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Cardiovascular Research 1997 33(1):71-81; doi:10.1016/S0008-6363(96)00185-X
© 1997 by European Society of Cardiology
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Copyright © 1997, European Society of Cardiology

Effects of low-flow ischemia on the positive inotropic action of angiotensin II in isolated rabbit and rat hearts

Joseph R Libonatib,*, Franz R Eberlia, Henry W Sesselberga and Carl S Apsteina

aCardiac Muscle Research Laboratory, Whitaker Cardiovascular Institute, Center for Advanced Biomedical Research, Boston University School of Medicine, 80 East Concord Street, W611, Boston, MA 02118, USA
bDepartment of Cardiopulmonary Sciences, Bouvé College of Pharmacy and Health Sciences, Northeastern University, 100 Dockser Hall, Boston, MA 02115, USA

Received 29 January 1996; accepted 12 August 1996


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: Angiotensin II (ANG II) has recently been reported to increase inotropy in adult rabbit myocytes by a mechanism of alkalinization and consequent increased myofilament sensitivity to calcium. Accordingly, we tested the hypothesis that ANG II would have a greater inotropic effect during ischemic conditions than it would during normoxia, since ischemia-induced intracellular acidosis contributes to ischemic contractile depression by decreasing myofilament calcium sensitivity. Methods: We studied the effects of ANG II in isolated, red-blood-cell-perfused, isovolumic rat and rabbit hearts during normoxic perfusion conditions and at graded reductions in coronary perfusion pressure (CPP). At each level of perfusion, ANG II was infused at progressively increasing concentrations ranging from 10–11 to 10–5 M. The maximal effective ANG II concentration was 10–7 M. Results: Our studies show that ANG II caused comparable absolute increases in isovolumic LV developed pressure in normoperfused and hypoperfused rabbit hearts. However, since contractile function was markedly depressed in ischemic hearts prior to ANG II administration, the relative inotropic response to ANG II was significantly greater during ischemia than normoxia. Similarly, ANG II had no positive inotropic effect in the rat during normoxia, but increased contractility during ischemia. To assess specifically the potential of ANG II to reverse the negative inotropy of acidosis, normoxic non-ischemic rat hearts were perfused with a hypercarbic acidotic perfusate (pH = 7.1). During the hypercarbic perfusion when contraction was depressed by acidosis, ANG II [10–7]M increased LV developed pressure by 19% and + dP/dt by 27% (P < 0.05), in contrast to its lack of inotropic effect at a normal pH. The positive inotropic effect observed in rat hearts with ANG II during ischemia was significantly attenuated (P < 0.001) by concomitant infusion with amiloride, 5-(N-ethyl-N-isopropyl) (EIPA), a Na+/H+ exchange inhibitor. Conclusions: We conclude that during normoxia, ANG II has a different inotropic potency in rabbits from that in rats. In both species, the relative inotropic responsiveness of ANG II is potentiated during low-flow ischemia. These results are consistent with a relative intracellular alkalinization that occurs secondary to ANG II's action to stimulate Na+/H+ exchange.

KEYWORDS Angiotensin II; Coronary artery tone; Contractile function; Rabbit, heart; Rat, heart; Myocardial ischemia


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Blockade of the renin-angiotensin system has been proven beneficial in the treatment of chronic heart failure and in groups of patients with acute myocardial infarction who were largely free of acute ischemic heart failure[21, 30]. However, despite the efficacy of its inhibition in these clinical settings, the renin-angiotensin system may have positive inotropic effects which are beneficial in the setting of acute ischemic cardiac failure.

Reversal of acute ischemic failure is an important therapeutic goal in several clinical settings. Myocardial contractility is largely determined by intracellular Ca2+ and myofilament sensitivity to Ca2+. The early decrease in contractile function with acute ischemia has been shown to result largely from a decreased myofilament Ca2+ sensitivity secondary to ischemia-induced intracellular acidosis and/or inorganic phosphate accumulation [14, 29, 31]. Angiotensin II (ANG II) increases myocardial contractility by increasing both [Ca2+]i and myofilament Ca2+ sensitivity[23, 33]. Recently, Ikenouchi et al. found that 10 nM ANG II increased inotropy in isolated rabbit hearts and dissociated myocytes without a concomitant increase in peak systolic [Ca2+]i or Ca2+ current (ICa) [20]. The proposed mechanism for the observed increase in inotropy was an increased myofilament Ca2+ sensitivity secondary to intracellular alkalosis (a 0.2 unit increase in pHi was reported). Matsui et al. recently showed that this alkalosis resulted from an ANG II stimulatory effect on Na+/H+ exchange[34]. However, higher levels of ANG II (100 nM) also increased ICa in adult rabbit myocytes [23].

In light of these previous studies, we hypothesized that ANG II would partially reverse the negative inotropic effects of ischemia, by increasing myofilament Ca2+ sensitivity and/or by increasing [Ca2+]i. We tested this hypothesis by infusing progressively increasing concentrations of ANG II into the coronary circulation of isolated, isovolumic hearts of rabbits and rats during normoxic perfusion conditions and at graded reductions in coronary perfusion pressure (CPP). To confirm the proposed mechanism of ANG II's improvement of ischemic contractile function we exposed the normally perfused, non-ischemic rat heart to acidosis without ischemia (hypercarbic perfusion), and in separate experiments we co-administered ANG II concomitantly with amiloride, 5-(N-ethyl-N-isopropyl) (EIPA), a specific Na+/H+ exchange inhibitor [34].


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1. Heart isolation
Male, Wistar rats (Charles River Farms) weighing 350–450 g and male, New Zealand White rabbits (Millbrook Farms) weighing 1.4–2.1 kg were housed 1 per cage under a 12 h light/dark cycle and fed ad libitum (Purina rat and rabbit chow). All rats received care in compliance with the ‘Principles of Laboratory Care’ formulated by the National Society for Medical Research. Rats were anesthetized intraperitoneally with sodium pentobarbital (0.5–1.0 ml), while rabbits were anesthetized with 0.3 ml ketamine followed by administration of intravenous sodium pentobarbital (1.5–2.0 ml) and heparin (0.1 ml). Once unresponsiveness was determined, animals were weighed and their thorax opened. Rabbits were mechanically ventilated during thoracotomy. Hearts were extracted, and within 20 s of withdrawal, mounted on a short perfusion cannula in a Langendorff fashion.

