzyme in the coronary vasculature more strongly than did 0.3 m g / k g of captopril administered i.v. as mentioned above. On the other hand, inhibition of pressor responses to i.c. angiotensin I by captopril infusion was only 29 ± 6% (n = 18). In contrast to the case of angiotensin I, the responses to angiotensin II were not inhibited by captopril although changes in coronary blood flow and aortic pressure induced by 50, 100 and 200 ng of angiotensin II similar to those with angiotensin I were noted before captopril (figs. 3 and 4). Dose-related increases in coronary blood flow induced by i.c. injections of bradykinin (10, 20 and 40 rig) were augmented by i.c. captopril (36 ± 11% to 66 ± 16%, 68 ± 10% to 101 + 17%, 102 _+ 15% to 141 ± 19%, respectively), as shown in figs. 5 and 6. The time required for returning to the half maximal increase in coronary blood flow after an injection of bradykinin was also prolonged by the inhibitor(ll ± 1 s t o 2 6 + 7 s , 1 4 ± l s t o 4 1 ± 8 s and 19 ± 2 s to 55 + 8 s, respectively). The coronary vasodilatation induced by acetylcholine (200, 400 and 800 ng i.c.), in contrast to bradykinin, was not significantly affected by captopril (figs. 5 and 6).
15
CONTROL
A. ANGIOTENSIN I (ng, i.c.)
200
4oo
5O
100
CAPTOPRIL 0.01 mg/min
i.c.
8oo
15
AoP m m Hg
5
CBF ml/min
Io
MCBF ml/min
lo
B. M ~ G I O T E N S I N I1
(ng, i.c.)
•
•
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5o
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v
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2OO •
1 AoP rnrn Hg
!I;
•
,~i
¸
1
CBF
ml/min
MCBF
ml/min It) L i 1 sec
I rain
Fig. 3. Responses of aortic pressure (AoP), coronary blood flow (CBF) and mean coronary blood flow (MCBF) to intracoronary injections of angiotensin I (200, 400 and 800 ng) and angiotensin II (50, 100 and 200 ng) before (left panels) and during (right panels) intracoronary infusion of captopril. ,'~ 20 o
3.3. Effect of intracoronary infusion of captopril on systemic and coronary circulation and regional myocardial function
:E
_z <
i 200
400
800
M~IGIOTENSIN I ( n g , i . c . ) g,.
z
200
400
800
50
100
200
/U~IGIOTENSIN I1 ( n g , I . c . ) 50
100
200
lo
T h e effect of infusing c a p t o p r i l (0.01 m g / m i n ) into the L C X for 30 min on aortic pressure, m e a n c o r o n a r y b l o o d flow, L V d P / d t , heart rate a n d m y o c a r d i a l segment length is shown in fig. 7 a n d the results are s u m m a r i z e d in table 1. As seen in table 1, no changes in h e m o d y n a m i c variables occurred d u r i n g the infusion of captopril. T h e p e r c e n t m y o c a r d i a l shortening within the LCXperfused a r e a as well as that within the L A D - p e r fused area d i d not change significantly (table 1). O n l y the e n d - d i a s t o l i c segment length of the d r u g - t r e a t e d m y o c a r d i u m decreased significantly 30 min after the start of the infusion, concom-
5O
Fig. 4. Percent changes in responses of mean aortic pressure (MAoP) and mean coronary blood flow (MCBF) to intracoronary injections of angiotensin 1 and angiotensin II be-
fore (open columns) and during (hatched columns) intracoronary infusion of captopril (0.01 mg/min). All values are expressed as means+ S.E. * P < 0.05; ** P < 0.01. n = 6.
16 CONTROL
CAPTOPRIL 0.01mg
A.
BRADYKIN IN (ng,
200
min
i.c.
(.9 _Z
i.c.)
10
20
10
40
20
40 loo
AoP mrnHg
50
o
IJ
CBF ml/min
lO
(ng,
200
i.e.)
m
150[ AoP mmHg
CBF ral/min
MCBF ral/min
~!iil
~I I
CAPTOPRIL
1
MCBF
0
LVP
~
~ --=
-
-
~-
L
.
0.O1
5 min
t
IOOF
ml/rnin
800
Fig. 5. Responses of aortic pressure (AoP), coronary blood flow (CBF) and mean coronary blood flow (MCBF) to intracoronary injections of bradykinin (10, 20 and 40 ng) and acetylcholine (200, 400 and 800 ng) before (left panels) and during (right panels) intracoronary infusion of captopril.
L-~ 1 rain
50
400
1 I
l
15° I
200
ACETYLCHOLINE (ng, i.c)
|1
50
AoP mmHg
40
Fig. 6. Percent changes in responses of mean coronary blood flow (MCBF) to intracoronary bradykinin and acetylcholine before (open columns) and during (hatched columns) intracoronary infusion of captopril (0.01 mg/min). All values are expressed as means+ S.E. * P < 0.05. n = 6.
MCBF rnl/min
I]. A C E T Y L C H O L I N E
20
13RADYKININ (n 9, i . c )
.
.
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.
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.
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.
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-
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.
.
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.
.
.
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--
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30 min
~
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i.c.
15 r n i n
~ -
-
20
mg/min
10 m i n
I . , ,
•
-
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LVd P/d t
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---
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LCX rn rn
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.-
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-
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.
.
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.
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lOO L_a I rain
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Fig. 7. Representative tracings of hemodynamic variables and myocardial segment shortening before and during infusion of captopril into the left circumflex coronary artery. LAD = left anterior descending coronary artery-perfused area. LCX = left circumflex coronary artery-perfused area. See the legend to fig. 2 for abbreviations.