Hearts were retrogradely perfused with a red-blood-cell perfusate developed by Marshall and Zhang [35] and in our laboratory [12, 13]. Briefly, fresh whole cow blood was collected at a local slaughterhouse in a vessel containing approximately 6000 units of sodium heparin and 100 000 units of penicillin per liter. The containers of blood were immediately placed on ice to facilitate rapid cooling for transportation. The whole blood was then spun in a refrigerated centrifuge (5°C) at a rotor speed of 3000 rpm for 15 min. The supernatant was aspirated and the resulting packed cells were mixed 1:1 with calcium-free Krebs-Henseleit buffer. The centrifugation and resuspension steps were repeated 3 times, resulting in packed red cells that were essentially free of white cells and platelets. The packed red blood cells were mixed 1:1 with calcium-free buffer and stored for future use at 4°C. Immediately prior to experimentation, blood was once again washed and mixed in a red blood cell perfusate consisting of bovine red-blood cells at a final hematocrit of 40% suspended in a Krebs-Henseleit buffer containing in mM: NaCl 118, KCl 4.7, KH2PO4 1.2, MgSO4 1.2, NaHCO3 25, glucose 5.5, lactate 1, palmitic acid 0.4, heparin 60 mU/ml, and 4 g% bovine serum albumin (Sigma Chemical Co., St. Louis, MO). To mimic in situ physiological extracellular Ca2+ concentrations as closely as possible, the perfusate ionized Ca2+ concentration was adjusted to 1 mM Ca2+ for rats and 1.5 mM Ca2+ for rabbits with a calcium sensitive electrode (Nova 6, Nova Biochemical). Gentamycin (0.2 mg/dl) was added to the red-blood-cell perfusate to retard bacterial growth. The perfusate was gassed with 20% O2, 3% CO2, and 77% N2, to achieve a Po2 of 100–160 mmHg and a pH of 7.35–7.4.

Following initial perfusion of the heart, a small apical drain was inserted into the left ventricle for collection of Thebesian drainage. A second cannula was inserted into apex of the right ventricle via the pulmonary artery for collection of coronary venous effluent. A pacing wire (model 59, Grass Instrument Co., Quincy, MA) and a thermistor (model 400, Yellow Springs, Boulder, CO) were inserted into the right ventricle through the superior vena cava and right atrium. A collapsed latex balloon was inserted into the left ventricle via the left atrium. The balloon was connected to a pressure transducer (Gould Statham P23dB) to measure left ventricular pressure and derive +dP/dt. All hearts were loaded with an initial balloon volume set to yield an end-diastolic pressure of approximately 10 mmHg, and that volume remained constant throughout the remainder of the experiment so that changes in diastolic chamber stiffness could be assessed by alterations in end-diastolic pressure [3].

Hearts were placed in a 0.9% saline bath and maintained at 37°C. All hearts were perfused at a constant level of coronary flow. Changes in coronary resistance were measured by alterations in coronary perfusion pressure via a pressure transducer (Gould-Statham P23dB, Gould Inc., Oxnard, CA) attached to a side-arm of the aortic cannula by short, inflexible tubing. Coronary flow was measured by timed collection of venous effluent. Coronary perfusion pressure (CPP), left ventricular pressure, and its first derivative were recorded continuously on a Gould physiologic recorder.

2.2. Equilibration
All hearts had an equilibration period of 30 min. During this time, coronary flow was set to a level that resulted in a coronary perfusion pressure of 80 mmHg. Rat hearts were paced at 5.5 Hz and rabbit hearts at 3 Hz. Following equilibration, baseline myocardial performance was assessed and hearts from both rabbits and rats were randomly assigned to either a normoxia or low-flow ischemia protocol. The total duration of equilibration plus either the normoxia or low-flow ischemia protocol was approximately 54–70 min.

2.3. Angiotensin II dose-response studies during normoxic perfusion levels
Rats (n = 6) and rabbits (n = 5) were perfused with a constant coronary blood flow, resulting in a coronary perfusion pressure (CPP) of 80 mmHg (CPP-80). Additional rabbit hearts were perfused with flows achieving a CPP = 120 mmHg (CPP-120; n = 3). ANG II (Sigma, St. Louis, MO) dissolved in 0.9% saline vehicle was then administered through a side-port of the aortic cannula by a constant flow pump (Harvard apparatus) in a dose-response manner yielding final blood concentrations ranging from 10–11 to 10–5 M. Preceded by a control saline infusion at 10% of the coronary flow rate, ANG II was administered at the same infusion rate of 10% of the coronary blood flow. Control rat hearts (Control; n = 5) were perfused with saline at intervals time-matched to ANG II infusions. Myocardial performance was allowed to stabilize during each infusion, and typically stabilized within 2–5 min.

2.4. Angiotensin II dose-response studies during low-flow ischemia
Following equilibration, separate groups of rat and rabbit hearts were exposed to low-flow ischemia by gradually decreasing coronary flow over a 10 min period. The final ischemic coronary flow rate was set to a level which yielded a CPP = 20 mmHg for rats (CPP-20; n = 6) and a CPP = 20 mmHg (CPP-20; n = 5) or CPP = 15 mmHg (CPP-15; n = 4) for rabbits. Myocardial performance was allowed to re-equilibrate during this time (10 min) and the functional characteristics of the heart were re-assessed. Hearts underwent an ANG II dose-response protocol as described above. Ischemic control rats (Control; n = 5) were perfused with saline at intervals time-matched to ANG II infusions. Each dose was terminated upon stabilization of myocardial performance, typically within 2–5 min of ANG II infusion.

2.5. Time-controlled ANG II infusion in rat hearts
The results of our dose-response experiments indicated that ANG II elicited a positive inotropic response in ischemic rat hearts (CPP = 20 mmHg) but did not alter contractility during normoxia (CPP = 80 mmHg) (see Section 3). To further confirm these findings, as well as to control for the variable ANG II infusion times in our dose-response experiments, additional studies were performed in isolated rat hearts. In these experiments, saline and ANG II [10–7]M were administered for 5 min during both normoxic (CPP = 80 mmHg) (n = 4) and ischemic (CPP = 20 mmHg) (n = 8) perfusion conditions. Normoxic and ischemic hearts were infused first with saline and then [10–7]M ANG II for 5 min at a rate equivalent to 10% of coronary flow. Each heart acted as its own control during both the normoxic and ischemic protocols, and myocardial performance during ANG II infusion was compared to its respective saline response.

2.6. Hypercarbic perfusion in non-ischemic rat hearts
Additional experiments were performed in order to test further the hypothesis that ANG II mediated its positive inotropic effect during ischemia by generating a relative intracellular alkalosis. In these studies, contractile function was depressed by perfusion with an acidotic perfusate during normoxic perfusion. The acidotic perfusate was a hypercarbic blood mixture that was identical to the normoxic blood mixture with the exception that it was gassed with 10% CO2. Rat hearts (n = 8) were first perfused with a normoxic blood mixture (as described above) during constant flow at pH 7.4 and a CPP = 85 mmHg. The hearts were then perfused with the hypercarbic blood medium (pH = 7.13) for 15 min. Following stabilization with the hypercarbic medium, hearts were infused with [10–7]M ANG II and saline at 10% of the coronary flow rate for 5 min. Myocardial performance during ANG II infusion was compared to its respective hypercarbic response.

2.7. Sodium-hydrogen exchange inhibition in ischemic rat hearts
In order to directly test the hypothesis that angiotensin II increases inotropy by stimulating Na+/H+ exchange, we performed additional experiments in which the Na+/H+ exchange inhibitor, EIPA [amiloride; 5-(N-ethyl-N-isopropyl), Research Biochemicals International, Natick, MA], was concomitantly infused with ANG II during ischemia. Rat hearts were allowed to equilibrate at constant flow eliciting CPP = 83 mmHg. Low-flow ischemia was imposed by reducing coronary flow to 15% of baseline for 10 min at which point hearts underwent treatment with either saline (n = 5), [10–7]M ANG II (n = 6), [5 x 10–5]M EIPA (n = 6) or [10–7]M ANG II + [5 x 10–5]M EIPA (n = 6). All treatments were infused at 5% of coronary flow. Control vehicles were infused during both equilibration and ischemia at 5% of coronary flow.

2.8. Data analysis
Data are reported as the mean ± s.e.m. Baseline characteristics of rabbits perfused at different coronary perfusion pressures were compared with one-way analysis of variance (ANOVA). Baseline characteristics of rats infused with ANG II and saline were compared with unpaired t-tests. Myocardial performance during ANG II infusion was tested by two-way ANOVA with repeated measures. When ANOVA indicated overall significance of groups or interaction, values at specific time points were examined by the method of least significant differences [44]. Rat hearts undergoing time controlled ANG II or saline infusions were analyzed with paired t-tests. Rat hearts perfused with a ANG II during hypercarbia were compared with ANOVA with repeated measures and Student-Neumann-Keuls multiple comparison tests. Sodium/hydrogen exchange inhibition experiments were analyzed with one-way analysis of variance and Student-Neumann-Keuls post-hoc analysis. All data were deemed significant at a P < 0.05 level.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1. Baseline hemodynamic characteristics of rabbit hearts
Baseline hemodynamics of rabbit hearts are listed in Table 1. All experimental groups had comparable baseline function prior to ANG II administration. In hearts subjected to low-flow ischemia (groups CPP-20 and CPP-15) left ventricular developed pressure decreased promptly to about 40% of the baseline value after flow reduction (Table 1).


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  		Table 1 Baseline pre-drug hemodynamics of rabbit hearts during normoxia and ischemia

 
3.2. ANG II dose-response in rabbit hearts
Fig. GR1 shows the hemodynamic responses of rabbit hearts subjected to ANG II at various levels of CPP. ANG II acted as a positive inotrope at all levels of coronary perfusion. This effect was demonstrated by significant positive changes in left ventricular developed pressure and +dP/dt at all levels of CPP (P < 0.05). The peak inotropic effect for all groups was most prominent at an ANG II concentration of 10–7 M. This increase in inotropy was of greatest relative value in ischemic hearts (CPP = 20 and 15 mmHg) where left ventricular developed pressure increased 33 ± 9% and 37 ± 9%, respectively (LV developed pressure increased 19 ± 3% at CPP = 80 mmHg and 12 ± 2% at CPP = 120 mmHg; P < 0.05 vs CPP-20, CPP-15). In all groups, the increase in inotropy was attained without any significant changes in left ventricular end-diastolic pressure or coronary perfusion pressure.


Figure 1
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  		Fig. GR1 ANG II dose-response curves in rabbit hearts. Isolated rabbit hearts were perfused throughout a range of coronary perfusion pressures (CPP = 120, 80, 20, and 15 mmHg) with coronary flow held constant during each experiment. ANG II was added at progressively increasing concentrations. At all CPPs concentrations of 10–8 M ANG II and above caused similar absolute changes in LV developed pressure and +dP/dt. ANG II had no effect on LV end-diastolic pressure or coronary perfusion pressure at any CPP. Values are mean ± s.e.m. * Indicates (P < 0.05) vs. baseline. {dagger} Indicates (P < 0.05) vs. baseline in CPP = 20 mmHg and CPP = 15 mmHg. {ddagger} Indicates (P < 0.05) vs. baseline in CPP = 80 mmHg.

 
3.3. Baseline hemodynamic characteristics of normoxic and ischemic rat hearts
The hemodynamic performance of normoxic rat hearts treated with ANG II was similar to saline controls at baseline (see Table 2). Baseline myocardial performance was also similar between saline controls and ANG II treated hearts undergoing the ischemic protocol. Hearts in the ANG II group had slightly lower baseline coronary flow rates during ischemia than saline controls (P < 0.05), but had similar myocardial performance.


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  		Table 2 Baseline pre-drug hemodynamics of ANG II treated and saline control rat hearts during normoxia and ischemia

 
3.4. ANG II dose-response in normoxic rat hearts
The hemodynamic responses of rat hearts to ANG II and saline were similar during normoxia (CPP = 80 mmHg). Hemodynamic performance (i.e., left ventricular developed pressure) declined approximately 10 mmHg relative to baseline in both saline controls and ANG II treated hearts during the 24–40 min dose-response protocol. There were no significant differences between saline controls and ANG II treated hearts for left ventricular developed pressure, +dP/dt, –dP/dt, left ventricular end-diastolic pressure or coronary perfusion pressure.