17 TABLE 1 Hemodynamic variables and regional myocardial performance during intracoronary infusion of captopril (0.01 mg/min). MAoP= mean aortic pressure; MCBF= mean coronary blood flow; CVR = coronary vascular resistance; HR = heart rate; LVdP/dt = left ventricular dP/dt; LVEDP= left ventricular end-diastolic pressure; LAD = left anterior descending coronary artery-perfused area; LCX = left circumflexcoronaryartery-perfused area; EDL = end-diastolic segment length. * P < 0.05. n = 6. MAoP M C B F CVR HR (mmHg) (ml/min) (mmHg/ml (beats/ per rain) rain) Control
LVdP/dt LVEDP LAD (mmHg/s) (mm Hg) EDL (mm)
LCX Shortening ~)
EDL (mm)
Shortening (4)
1035:6 26.5+_4.2 4.275:0.55
128+-10 2026+160 4.15:0.7 9.29+_1.01 28.8+_2.2 9.03+_1.40 15.4+_1.7
5 min
103+_6 26.7+_4.4 4.275:0.56
128+10 20285:156 4.0+_0.7 9.24+_1.01 28.7+_2.2 9.01+_1,40 15.45:1.8
10min
103+_6 27.0+_4.0 4.165:0.54
1295:10 20355:137 4.2+_0.7 9.26+1.01 29.2+2.1 8.98+-1.40 15.25:1.8
15 min
103+7
27.1+-3.8 4.145:0.53
1305:10 2031+_137 3.9+_0.7 9.20+-0.99 29.05:2.2 8.99+-1.40 15.35:1.9
30min
102+7
26.4+3.3 4.165:0.56
1315:10 1965+_ 98 3.85:0.7 9.15+_0.99 29.1+2.0 8.94+-1.39" 14.65:1.9
Captopril
itantly with an insignificant decline of that in the L A D area (table 1).
4. Discussion The major purpose of this study was to evaluate the effect of an angiotensin-converting enzyme inhibitor on coronary vascular tone and myocardial contraction. If the angiotensin-converting enzyme within one vasculature plays physiologically significant roles, blockade of the enzyme would result in vasodilatation of the vessel because tonic formation of angiotensin II, a potent vasoconstrictor peptide, would decrease. Although several studies have shown that i.v. administration of angiotensin-converting enzyme inhibitors produced coronary vasodilatation in patients without coronary artery disease (Faxon et al., 1982), sodium-depleted dogs (Liang et al., 1978) and anesthetized normal dogs (Maxwell et al., 1981; Noguchi et al., 1983), there has been no study in which the inhibitor was applied directly into the coronary artery. Additionally, it is meaningful to examine whether blockade of locally formed angiotensin II could influence myocardial function, since it has been reported that angiotensin II has a direct inotropic effect (Fowler and Holmes, 1964; Koch-Weser, 1965). Thus, in this experiment
the effect of i.c. as well as i.v. captopril on coronary blood flow and regional myocardial function was evaluated. Most of the exogenous angiotensin I is converted to angiotensin II by the converting enzyme on the endothelium of coronary vascular beds then causes vasoconstriction, as demonstrated by Britton and Di Salvo (1972), since angiotensin I conversion in plasma has been shown not to play an important role (Oparil et al., 1970). Reductions in coronary blood flow induced by i.c. injections of angiotensin I with a similar pattern to those of angiotensin II were potently inhibited by captopril. Moreover, enhancement of the response to bradykinin was observed during captopril infusion, probably due to inhibition of kinin degradation by kininase II because this enzyme is considered to be identical to an angiotensin-converting enzyme (Engel et al., 1972). It is unlikely that the alteration in the response to angiotensin I or bradykinin caused by captopril was derived from changes in reactivity of the vessel, since responses to angiotensin II and acetylcholine were almost unchanged before and after captopril. Therefore, these results indicate that the angiotensin-converting enzyme certainly exists in the coronary vasculature and causes coronary vasoconstriction by converting exogenous angiotensin I to angiotensin II.
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The present experiments showed that 0.01 m g / m i n i.c. infusion of captopril failed to alter resting coronary blood flow, aortic pressure or regional myocardial shortening while i.v. administration of captopril (0.3 mg/kg) produced an immediate decrease in aortic pressure and an increase in myocardial shortening. Nevertheless, the dose of i.c. captopril used in this study may be regarded as sufficient to inhibit angiotensin-converting enzyme in coronary circulation, because reductions in coronary blood flow induced by i.c. angiotensin I were potently suppressed by the i.c. infusion of captopril (81%) rather than by i.v. administration of the inhibitor (65%). Thus, local blockade of angiotensin-converting enzyme in coronary beds did not affect resting coronary blood flow and regional segment shortening. The lack of vasodi[ator action of captopril applied directly into the coronary artery has also been observed in our previous study (Noguchi et al., 1983) where captopril (up to 0.1 raM) did not relax the potassium contracture of isolated canine coronary strips. Recently, Liang et al. (1982) have shown that direct effects of renin-angiotensin system blockade upon myocardial performance were lacking in conscious dogs with coronary occlusion. Chiba et al. (1983) have also demonstrated that a dose level of captopril which caused significant hypotension had no direct cardiac effect. In contrast to the case of i.c. administration, i.v. captopril caused significant decreases in aortic pressure and calculated coronary vascular resistance and increases in heart rate and LVdP/dt. These latter two changes may have been primarily due to baroreflex-mediated secondary effects, because the positive chronotropic and inotropic effects were invariably seen following hypotension and the drug was shown to have little direct cardiac action as described above. An increase in percent segment shortening, which was due to a larger decrease in end-systolic length than in end-diastolic length, coincided chronologically with a decrease in aortic pressure, and hence a major part of this change may have been due to an indirect effect caused by a fall in blood pressure which would lessen the afterload during systolic shortening. In the previous study we observed that the hypotension and the decrease in coronary vascular