3.5. ANG II dose-response in ischemic rats hearts
Fig. GR2 demonstrates the hemodynamic performance of ischemic rat hearts during ANG II infusion. ANG II infusion significantly increased myocardial contractility at concentrations of 10–9 M and greater. ANG II elicited significant positive changes in left ventricular developed pressure, +dP/dt, and –dP/dt vs. saline controls (P < 0.01). During ischemia, ANG-II-treated hearts demonstrated a trend towards higher left ventricular end-diastolic pressure which was not statistically significant. Coronary perfusion pressure at a constant flow rate, significantly increased with ANG II vs. saline control (P < 0.01), indicating an increase in this index of coronary resistance.


Figure 2
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  		Fig. GR2 Hemodynamic changes in ischemic rat hearts treated with ANG II. Isolated rat hearts were exposed to low-flow ischemia by reducing coronary perfusion pressure from 80 to 20 mmHg. Hearts were either infused with ANG II at increasing concentrations or 0.9% saline. ANG II significantly increased LV developed pressure, +dP/dt and coronary resistance at concentrations of 10–8 M and higher. Therefore, in contrast to normoxic conditions, during low-flow ischemia ANG II exerted a positive inotropic effect. Values are mean ± s.e.m. * Indicates (P < 0.05) vs. saline control.

 
3.6. Five-minute infusions with [10–7]M ANG II
Five-minute infusion experiments in the rat demonstrated results similar to the dose-response experiments reported in Fig. GR2. During ischemia (CPP = 20 mmHg), 5 min of ANG II infusion resulted in an 11% increase in left ventricular developed pressure and +dP/dt. Left ventricular developed pressure significantly increased from 34 ± 1 mmHg during saline infusion to 38 ± 1 mmHg with ANG II (P < 0.001), and +dP/dt significantly increased from 937 ± 40 mmHg/s during saline infusion to 1040 ± 46 mmHg/s with ANG II (P < 0.01). During this relatively brief ischemic period, ANG II did not affect left ventricular end-diastolic pressure. ANG II infusion had no effect on hemodynamic performance during normoxia. However, the prolonged infusion of the high dose ANG II resulted in coronary vasoconstriction during both normoxia and ischemia. CPP increased 14.5 ± 5.2% with ANG II vs. saline during normoxia (P < 0.05) and 11 ± 2% vs. saline during ischemia (P < 0.001).

3.7. Hypercarbic perfusion in non-ischemic rat hearts
Fig. GR3 shows that perfusion with the hypercarbic medium (pH = 7.13) resulted in a prompt decrease in left ventricular developed pressure from 117 ± 6 mmHg during normoperfusion to 86 ± 5 mmHg (P < 0.001). Positive dP/dt also decreased significantly from 2562 ± 151 mmHg/s during normoperfusion to 1782 ± 106 mmHg/s with hypercarbia (P < 0.001). Infusion with [10–7]M ANG II increased left ventricular developed pressure by 17 mmHg to 103 ± 10 mmHg (P < 0.05) and +dP/dt to 2270 ± 262 (P < 0.05). A matched saline infusion had no effect on inotropy. Hypercarbia caused an insignificant increase in left ventricular end-diastolic pressure from 11 ± 1 mmHg during normoperfusion to 14 ± 1 mmHg during hypercarbia. ANG II infusion did not affect left ventricular end-diastolic pressure. Coronary perfusion pressure was relatively constant throughout normoperfusion and hypercarbia (85 ± 2 mmHg during normoperfusion; 80 ± 2 mmHg during hypercarbia; 84 ± 4 mmHg with saline) and increased slightly with ANG II (90 ± 4 mmHg) (P = n.s.).


Figure 3
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  		Fig. GR3 Changes in hemodynamic performance in rat hearts (n = 8) perfused with a hypercarbic blood perfusate at CPP = 80 mmHg. Rat hearts were first infused with a normoxic blood mixture at pH = 7.4. Hearts were then transferred to a hypercarbic blood (HC) mixture at pH = 7.1. Left ventricular developed pressure and +dP/dt significantly decreased (P < 0.001) with hypercarbia vs. normoxia. The addition of ANG II [10–7]M (HC + ANG II) caused a positive inotropic response in hypercarbic perfused hearts (P < 0.05). Saline infusion (HC + Saline) had no inotropic effect. Values are mean ± s.e.m. * Indicates (P < 0.05) vs. hypercarbia and saline. * * Indicates (P < 0.001) vs. normoxia. {dagger} Indicates (P < 0.01) vs. normoxia.

 
3.8. Sodium-hydrogen exchange inhibition in ischemic rat hearts
Fig. GR4 shows that during low-flow ischemia [10–7]M ANG II significantly increased LV developed pressure (P < 0.001), but concomitant infusion with EIPA significantly attenuated this inotropic effect (P < 0.001). In this series of experiments, at baseline CPP was approximately 85 mmHg in all groups (saline, 87 ± 3 mmHg; EIPA, 86 ± 3 mmHg; ANG II, 86 ± 4 mmHg; ANG II + EIPA, 82 ± 4 mmHg; P = ns) and left ventricular developed pressure was approximately 130 mmHg and similar in all groups (saline, 132 ± 15 mmHg; EIPA, 137 ± 11 mmHg; ANG II, 144 ± 4 mmHg; ANG II + EIPA, 105 ± 21 mmHg). During low-flow ischemia when CPP was reduced to comparable levels in all groups (saline, 15 ± 2 mmHg; EIPA, 15 ± 2 mmHg; ANG II, 14 ± 1 mmHg; ANG II + EIPA, 12 ± 1 mmHg) left ventricular developed pressure fell to approximately 35 mmHg and was similar in all groups (saline, 39 ± 5 mmHg; EIPA, 33 ± 3 mmHg; ANG II, 39 ± 2 mmHg; ANG II + EIPA, 31 ± 2 mmHg).