resistance induced by captopril (0.01-0.3 m g / k g i.v.) were significantly correlated with inhibition of the angiotensin I-pressor response (Noguchi et al. 1983). Thus, vasodilatation following i.v. captopril appears to be mainly due to lowering of the circulating level of angiotensin II which was induced by inhibition of angiotensin I-converting enzyme in systemic vascular beds, primarily in lung (Ng and Vane, 1967), as suggested in several reports (Murthy et al., 1978; Sato et al., 1980; Zimmerman et al., 1982; Chen et al., 1984). However, other mechanisms may be involved in i.v. captopril-induced vasodilatation, because it has been reported that angiotensin-converting enzyme inhibitors have various actions such as augmentation of the kinin level through inhibition of kininase I1 (Engel et al., 1972), facilitation of PGI 2 synthesis (Swartz et al., 1980; Di)sing et al., 1983), and suppression of sympathetic nerve transmission (Antonaccio and Kerwin, 1980; Clough et al., 1982). Thus, the precise mechanism responsible for the hypotension remains unclear. Marked differences in hemodynamic responses to i.v. and i.c. administered captopril were observed in this study. The relatively weak inhibition (29%) of the angiotensin I-induced pressor response observed during i.c. infusion of captopril appears to indicate that the quantities of infused captopril were too small to cause sufficient blockade of angiotensin 1 conversion in the extracardiac circulation. Thus, the reason why i.c. captopril did not produce marked changes in hemodynamics may be that there was only slight inhibition of the enzyme in the systemic vasculature. The finding that the cardiac action of i.c. captopril was quiescent, despite the strongly inhibitor-blocked angiotensin I conversion within the coronary vasculature, may imply that reninangiotensin system in the canine heart plays only a minor if any part in the regulation of resting coronary hemodynamics. Additionally, the present study also suggests that the acute beneficial effect of an angiotensin-converting enzyme inhibitor on cardiac performance seen in patients with heart failure (Antonaccio, 1982) may result mainly from its peripheral effect rather than from the direct cardiac effect.
19
Acknowledgments We thank Junko Nakasone for secretarial assistance, and Sankyo Pharmaceutical Company for supplying captopril. This work was supported in part by a Developmental Scientific Research Grant (No. 58770170) from the Ministry of the Education, Science and Culture, Japan.
References Antonaccio, M.J., 1982, Angiotensin converting enzyme (ACE) inhibitors, Ann. Rev. Pharmacol. Toxicol. 22, 57. Antonaccio, M.J. and L. Kerwin, 1980, Evidence for prejunctional inhibition of norepinephrine release by captopril in spontaneously hypertensive rats, European J. Pharmacol. 68, 209. Britton, S. and J. Di Salvo, 1972, Effects of angiotensin I and angiotensin I1 on hindlimb and coronary vascular resistance, Am. J. Physiol. 225, 1226. Caldwell, P.R.B., B.C. Seegal, K.C. Hsu, M. Das and R.L. Softer, 1976, Angiotensin-converting enzyme: Vascular endothelial localization, Science 191, 1050. Chen, X., B.R. Pitt, R. Moalli and C.N. Gillis, 1984, Correlation between lung and plasma angiotensin converting enzyme and the hypotensive effect of captopril in conscious rabbits, J. Pharmacol. Exp. Ther. 229, 649. Chiba, S., H. Watanabe and M. Kobayashi, 1983, Chronotropic and inotropic effects of captopril on the dog heart, Jpn. Heart J. 24, 269. Clough, D.P., M.G. Collis, J. Conway, R. Hatton and J.R. Keddie, 1982, Interaction of angiotensin-convertingenzyme inhibitors with the function of the sympathetic nervous system, Am. J. Cardiol. 49, 1410. Cohen, M.L. and K.D. Kurz, 1982, Angiotensin converting enzyme inhibition in tissues from spontaneously hypertensive rats after treatment with captopril or MK-421, J. Pharmacol. Exp. Ther. 220, 63. Cohn, J.N. and T.B. Levine, 1982, Angiotensin-converting enzyme inhibition in congestive heart failure: The concept, Am. J. Cardiol. 49, 1480. Di~sing, R., R. Scherhag, G. Landsberg, K. Glanzer and H.J. Kramer, 1983, The converting enzyme inhibitor captopril stimulates prostacyclin synthesis by isolated rat aorta, European J. Pharmacol. 91, 501. Engel, S.L, T.R. Schaeffer, B.I. Gold and B. Rubin, 1972, Inhibition of pressor effects of angiotensin I and augmentation of depressor effects of bradykinin by synthetic peptides, Proc. Soc. Exp. Biol. Med. 140, 240. Faxon, D.P., M.A. Creager, J.L. Halperin, H.A. Sussman, H. Gavras and T.J. Ryan, 1982, The effect of angiotensin converting enzyme inhibition on coronary blood flow and hemodynamics in patients without coronary artery disease, Int. J. Cardiol. 2, 251.