Figure 4
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  		Fig. GR4 Changes in left ventricular developed pressure in rat hearts during low-flow ischemia following a 5 min infusion with saline (n = 5), ANG II [10–7]M (n = 6), EIPA [5 – 10–5]M (n = 6), or ANG II [10–7]M + EIPA [5 – 10–5]M (n = 6). ANG II significantly increased left ventricular developed pressure relative to saline control and EIPA alone (P < 0.001) The positive inotropic action of ANG II was significantly reduced by concomitant infusion with EIPA (P < 0.001). Values are mean ± s.e.m. *Indicates (P < 0.001) vs. all other groups.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
The importance of the renin-angiotensin system in cardiac disease has recently become apparent. The renin-angiotensin system contributes significantly to the pathophysiology of several aspects of heart failure, remodeling, and myocardial loading conditions [33]. Angiotensin-converting-enzyme inhibition has successfully been used to improve both systolic and diastolic dysfunction and reduce left ventricular dilation after myocardial infarction [13, 33].

However, a potentially important role for the renin-angiotensin system in acute ischemic syndromes has received relatively little attention. In the current study, we utilized a model of low-flow ischemia, where the myocardium is hypoperfused but still functional, albeit at a reduced level. Such a low-flow ischemic condition is frequent and common to several coronary artery disease syndromes such as angina pectoris, unstable angina, myocardial infarction, and cardiogenic shock. The interaction between low-flow ischemia and the inotropic action of ANG II is therefore of potential clinical and physiologic significance.

4.1. Mechanism of inotropic action
Recently, 10 nM ANG II has been shown to exert a positive inotropic action in the rabbit by a mechanism of intracellular alkalinization and an increased myofilament sensitivity to calcium, rather than by increasing peak systolic [Ca2+]i or ICa [20, 34], whereas higher concentrations of ANG II (100 nM) also increased ICa [23]. Since the ischemic condition has been reported to acutely decrease myocardial contractility by decreasing myofilament sensitivity to calcium, at least in part by intracellular acidosis[8, 14, 31], we hypothesized that ANG II may potentially reverse the ischemia-induced depression of calcium sensitivity and have a more pronounced inotropic effect during ischemia.

Our results support this hypothesis. The results of our rabbit experiments show that although ANG II increased developed pressure and +dP/dt by a similar absolute amount at all CPPs in the rabbit, its effect was of a greater relative degree in hypoperfused rabbit hearts. The increased contractility observed with ANG II in the current study was not accompanied by significant changes in coronary perfusion pressure or isovolumic left ventricular end-diastolic pressure, suggesting that the duration of ischemia is critical in determining the extent of diastolic dysfunction reported to occur with ANG II in prior studies of longer periods of ischemia [36]. Our observations of enhanced inotropy with ANG II are consistent with previous reports in rabbits [9, 20, 34], humans [37], and most other species [5, 11, 33].

Our results provide new information by showing that ANG II augments contractility to a relatively greater extent during low-flow ischemia than during normoperfusion in the rabbit heart. Furthermore, in the rat, ANG II had a positive inotropic effect only during ischemia or during non-ischemic acidosis, but in contrast with the rabbit, not during normoxic perfusion at normal pH. The mechanisms responsible for these differences in inotropy between species and perfusion conditions are probably related to differences in one or more steps in the ANG II pathway.

However, the positive inotropic response of ANG II was somewhat less than that of dobutamine in this model. Recent studies from our laboratory have shown that [10–5]M dobutamine increases left ventricular developed pressure by approximately 20 mmHg during ischemic (CPP = 20 mmHg) and normoxic (CPP = 80 mmHg) perfusion conditions [7]. Similar results have been reported in the rat [26].

ANG II initiates its physiological cascade by receptor stimulation of well-defined ANG II receptors[22, 39, 41, 42, 49]. Following receptor activation, ANG II acts through the second messengers inositol 1,4,5-triphosphate (IP3) and diacylglycerol, which increase contractility by increasing sarcolemmal ICa2+ and [Ca2+]i [1, 16, 24] and/or by alkalinizing the intracellular milieu of the myocyte via protein kinase C stimulation of Na+/H+ exchange[20, 34](Fig. GR5). It is well recognized that changes in pHi can alter the inotropic state of the heart, with intracellular alkalosis providing a positive inotropic effect via an enhanced sensitivity of troponin C to [Ca2+]i [8]. Since pHi is known to be reduced during ischemia[10, 14, 31], we hypothesized that the inotropic effects of ANG II during ischemia were secondary to ANG II's alkalinizing actions. We addressed this hypothesis by assessing the potential of ANG II to reverse the negative inotropy associated with non-ischemic acidosis.


Figure 5
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  		Fig. GR5 Mechanisms of action for ANG II in a cardiac muscle cell. This figure indicates two general mechanisms of increased contractile force following ANG II infusion. Following its binding to well-defined receptors, ANG II activates the phosphoinositol pathway and leads to activation of inositol triphosphate (IP3) and diacylglycerol. IP3 causes release of Ca2+ from the sarcoplasmic reticulum which directly increases the force generation of the myofilament. Diacylglycerol activates protein kinase C, which in turn phosphorylates the Na+/H+ anti-porter resulting in H+ extrusion from the cell. H+ extrusion leads to a relative intracellular alkalosis (increased pHi) and increased myofilament-generated force. Additionally, the accumulation of intracellular Na+ in exchange for H+ may activate Na+/Ca2+ exchange and thereby increase myofilament force (adapted from [20, 33]).

 
4.2. Effects of ANG II in ischemic, non-ischemic, acidotic, and normal pH rat hearts
At normal perfusate pH [10–7]M ANG II did not affect developed pressure, but in non-ischemic acidotic rat hearts (hypercarbic perfusate) it caused a 19% increase, similar to that observed during low-flow ischemia. The lack of an ANG II effect at normal pH suggests that, in the rat, ANG II stimulates Na+/H+ exchange significantly only when Na+/H+ exchange has been already stimulated by intracellular acidosis. Thus during ischemia and its associated acidosis ANG II's effects on pHi and myofilament Ca2+ sensitivity were probably partly responsible for its greater inotropic effect relative to normoperfusion. ANG II alone (i.e., in the absence of intracellular acidosis) may not be capable of stimulating the rat's Na+/H+ exchange because the rat's relatively high intracellular Na+ concentration opposes Na+ entry more than the lower intracellular Na+ of the rabbit [43](see discussion below).

To test whether ANG II increased inotropy by inducing intracellular alkalosis by extrusion of protons via stimulation of Na+/H+ exchange, we infused the Na+/H+ exchange inhibitor EIPA during low-flow ischemia. EIPA negated the inotropic action of ANG II. These results suggest that stimulation of Na+/H+ exchange mediates the positive inotropic response of ANG II in ischemic rat hearts.