Fowler, N.O. and J.C. Holmes, 1964, Coronary and myocardial actions of angiotensin, Circ. Res. 14, 191. Koch-Weser, J., 1965, Nature of the inotropic action of angiotensin in ventricular myocardium, Circ. Res. 16, 230. Liang, C.S., H. Gavras, J. Black, L.G. Sherman and W.B. Hood, Jr., 1982, Renin-angiotensin system inhibition in acute myocardial infarction in dogs. Effects on systemic hemodynamics, myocardial blood flow, segmental myocardial function and infarct size, Circulation 66, 1249. Liang, C.S., H. Gavras and W.B. Hood, Jr., 1978, Reninangiotensin system inhibition in conscious sodium-depleted dogs. Effects on systemic and coronary hemodynamics, J. Clin. Invest. 61,874. Maxwell, G.M., V. Rencis and D. Ness, 1981, The effects of captopril (SQ14225) on the systemic and coronary haemodynamics of the intact anesthetized dog, Clin. Exp. Pharmacol. Physiol. 8, 17. Miyazaki, M., H. Okunishi, K. Nishimura and N. Toda, 1984, Vascular angiotensin-converting enzyme activity in man and other species, Clin. Sci. 66, 39. Murthy, V.S., T.L. Waldon and M.E. Goldberg, 1978, Inhibition of angiotensin-converting enzyme by SQ14,225 in anesthetized dogs: hemodynamic and renal vascular effects. Proc. Soc. Exp. Biol. Med. 157, 121. Ng, K.K.F. and J.R. Vane, 1967, Conversion of angiotensin I to angiotensin II, Nature (London) 216, 762. Noguchi, K., Y. Miyamoto and M. Sakanashi, 1983, Acute hemodynamic effects of captopril in the anesthetized openchest dog, Jpn. J. Pharmacol. 33 Suppl., 254P. Oparil, S., C.A. Sanders and E. Haber, 1970, In vivo and in vitro conversion of angiotensin I to angiotensin II in dog blood, Circ. Res. 26, 591. Rushmer, R.F., D.L. Franklin and R.M. Ellis, 1956, Left ventricular dimensions recorded by sonocardiometry, Circ. Res. 4, 684. Ryan, J.W., U.S. Ryan, D.R. Schultz, C. Whitaker, A. Chung and F.E. Dorer, 1975, Subcellular localization of pulmonary angiotensin-converting enzyme (Kininase II), Biochem. J. 146, 497. Sato, S., S. Fujisawa, R. Tanaka and K. Nakai, 1980, Hypotensive effect of captopril, an angiotensin-converting enzyme inhibitor in pentobarbital anesthetized dogs, Jpn. J. Pharmacol. 30, 112. Swartz, S.L., G.H. Williams, N.K. Hollenberg, L. Levine, R.G. Dluhy and T.J. Moore, 1980, Captopril-induced changes in prostaglandin production. Relationship to vascular responses in normal man, J. Clin. Invest. 65, 1257. Theroux, P., D. Franklin, J. Ross, Jr. and W.S. Kemper, 1974, Regional myocardial function during acute coronary artery occlusion and its modification by pharmacologic agents in the dog, Circ. Res. 35, 896. Zimmerman, B.G., P.C. Wong, G.K. Kounenis and E.J. Kraft, 1982, No effect of intrarenal converting enzyme inhibition on canine renal blood flow, Am. J. Physiol. 243, H277.
11
Elsevier
EFFECT OF I N T R A C O R O N A R Y C A P T O P R I L O N C O R O N A R Y B L O O D F L O W AND R E G I O N A L M Y O C A R D I A L F U N C T I O N IN D O G S KATSUHIKO NOGUCH1 *, TAKAYUKI KATO, HIROSUM1 ITO, YOKO ANIYA ** and MATAO SAKANASHI Department of Pharmacology, School of Medicine, Faculty of Medicine, University of the Ryukyus, and ** Laboratory of Pharmacology and Toxicology, School of Health Sciences, Faculty of Medicine, University of the Ryukyus, Nishihara, Okinawa 903-01, Japan
Received 15 October 1984, revised MS received 27 November 1984, accepted 18 December 1984
K. NOGUCHI, T. KATO, H. 1TO, Y. ANIYA and M. SAKANASHI, Effect of intracoronary captopril on coronary blood flow and regional myocardial function in dogs, European J. Pharmacol. 110 (1985 ) 11 - 19. To evaluate the cardiac effect of an inhibitor of angiotensin-converting enzyme, the effect of intracoronary (i.c.) captopril on coronary blood flow and regional myocardial function was examined in the anesthetized open-chest dog. Blood flow of the left circumflex coronary artery (LCX), left ventricular pressure (LVP), aortic pressure (AoP) and regional myocardial segment length were measured continuously. Captopril i.v. (0.3 mg/kg) produced an immediate reduction in AoP and an increase in percent shortening of myocardial segments followed by a decrease in coronary vascular resistance and increases in heart rate and LVdP/dt. Reductions in LCX flow induced by i.c. angiotensin were attenuated and i.c. bradykinin-induced increases in LCX flow were augmented after captopril. On the contrary, i.c. infusion of captopril (0.01 mg/min) into the LCX caused no change in hemodynamic variables and myocardial shortening although responses to angiotensin I and bradykinin were markedly modified. These results suggest that captopril may have no direct cardiac effect. Angiotensin-converting enzyme Captopril
Regional myocardial function
1. Introduction Although captopril, an angiotensin-converting enzyme inhibitor, has consistently been shown to improve the left ventricular performance of patients with severe congestive heart failure (Antonaccio, 1982), the mechanism(s) responsible for the salutary effect of captopril is not yet fully understood (Cohn and Levine, 1982). Angiotensin-converting enzyme is thought to be localized on the endothelium of various blood vessels (Ryan et al., 1975; Caldwell et al., 1976; Miyazaki et al., 1984), and might serve to maintain the tone of the respective vascular beds through
* To whom all correspondence should be addressed. 0014-2999/85/$03,30 © 1985 Elsevier Science Publishers B.V.
Intracoronary infusion
Coronary blood flow
locally formed angiotensin II (Britton and Di Salvo, 1972; Cohen and Kurz, 1982). However, it is not established whether local inhibition of angiotensin-converting enzyme in the coronary vasculature influences coronary vascular tone a n d / o r myocardial contraction. To understand the mechanism by which captopril exerts beneficial effects in congestive heart failure, it is important to know to what extent the direct effect of captopril on coronary circulation and myocardial contraction participates in the hemodynamic response to systemically given captopril. The present study aimed to examine the direct effect of an angiotensin-converting enzyme inhibitor on the heart. To this end we evaluated the effects of i.v. and i.c. captopril on coronary blood flow and regional myocardial function in anesthetized dogs.