The greater inotropic potency of ANG II in the ischemic rat heart compared to the non-ischemic state may also be related to the effects of coronary turgor on myocyte stretch. During normal perfusion, coronary flow and vascular distension contribute to preload and sarcomere length via the effect of coronary turgor [17, 27, 40, 48]. Li et al. [32] have shown that rat papillary muscles preloaded at maximal length have an attenuated force production with ANG II, whereas if muscle length falls on the ascending limb of the length-tension relationship, ANG II induces a positive inotropic effect. Thus during ischemia, when coronary flow and turgor were reduced, myocyte stretch probably decreased and may have shifted to a point on the ascending limb of the length-tension curve where ANG II increases myocardial contractility.

4.3. Rat-rabbit differences
Both ANG II and endothelin increased contraction amplitude of adult rabbit myocytes, concomitant with an increase in intracellular pH and with no change in the Ca2+ transient. However, both ANG II and endothelin decreased the contraction magnitude in neonatal rat myocytes together with a reduced inward Ca2+ transient and intracellular acidification [25]. These observations are consistent with our results where the normoxic adult rabbit heart had a greater inotropic response than the adult rat heart.

Species differences in myocyte Na+ and Ca2+ regulation may be responsible for this differential inotropic response. Although intracellular pH is similar in rat and rabbit myocytes [20, 28], Shattock and Bers [43] have shown that intracellular Na+ is higher in rat than in rabbit myocytes. Therefore Na+/H+ exchange stimulation by ANG II may be relatively ineffective during normoxia and normal pH in the rat because Na+ entry is opposed by a relatively higher intracellular [Na+]. However, an increased intracellular H+ concentration may provide a further stimulus to Na+/H+ exchange to which the ANG II effect is additive, resulting in ANG II's positive inotropic action during ischemia, or during non-ischemic acidosis, as in our hypercarbia experiments.

Rat-rabbit differences in Na+/Ca2+ exchange may also contribute to the differential inotropic response to ANG II between these species. The increase in intracellular Na+ resulting from Na+/H+ exchange stimulation by ANG II can increase contractile function by increasing intracellular Ca2+ via Na+/Ca2+ exchange. The rates and direction of Na+/Ca2+ exchange are quite different in the rat and rabbit heart [6, 43]. During twitching Na+/Ca2+ exchange is in the direction of Ca2+ uptake in the rabbit, but in the direction of Ca2+ release in the rat, with the reverse occurring at rest. In our experiments at relatively rapid pacing rates, Na+/Ca2+ exchanges of the twitch state may predominate and contribute to the species differences in inotropic response to ANG II; ANG II may be able to increase intracellular Na+ and secondarily increase Ca2+ entry in the rabbit, but not in the rat where Na+/Ca2+ exchange is operating in the direction of Ca2+ efflux. In the study by Ikenouchi et al. [20] an increase in [Ca2+]i was not observed with 10–8 M ANG II, but Ca2+ influx did increase at 10–7 M ANG II in the rabbit, consistent with increased Na+/Ca2+ exchange [23]. Other species differences may be related to ANG II receptor subtypes [4, 22], ANG II binding capacity [33, 39, 49], and/or second messenger cross-talk [2]. Rat hearts have been reported to express two AT1 receptors (AT1a and AT1b) [22] and have greater ANG II binding capacities than rabbit [39, 49]. Also, rat sarcolemma contains ANG II receptors that are negatively coupled to adenylate cyclase[2], which may thereby decrease cAMP levels and reduce contractile function in the rat heart.

4.4. Study limitations
Since peak systolic [Ca2+]i and ICa2+ were not measured in our study, we cannot assess a role for reverse-mode Na+/Ca2+ exchange or for involvement of the Na+/H+ antiporter in regulating L-type Ca2+ current [23] and contractility in experiments where we used 10–7 M ANG II. However, Ikenouchi et al. did not report any changes in peak systolic [Ca2+]i or ICa2+ with 10–8 M ANG II in rabbit myocytes, thereby suggesting that the role of Na+/Ca2+ exchange and L-type Ca2+ flux are not required for mediating ANG II's effect in the rabbit [20, 34]. Experiments which have examined the actions of endothelin-1 on [Ca2+]i, pHi, and cell contracture have reported similar results [25]. Endothelin-1 has an intracellular cascade similar to ANG II and has been shown to increase cell contraction and pHi without altering [Ca2+]i in adult rabbit myocytes [25] as well as stimulating Na+/H+ exchange by a protein kinase C mediated pathway in adult rat ventricular myocytes [28]. In a recent study in adult rat cardiomyocytes [46] endothelin increased [Ca2+]i in a dose-dependent manner, but ANG II did not, consistent with our observed lack of inotropy under normoxic and normal pH conditions in the rat.

4.5. Potential significance
In our studies, ANG II's action of increasing contractility during low-flow ischemia may represent a compensatory mechanism which improves global contractile function during periods of limited coronary supply. Similarly, rats exposed to chronic ischemia (1 week) have a 3–4-fold increase in the expression and density of left ventricular ANG II receptors. Subsequent ANG II exposure to these myocytes produced a relatively large increase in contraction magnitude with concomitant increases in peak systolic [Ca2+]i [19]. Thus the renin-angiotensin system may oppose ischemic contractile dysfunction by virtue of an inotropic effect which becomes amplified during both acute and more chronic ischemia. These observations should be considered as anti-renin-angiotensin therapies are expanded in the treatment of cardiac disease.

However, our results do not contradict the reported benefit of angiotensin-converting-enzyme inhibitors in large trials of patients with myocardial infarction where hypotension was absent in patients receiving these agents[30]. Our results suggest that angiotensin II may be useful when diffuse myocardial ischemia is present (e.g., with severe hypotension or cardiogenic shock).

While human hearts express cardiac Na+/Ca2+ [45] and Na+/H+ [15] exchangers, it is difficult to determine whether ANG II mediates its inotropic effects in human myocardium via these subcellular pathways. ANG II receptor binding sites have been reported in human hearts[38, 47], but in the few studies which have addressed the inotropic effects of ANG II in human myocardium[18, 37, 47], the positive inotropic action of ANG II appears to be more pronounced in atrial than in ventricular tissue[18, 37]. Thus, while ANG II tends to increase contractility in the human heart, further study is needed to understand the inotropic action of ANG II in humans.