12
2. Materials and methods
Experiments were conducted on 24 mongrel dogs of either sex weighing 10 to 24 kg (mean = 16.0 kg). The dogs were anesthetized with pentobarbital sodium (25 m g / k g i.v.) and ventilated with room air by a Harvard respirator (model 607). Left thoracotomy was performed through the fifth intercostal space. The pericardium was opened and the heart was suspended in a pericardial cradle. A catheter tip manometer (Millar Instrument Co., Houston, TX) was introduced through the right femoral artery into the left ventricle to measure left ventricular pressure. A cannula filled with heparinized saline (0.9% NaCI) was inserted through the left carotid artery into the aortic root and connected to a Statham pressure transducer (P23ID) to measure the aortic pressure. An appropriately sized electromagnetic flow probe (Statham, SP7515) was placed around the left circumflex coronary artery (LCX) and connected to an electromagnetic flowmeter (Statham, SP2204) for the measurement of coronary blood flow. Zero flow was determined by temporary occlusions of the artery just distal to the flow probe with a snare occluder.
Y
SEGMENT LAD
IIM.:NXIT:
Fig. 1. Schematic representation of the experimental preparation. The left circumflex coronary artery (LCX) w a s d i s s e c t e d free near its origin. An electromagnetic flow (EMF) probe w a s placed around LCX, and a 27-gauge hypodermic needle connected to a fine polyethylene tube was inserted distal to the flow probe for constant infusion and injections of drugs. Two pairs of piezoelectric crystals were implanted within the LCXperfused area and the left anterior descending coronary artery (LAD)-perfused area, respectively.
As shown in fig. 1, a siliconized 27-gauge hypodermic needle connected to a fine polyethylene tube (3 cm long) was inserted into the LCX 0.5 to 1 cm distal to the flow probe for the i.c. administration of drugs. The distal end of the tube was connected to two other polyethylene tubes for constant infusion of either saline or captopril and for the injection of drugs, respectively. To prevent coagulation within the tube, saline was infused by means of a Harvard infusion pump (model 975) at a constant rate of 0.1 m l / m i n throughout the experiments. Regional myocardial segment length was measured by sonomicrometry (Rushmer et al., 1956; Theroux et al., 1974) which provides continuous quantification of regional myocardial function (Theroux et al., 1974). Two pairs of 5 MHz piezoelectric crystals (2 mm in diameter) were implanted in the left ventricular wall close to the endocardium in a circumferential plane. One pair was placed in the area perfused by the LCX to estimate the effect of drugs on myocardial segment length, and another pair was placed in the area perfused by the anterior descending coronary artery (LAD) to serve as a control segment (fig. 1). The leads of the crystals were connected to a four-channel ultrasonic amplifier (Mecc, UDM-5B) which generated voltages linearly proportional to the transit time of acoustic impulses travelling between each pair of crystals, and thus giving electrical signals proportional to the distance between the paired crystals. The system was precalibrated in vitro at 37°C and was repeatedly calibrated by substituting signals of known time duration from a calibrated pulse generator to eliminate any drift. Mean coronary blood flow was measured by using an electronic resistance-capacitance filter with a 2 s time constant. Left ventricular d P / d t ( L V d P / d t ) was derived from differentiating the left ventricular pressure signal using an electronic differentiator (Nihon Kohden, ED-601G). The heart rate was counted continuously with a cardiotachometer (Nihon Kohden, AT-600G) triggered by the pressure pulse. Data were recorded continuously on a multichannel pen recorder (Nihon Kohden, WT-685G) and stored on tape
13
using an FM data recorder (TEAC, R-81) for subsequent data analysis. Experiments were started at least 30 min after instrumentation. The dogs were divided into four groups. In the first group (n = 6), the animals received i.v. captopril (0.3 mg/kg), which has been shown to cause significant hypotension and more than 70% inhibition of pressor response to i.v. angiotensin I (Noguchi et al., 1983). To evaluate the extent of inhibition of angiotensin-converting enzyme within the coronary vasculature, 200, 400 and 800 ng of angiotensin I (Peptide Institute Protein Research Foundation) and 50, 100 and 200 ng of angiotensin II (Peptide Institute Protein Research Foundation) were injected into the LCX before and 10 min after captopril administration. In the second (n = 6) and third (n = 6) groups, responses to i.c. angiotensin I and angiotensin II, or bradykinin (Peptide Institute Protein Research Foundation) and acetylcholine chloride (Daiichi Seiyaku Co. Ltd.) were examined respectively before and during i.c. infusion of captopril (0.01 mg/min) to estimate the efficacy of inhibition of converting enzyme in the heart, since blockade of this enzyme is expected to result in attenuation of vasoconstrictor response to angiotensin I or augmentation of vasodilatory response to bradykinin. The percent inhibition of coronary vasoconstrictor and systemic pressor response to angiotensin I was calculated as the difference between values before and after captopril administration × 100 divided by the respective control value. In the fourth group (n = 6), the effect of i.c. infusion of captopril (0.01 m g / m i n ) into the LCX on coronary blood flow and regional myocardial function was observed for 30 min. The reason why 0.01 m g / m i n of captopril was employed in this study was that it was the maximal dosage which had not affected the systemic blood pressure in pilot studies. All drugs were dissolved in physiological saline. A final volume of 0.1 to 0.4 ml angiotensin I, angiotensin II, bradykinin or acetylcholine was injected into the LCX for 10 to 15 s. The i.c. injection of the corresponding volume of physiological saline did not cause any significant changes in the parameters measured. Percent regional myocardial shortening was assessed as the percent ratio of end-diastolic length
minus end-systolic length to end-diastolic length. Coronary vascular resistance was calculated as the quotient of mean aortic pressure and mean coronary blood flow. Time sequence data were analyzed by two-way analysis of variance and paired data were analyzed with the paired t-test. The level for statistical significance was P < 0.05. All results were expressed as means 5- standard error.