   Acknowledgements
 
This study was supported in part by National Heart, Lung, and Blood Institute Grant HL-48715 (C.S.A.) and by a grant-in-aid from the American Heart Association (F.R.E.).


   Notes
 
* Corresponding author. Tel. + 1 617 638-4037; Fax + 1 617 638-4031. Back


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

  1. Allen IS, Cohen NM, Dhallan RS, Gaa ST, Lederer WJ, Rogers TB. Angiotensin II increases spontaneous contractile frequency and stimulates calcium current in cultured neonatal rat, heart myocytes insights into the underlying mechanisms. Circ Res (1988) 62:524–534.[Abstract/Free Full Text]
  2. Anand-Srivastava MB. Angiotensin II receptors negatively coupled to adenylate cyclase in rat myocardial sarcolemma. Biochem Pharmacol (1989) 38:489–496.[CrossRef][Web of Science][Medline]
  3. Apstein CS, Mueller M, Hood WB. Ventricular contracture and compliance changes with global ischemia and reperfusion and their effect on coronary resistance in the rat. Circ Res (1977) 41:206–217.[Free Full Text]
  4. Baker KM, Campanile CP, Trachte GJ, Peach MJ. Identification and characterization of rabbit angiotensin II myocardial receptor. Circ Res (1984) 54:286–293.[Abstract/Free Full Text]
  5. Baker KM, Aceto JA. Characterization of avian angiotensin II cardiac receptors: coupling to mechanical activity and phosphoinositide metabolism. J Mol Cell Cardiol (1989) 21:375–382.[CrossRef][Web of Science][Medline]
  6. Bassani JW, Bassani RA, Bers DM. Relaxation in rabbit and rat cardiac cells: species-dependent differences in cellular mechanisms. J Physiol (1994) 476:279–293.[Abstract/Free Full Text]
  7. Bernstein EA, Eberli FR, Libonati JR, Horowitz GL, Apstein CS. Can dobutamine stimulate ischemic and stressed heart without increasing injury? J Mol Cell Cardiol (1995) 27(suppl_5):A45.
  8. Blanchard EM, Solaro JR. Inhibition of the activation and troponin calcium binding of dog cardiac myofibrils by acidic pH. Circ Res (1984) 55:382–391.[Abstract/Free Full Text]
  9. Blumberg AL, Ackerly JA, Peach MJ. Differentiation of neurogenic and myocardial angiotensin II receptors in isolated rabbit atria. Circ Res (1976) 36:719–726.[Web of Science]
  10. Cave AC, Eberli FR, Ngoy S, Rose J, Ingwall JS, Apstein CS. Increased NMR studies in the isolated blood perfused rat heart. Circulation (1993) 88(4):1–43. (Abstract).
  11. Dempsey PJ, McCallum ZT, Kent KM, Cooper T. Direct myocardial effects of angiotensin II. Am J Physiol (1971) 220:477–481.[Free Full Text]
  12. Eberli FR, Weinberg EO, Grice WM, Horowitz G, Apstein CS. Protective effect of increased glycolytic substrate against systolic and diastolic dysfunction and increased coronary resistance from prolonged global underperfusion and reperfusion in isolated rabbit hearts perfused with erythrocyte suspensions. Circ Res (1991) 68:466–481.[Abstract/Free Full Text]
  13. Eberli FR, Apstein CS, Ngoy S, Lorell BH. Exacerbation of left ventricular ischemic diastolic dysfunction by pressure overload hypertrophy: prevention by specific inhibition of intrinsic cardiac renin-angiotensin system. Circ Res (1992) 70:931–943.[Abstract/Free Full Text]
  14. Figueredo VM, Brandes R, Weiner MW, Massie BM, Camacho SA. Cardiac contractile dysfunction during mild coronary flow reductions is due to an altered calcium-pressure relationship in rat hearts. J Clin Invest (1992) 90:1794–1802.[Web of Science][Medline]
  15. Fliegel L, Dyck JR, Wang H, Fong C, Haworth RS. Cloning and analysis of the human myocardial Na+/H+ exchanger. Mol Cell Biochem (1993) 125(2):137–143.[CrossRef][Web of Science][Medline]
  16. Freer RJ, Pappano AJ, Peach MJ, et al. Mechanism for the positive inotropic effect of angiotensin II on isolated cardiac muscle. Circ Res (1976) 39:178–183.[Abstract/Free Full Text]
  17. Gaasch WH, Bing OH, Franklin A, Rhodes D, Bernard SA, Weintraub RM. The influence of acute alterations in coronary blood flow on left ventricular diastolic compliance and wall thickness. Eur J Cardiol (1978) 7:147–161.[Web of Science][Medline]
  18. Holubarsch C, Hasenfuss G, Schmidt-Schweda S, et al. Angiotensin I and II exert inotropic effects in atrial but not in ventricular human myocardium;An in vitro study under physiological experimental conditions. Circulation (1993) 88:1228–1237.[Abstract/Free Full Text]
  19. Huang H, Li P, Hamby CV, Reiss K, Meggs LG, Anversa P. Alterations in angiotensin II receptor mediated signal transduction shortly after coronary artery constriction in the rat. Cardiovasc Res (1994) 28:1564–1573.[Abstract/Free Full Text]
  20. Ikenouchi H, Barry WH, Bridge JHB, Weinberg EO, Apstein CS, Lorell BH. Effects of angiotensin II on intracellular Ca2+ and pH in isolated beating rabbit hearts and myocytes loaded with the indicator Indo-1. J Physiol (1994) 480:203–215.[Abstract/Free Full Text]
  21. ISIS IV Collaborative Group. A randomised factorial trial assessing early oral captopril, oral mononitrate, and intravenous magnesium sulfate in 58,050 patients with suspected acute myocardial infarction Lancet 1995;345:669–685.
  22. Iwai N, Inagami T. Identification of two subtypes in the rat type I angiotensin II receptor. FEBS Lett (1992) 298:257–260.[CrossRef][Web of Science][Medline]
  23. Kaibara M, Mitarai S, Yano K, Kameyama M. Involvement of Na+-H+ antiporter in regulation of L-type Ca2+ channel current by angiotensin II in rabbit ventricular myocytes. Circ Res (1994) 75:1121–1125.[Abstract/Free Full Text]