3. Results
3.1. Effect of intravenous captopril on systemic and coronary circulation and regional myocardial function Captopril (0.3 m g / k g i.v.) produced an immediate and sustained reduction in mean aortic pressure, followed by a decline of calculated coronary vascular resistance and by increases in L V d P / d t and heart rate. Mean coronary blood flow was practically unchanged in spite of a significant reduction in mean aortic pressure (fig. 2). Left ventricular end-diastolic pressure decreased from a control value of 2.5 5- 0.2 mm Hg to 2.0 50.3 mm Hg 10 min after the injection but the change was not significant. A significant decrease in end-systolic segment length occurred within 1 min of the administration, corresponding to the change in aortic pressure. End-diastolic segment length also decreased only slightly. Thus, the resultant percent myocardial shortening calculated from the end-systolic and diastolic length was significantly increased to 22.6 5- 2.2% at 3 min after the injection, from a control value of 19.9 _ 1.5% (fig. 2). Intracoronary injections of angiotensin I 200, 400 and 800 ng decreased coronary blood flow by 13.9 + 2.2%, 22.4 + 4.5% and 34.8 5- 6.3% under control conditions. After i.v. administration of captopril (0.3 mg/kg), these responses were significantly attenuated to 5.5 ± 1.5%, 6.4 5- 2.0% and 11.5 5- 2.7%, respectively. Thus, 0.3 m g / k g i.v. of captopril inhibited the coronary vasoconstrictor response to i.c. angiotensin I by 65.0 + 5.3% (n = 18) of the control response. The maximal rise in aortic pressure induced by angiotensin I, which
14
CAPTOPRIL O,3mg/kg
i.v.
3.2. Responses to angiotensin I and l I or bradykmin and acetylcholine injected into the L C X during intracoronary infusion of captopril
l 11o
MAoP mmHg
lOO
I
go MCBF ml/min
35[ 25
e.o[
CVR
i
mm H g / s e e
MR
-~*
leo[
[
**
LVEDP
mrnHg SEGMENT LENGTH mm
II
e
F
[
,
'
1"*
T"~
T-
| 1
I 3
I 5
T**
I
18
I BEFORE
I 0
~ rain
I 10
Fig. 2. Time courses of mean aortic pressure (MAoP), mean coronary blood flow (MCBF), coronary vascular resistance (CVR), left ventricular dP/dt (LVdP/dt), heart rate (HR), left ventricular end-diastolic pressure (LVEDP), myocardial segment length and percent myocardial shortening after an intravenous injection of captopril (0.3 mg/kg). All values are expressed as means_+S.E. * P < 0.05; ** P < 0.01. n = 6.
was noted following the maximal decrease in coronary blood flow, was also reduced by 65.2 ± 5.5% of the control response, indicating that blockade of angiotensin conversion in the systemic circulation was comparable to that in the coronary vascular beds. On the contrary, the responses of coronary blood flow to i.c. angiotensin II (50 and 100 rig) were significantly enhanced by captopril (15.5 ± 4.0% and 21.5 ± 3.5% to 21.8 ± 3.2% and 29.7 ± 4.6%, respectively).
Under control conditions, i.c. injections of angiotensin I (200, 400 and 800 ng) elicited dose-related and transient decreases in coronary blood flow 16 to 26 s after the injection followed by increases in aortic pressure (figs. 3 and 4). During the infusion of captopril (0.01 mg/min), the percent reductions in coronary blood flow induced by 200, 400 and 800 ng of angiotensin I were significantly attenuated from the control response of 11.4 _+ 2.4%, 21.3 + 3.7% and 26.4 ± 5.6% to 1.2 ± 0.8%, 4.5 ± 1.5% and 6.6 ± 2.0%, respectively (fig. 4). Thus, captopril (0.01 rag/rain) inhibited i.c. angiotensin l-induced decreases in coronary blood flow by 81 + 4% (n = 18) of the control response; this dose of i.c. captopril blocked angiotensin-converting enzyme in the coronary vasculature more strongly than did 0.3 m g / k g of captopril administered i.v. as mentioned above. On the other hand, inhibition of pressor responses to i.c. angiotensin I by captopril infusion was only 29 ± 6% (n = 18). In contrast to the case of angiotensin I, the responses to angiotensin II were not inhibited by captopril although changes in coronary blood flow and aortic pressure induced by 50, 100 and 200 ng of angiotensin II similar to those with angiotensin I were noted before captopril (figs. 3 and 4). Dose-related increases in coronary blood flow induced by i.c. injections of bradykinin (10, 20 and 40 rig) were augmented by i.c. captopril (36 ± 11% to 66 ± 16%, 68 ± 10% to 101 + 17%, 102 _+ 15% to 141 ± 19%, respectively), as shown in figs. 5 and 6. The time required for returning to the half maximal increase in coronary blood flow after an injection of bradykinin was also prolonged by the inhibitor(ll ± 1 s t o 2 6 + 7 s , 1 4 ± l s t o 4 1 ± 8 s and 19 ± 2 s to 55 + 8 s, respectively). The coronary vasodilatation induced by acetylcholine (200, 400 and 800 ng i.c.), in contrast to bradykinin, was not significantly affected by captopril (figs. 5 and 6).
15
CONTROL
A. ANGIOTENSIN I (ng, i.c.)
200
4oo
5O
100
CAPTOPRIL 0.01 mg/min
i.c.
8oo
15
AoP m m Hg
5
CBF ml/min
Io
MCBF ml/min
lo
B. M ~ G I O T E N S I N I1
(ng, i.c.)
•
•
20O
5o
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v
lO0 •
2OO •
1 AoP rnrn Hg
!I;
•
,~i
¸
1
CBF
ml/min
MCBF
ml/min It) L i 1 sec
I rain
Fig. 3. Responses of aortic pressure (AoP), coronary blood flow (CBF) and mean coronary blood flow (MCBF) to intracoronary injections of angiotensin I (200, 400 and 800 ng) and angiotensin II (50, 100 and 200 ng) before (left panels) and during (right panels) intracoronary infusion of captopril. ,'~ 20 o
3.3. Effect of intracoronary infusion of captopril on systemic and coronary circulation and regional myocardial function
:E
_z <
i 200
400
800
M~IGIOTENSIN I ( n g , i . c . ) g,.
z
200
400
800
50
100
200
/U~IGIOTENSIN I1 ( n g , I . c . ) 50
100
200
lo
T h e effect of infusing c a p t o p r i l (0.01 m g / m i n ) into the L C X for 30 min on aortic pressure, m e a n c o r o n a r y b l o o d flow, L V d P / d t , heart rate a n d m y o c a r d i a l segment length is shown in fig. 7 a n d the results are s u m m a r i z e d in table 1. As seen in table 1, no changes in h e m o d y n a m i c variables occurred d u r i n g the infusion of captopril. T h e p e r c e n t m y o c a r d i a l shortening within the LCXperfused a r e a as well as that within the L A D - p e r fused area d i d not change significantly (table 1). O n l y the e n d - d i a s t o l i c segment length of the d r u g - t r e a t e d m y o c a r d i u m decreased significantly 30 min after the start of the infusion, concom-
5O
Fig. 4. Percent changes in responses of mean aortic pressure (MAoP) and mean coronary blood flow (MCBF) to intracoronary injections of angiotensin 1 and angiotensin II be-
fore (open columns) and during (hatched columns) intracoronary infusion of captopril (0.01 mg/min). All values are expressed as means+ S.E. * P < 0.05; ** P < 0.01. n = 6.
16 CONTROL
CAPTOPRIL 0.01mg
A.
BRADYKIN IN (ng,
200
min
i.c.
(.9 _Z
i.c.)
10
20
10
40
20
40 loo
AoP mrnHg
50
o
IJ
CBF ml/min
lO
(ng,
200
i.e.)
m
150[ AoP mmHg
CBF ral/min
MCBF ral/min
~!iil
~I I
CAPTOPRIL
1
MCBF
0
LVP
~
~ --=
-
-
~-
L
.
0.O1
5 min
t
IOOF
ml/rnin
800
Fig. 5. Responses of aortic pressure (AoP), coronary blood flow (CBF) and mean coronary blood flow (MCBF) to intracoronary injections of bradykinin (10, 20 and 40 ng) and acetylcholine (200, 400 and 800 ng) before (left panels) and during (right panels) intracoronary infusion of captopril.
L-~ 1 rain
50
400
1 I
l
15° I
200
ACETYLCHOLINE (ng, i.c)
|1
50
AoP mmHg
40
Fig. 6. Percent changes in responses of mean coronary blood flow (MCBF) to intracoronary bradykinin and acetylcholine before (open columns) and during (hatched columns) intracoronary infusion of captopril (0.01 mg/min). All values are expressed as means+ S.E. * P < 0.05. n = 6.
MCBF rnl/min
I]. A C E T Y L C H O L I N E
20
13RADYKININ (n 9, i . c )
.
.
--.
.
.
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.
.
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.
.
.
.
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-
-
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.
.
"
.
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.
.
.
.
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--
-
1
30 min
~
U.
i.c.
15 r n i n
~ -
-
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mg/min
10 m i n
I . , ,
•
-
. r
0
LVd P/d t
3( -3(
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---
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LCX rn rn
_
.-
- _ _- -
i.
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-
.
.
.
.
.
.
.
.
.
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Fig. 7. Representative tracings of hemodynamic variables and myocardial segment shortening before and during infusion of captopril into the left circumflex coronary artery. LAD = left anterior descending coronary artery-perfused area. LCX = left circumflex coronary artery-perfused area. See the legend to fig. 2 for abbreviations.
17 TABLE 1 Hemodynamic variables and regional myocardial performance during intracoronary infusion of captopril (0.01 mg/min). MAoP= mean aortic pressure; MCBF= mean coronary blood flow; CVR = coronary vascular resistance; HR = heart rate; LVdP/dt = left ventricular dP/dt; LVEDP= left ventricular end-diastolic pressure; LAD = left anterior descending coronary artery-perfused area; LCX = left circumflexcoronaryartery-perfused area; EDL = end-diastolic segment length. * P < 0.05. n = 6. MAoP M C B F CVR HR (mmHg) (ml/min) (mmHg/ml (beats/ per rain) rain) Control
LVdP/dt LVEDP LAD (mmHg/s) (mm Hg) EDL (mm)
LCX Shortening ~)
EDL (mm)
Shortening (4)
1035:6 26.5+_4.2 4.275:0.55
128+-10 2026+160 4.15:0.7 9.29+_1.01 28.8+_2.2 9.03+_1.40 15.4+_1.7
5 min
103+_6 26.7+_4.4 4.275:0.56
128+10 20285:156 4.0+_0.7 9.24+_1.01 28.7+_2.2 9.01+_1,40 15.45:1.8
10min
103+_6 27.0+_4.0 4.165:0.54
1295:10 20355:137 4.2+_0.7 9.26+1.01 29.2+2.1 8.98+-1.40 15.25:1.8
15 min
103+7
27.1+-3.8 4.145:0.53
1305:10 2031+_137 3.9+_0.7 9.20+-0.99 29.05:2.2 8.99+-1.40 15.35:1.9
30min
102+7
26.4+3.3 4.165:0.56
1315:10 1965+_ 98 3.85:0.7 9.15+_0.99 29.1+2.0 8.94+-1.39" 14.65:1.9
Captopril
itantly with an insignificant decline of that in the L A D area (table 1).