  24. Kass RS, Blair ML. Effects of ANG II on membrane current in cardiac Purkinje fibers. J Mol Cell Cardiol (1981) 13:797–809.[CrossRef][Web of Science][Medline]
  25. Kohmoto OH, Ikenouchi H, Hiata Y, Momomura S, Serizawa T, Barry WH. Variable effects of endothelin-1 on [Ca2+]i transients, pHi, and contraction in ventricular myocytes. Am J Physiol (1993) 265:H793–H800.[Web of Science][Medline]
  26. Kojima S, Wu ST, Parmley WW, Wikman-Coffelt JW. Relationship between intracellular calcium and oxygen consumption: effects of perfusion pressure, extracellular calcium, dobutamine, and nifedipine. Am Heart J (1994) 127:386–391.[CrossRef][Web of Science][Medline]
  27. Koretsune Y, Corretti MC, Kusuoka H, Marban E. Mechanism of early ischemic contractile failure. Inexcitability, metabolite accumulation, or vascular collapse? Circ Res (1991) 68:255–262.[Abstract/Free Full Text]
  28. Kramer BK, Smith TW, Kelly RA. Endothelin and increased contractility in adult rat ventricular myocytes;role of intracellular alkalosis induced by activation of the protein kinase C-dependent Na+-H+ exchanger. Circ Res (1991) 68:269–279.[Abstract/Free Full Text]
  29. Kubler W, Katz A. Mechanism of early ‘pump’ failure of the ischemic heart. Possible role of adenosine triphosphate depletion and inorganic phosphate accumulation. Am J Cardiol (1977) 40:467–471.[CrossRef][Web of Science][Medline]
  30. Latini R, Maggioni AP, Flather M, Sleight P, Tognoni G. ACE inhibitor use in patients with myocardial infarction;Summary of evidence from clinical trails. Circulation (1995) 92:3132–3137.[Free Full Text]
  31. Lee JA, Allen DG. Mechanisms of acute ischemic contractile failure of the heart: role of intracellular calcium. J Clin Invest (1991) 88:361–367.[Web of Science][Medline]
  32. Li P, Sonnenblick EH, Anversa P, Capasso JM. Length-dependent modulation of ANG II inotropism in rat myocardium: effects of myocardial infarction. Am J Physiol (1994) 266:H779–H786.[Web of Science][Medline]
  33. Lindpainter K, Ganten D. The renin-angiotensin system. An appraisal of present experimental and clinical evidence. Circ Res (1991) 68:905–921.[Free Full Text]
  34. Matsui H, Barry WH, Livsey C, Spitzer KW. Angiotensin II stimulates sodium-hydrogen exchange in adult rabbit ventricular myocytes. Cardiovasc Res (1995) 29:215–221.[Abstract/Free Full Text]
  35. Marshall RC, Zhang DY. Correlation of contractile dysfunction with oxidative energy production and tissue high energy phosphate stores during partial coronary flow disruption in rabbit heart. J Clin Invest (1988) 82:86–95.[Web of Science][Medline]
  36. Mochizuki T, Eberli FR, Apstein CS, Lorell BH. Exacerbation of ischemic dysfunction by angiotensin II in red blood cell-perfused rabbit hearts. J Clin Invest (1992) 89:490–498.[Web of Science][Medline]
  37. Moravec CS, Schluter MD, Paranandi L, et al. Inotropic effects of angiotensin II on human cardiac muscle in vitro. Circulation (1990) 82:1973–1984.[Abstract/Free Full Text]
  38. Regitz-Zagrosek V, Friedel N, Heymann A, et al. Regulation, chamber localization, and subtype distribution of angiotensin II receptors in human hearts. Circulation (1995) 91:1461–1471.[Abstract/Free Full Text]
  39. Rogers TB, Gaa ST, Allen IS. Identification and characterization of functional angiotensin II receptors on cultured heart myocytes. J Pharmacol Exp Ther (1986) 236:438–444.[Abstract/Free Full Text]
  40. Salisbury PF, Cross CE, Rieben PA. Influence of coronary artery pressure upon myocardial elasticity. Circ Res (1960) 8:794–800.[Abstract/Free Full Text]
  41. Sadoshima J, Izumo S. Signal transduction pathways of angiotensin II-induced c-fos gene expression in cardiac myocytes in vitro: roles of phospholipid-derived second messengers. Circ Res (1993) 73:424–438.[Abstract/Free Full Text]
  42. Sadoshima J, Izumo S. Molecular characterization of angiotensin II-induced hypertrophy of cardiac myocytes and hyperplasia of cardiac fibroblasts: critical role of the AT1 receptor subtype. Circ Res (1993) 73:413–423.[Abstract/Free Full Text]
  43. Shattock MJ, Bers DM. Rat vs. rabbit ventricle: Ca flux and intracellular Na assessed by ion-selective microelectrodes. Am J Physiol (1989) 256:C813–C822.[Web of Science][Medline]
  44. Snedocor GW, Lochran WG. Statistical Methods. Ames, IO: Iowa State University Press, 1980;215–237.
  45. Struder R, Reinecke H, Bilger J, et al. Gene expression of the cardiac Na+-Ca2+ exchanger in end-stage human heart failure. Circ Res (1994) 75:443–453.[Abstract/Free Full Text]
  46. Touyz RM, Fareh J, Thibault G, Tolloczko B, Lariviere R, Schiffrin EL. Modulation of Ca2+ transients in neonatal and adult rat cardiomyocytes by angiotensin II and endothelin-1. Am J Physiol (1996) 270:H857–H868.[Medline]
  47. Urata H, Healy B, Stewart RW, Bumpus MF, Husain A. Angiotensin II forming pathways in normal and failing human hearts. Circ Res (1990) 66:883–890.[Abstract/Free Full Text]
  48. Vogel WM, Apstein CS, Briggs LL, Gaasch WH. Acute alterations in left ventricular diastolic chamber stiffness: role of the ‘erectile’ effect of coronary arterial pressure and flow in normal and damaged hearts. Circ Res (1982) 51:465–478.[Free Full Text]
  49. Wright GB, Alexander RW, Eckstein LS, Gimbrone MA. Characterization of the rabbit ventricular myocardial receptor for angiotensin II: evidence for two sites of different affinities and specificities. Mol Pharmacol (1983) 24:213–221.[Abstract]

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