4. Discussion The major purpose of this study was to evaluate the effect of an angiotensin-converting enzyme inhibitor on coronary vascular tone and myocardial contraction. If the angiotensin-converting enzyme within one vasculature plays physiologically significant roles, blockade of the enzyme would result in vasodilatation of the vessel because tonic formation of angiotensin II, a potent vasoconstrictor peptide, would decrease. Although several studies have shown that i.v. administration of angiotensin-converting enzyme inhibitors produced coronary vasodilatation in patients without coronary artery disease (Faxon et al., 1982), sodium-depleted dogs (Liang et al., 1978) and anesthetized normal dogs (Maxwell et al., 1981; Noguchi et al., 1983), there has been no study in which the inhibitor was applied directly into the coronary artery. Additionally, it is meaningful to examine whether blockade of locally formed angiotensin II could influence myocardial function, since it has been reported that angiotensin II has a direct inotropic effect (Fowler and Holmes, 1964; Koch-Weser, 1965). Thus, in this experiment
the effect of i.c. as well as i.v. captopril on coronary blood flow and regional myocardial function was evaluated. Most of the exogenous angiotensin I is converted to angiotensin II by the converting enzyme on the endothelium of coronary vascular beds then causes vasoconstriction, as demonstrated by Britton and Di Salvo (1972), since angiotensin I conversion in plasma has been shown not to play an important role (Oparil et al., 1970). Reductions in coronary blood flow induced by i.c. injections of angiotensin I with a similar pattern to those of angiotensin II were potently inhibited by captopril. Moreover, enhancement of the response to bradykinin was observed during captopril infusion, probably due to inhibition of kinin degradation by kininase II because this enzyme is considered to be identical to an angiotensin-converting enzyme (Engel et al., 1972). It is unlikely that the alteration in the response to angiotensin I or bradykinin caused by captopril was derived from changes in reactivity of the vessel, since responses to angiotensin II and acetylcholine were almost unchanged before and after captopril. Therefore, these results indicate that the angiotensin-converting enzyme certainly exists in the coronary vasculature and causes coronary vasoconstriction by converting exogenous angiotensin I to angiotensin II.
18
The present experiments showed that 0.01 m g / m i n i.c. infusion of captopril failed to alter resting coronary blood flow, aortic pressure or regional myocardial shortening while i.v. administration of captopril (0.3 mg/kg) produced an immediate decrease in aortic pressure and an increase in myocardial shortening. Nevertheless, the dose of i.c. captopril used in this study may be regarded as sufficient to inhibit angiotensin-converting enzyme in coronary circulation, because reductions in coronary blood flow induced by i.c. angiotensin I were potently suppressed by the i.c. infusion of captopril (81%) rather than by i.v. administration of the inhibitor (65%). Thus, local blockade of angiotensin-converting enzyme in coronary beds did not affect resting coronary blood flow and regional segment shortening. The lack of vasodi[ator action of captopril applied directly into the coronary artery has also been observed in our previous study (Noguchi et al., 1983) where captopril (up to 0.1 raM) did not relax the potassium contracture of isolated canine coronary strips. Recently, Liang et al. (1982) have shown that direct effects of renin-angiotensin system blockade upon myocardial performance were lacking in conscious dogs with coronary occlusion. Chiba et al. (1983) have also demonstrated that a dose level of captopril which caused significant hypotension had no direct cardiac effect. In contrast to the case of i.c. administration, i.v. captopril caused significant decreases in aortic pressure and calculated coronary vascular resistance and increases in heart rate and LVdP/dt. These latter two changes may have been primarily due to baroreflex-mediated secondary effects, because the positive chronotropic and inotropic effects were invariably seen following hypotension and the drug was shown to have little direct cardiac action as described above. An increase in percent segment shortening, which was due to a larger decrease in end-systolic length than in end-diastolic length, coincided chronologically with a decrease in aortic pressure, and hence a major part of this change may have been due to an indirect effect caused by a fall in blood pressure which would lessen the afterload during systolic shortening. In the previous study we observed that the hypotension and the decrease in coronary vascular
resistance induced by captopril (0.01-0.3 m g / k g i.v.) were significantly correlated with inhibition of the angiotensin I-pressor response (Noguchi et al. 1983). Thus, vasodilatation following i.v. captopril appears to be mainly due to lowering of the circulating level of angiotensin II which was induced by inhibition of angiotensin I-converting enzyme in systemic vascular beds, primarily in lung (Ng and Vane, 1967), as suggested in several reports (Murthy et al., 1978; Sato et al., 1980; Zimmerman et al., 1982; Chen et al., 1984). However, other mechanisms may be involved in i.v. captopril-induced vasodilatation, because it has been reported that angiotensin-converting enzyme inhibitors have various actions such as augmentation of the kinin level through inhibition of kininase I1 (Engel et al., 1972), facilitation of PGI 2 synthesis (Swartz et al., 1980; Di)sing et al., 1983), and suppression of sympathetic nerve transmission (Antonaccio and Kerwin, 1980; Clough et al., 1982). Thus, the precise mechanism responsible for the hypotension remains unclear. Marked differences in hemodynamic responses to i.v. and i.c. administered captopril were observed in this study. The relatively weak inhibition (29%) of the angiotensin I-induced pressor response observed during i.c. infusion of captopril appears to indicate that the quantities of infused captopril were too small to cause sufficient blockade of angiotensin 1 conversion in the extracardiac circulation. Thus, the reason why i.c. captopril did not produce marked changes in hemodynamics may be that there was only slight inhibition of the enzyme in the systemic vasculature. The finding that the cardiac action of i.c. captopril was quiescent, despite the strongly inhibitor-blocked angiotensin I conversion within the coronary vasculature, may imply that reninangiotensin system in the canine heart plays only a minor if any part in the regulation of resting coronary hemodynamics. Additionally, the present study also suggests that the acute beneficial effect of an angiotensin-converting enzyme inhibitor on cardiac performance seen in patients with heart failure (Antonaccio, 1982) may result mainly from its peripheral effect rather than from the direct cardiac effect.
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Acknowledgments We thank Junko Nakasone for secretarial assistance, and Sankyo Pharmaceutical Company for supplying captopril. This work was supported in part by a Developmental Scientific Research Grant (No. 58770170) from the Ministry of the Education, Science and Culture, Japan.
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