Effects of Angiotensin II on Cardiac Function and Peripheral Vascular Structure During Compensated Heart Failure in the Rat
Abstract The present experiments were designed to test the hypothesis that the activation of the renin-angiotensin system during compensated heart failure may have adverse effects on cardiac function and change the peripheral vascular structure. ANG II (250 ng/kg/min) or saline (0.9% NaCl) were infused in myocardial-infarcted and sham-operated rats. After 2 weeks, cardiac function and peripheral vascular changes were investigated. Results: ANG II infusion reduced baseline cardiac index in sham rats but did not further reduce this index in ANG II-infused MI rats. Total peripheral resistance was similarly increased in ANG II-infused infarcted and sham rats, and also plasma ANG II concentrations were comparable. ANG II elevated systolic blood pressure by approximately 70 mm Hg in sham rats and increased the medial cross-sectional area of the superior mesenteric artery by 33%. However, ANG II infusions in MI rats resulted in only a minor increase in blood pressure, whereas the cross-sectional area of the superior mesenteric artery did not change. ANG II infusion had no effect on vessel dimensions of the resistance arteries of the pulmonary and mesenteric vascular bed of either group. Calculated ED50 and peak pressor response to acute ANG II injections were comparable in all groups, confirming the presence of functionally intact AT1 receptors. The increases in plasma atrial natriuretic peptide (ANP) and nitric oxide (NO) synthase activity (estimated by aortic cyclic GMP concentrations) were higher in ANG II-infused MI rats than in ANG II-infused sham rats. Conclusion: ANG II infusion in rats with and without MI has comparable negative effects on cardiac function but has different effects on blood pressure and vascular structure. The concomitant increases in plasma ANP and NO synthase activity in ANG II-infused MI rats suggest that the growth stimulatory and hypertensive actions of ANG II in sham rats may be counter-regulated by activation of inhibitory neurohumoral systems such as ANP or NO in MI rats.
- Received November 26, 1996.
- Accepted February 4, 1997.
The renin-angiotensin system (RAS) is one of the most extensively studied neurohumoral systems in the pathophysiology of congestive heart failure. Angiotensin II (ANG II), the effector peptide of the RAS, is involved in the regulation of cardiovascular function and volume homeostasis by raising peripheral resistance through vasoconstriction, stimulating aldosterone release, and enhancing renal sodium reabsorption.1 Given its important role in maintaining circulatory homeostasis, one can expect an activation of the RAS in a situation of decreased cardiac output, as seen during heart failure. An interesting aspect of RAS activation during heart failure is the timing. Plasma concentrations of ANG II are increased in the acute phase after myocardial infarction, normalize in the compensated phase, and increase again in overt heart failure.2 3 4 This biphasic activation is considered to be a compensatory mechanism in response to decreased cardiac function. Although this may well be true for the first phase, it may not be true for the second phase in which activation of the renin angiotensin system may even have adverse effects on the function and/or structure of the cardiovascular system. Indeed, a significant positive correlation between mortality and plasma levels of ANG II has been shown in patients with heart failure,5 6 and it has been suggested that the effect of angiotensin-converting enzyme inhibition is related to neurohormonal activation in general and the RAS in particular.6
To test the hypothesis that increased plasma concentrations of ANG II have adverse effects on cardiac function in conditions of an already reduced cardiac output, we infused ANG II in rats 2 weeks after the induction of a myocardial infarction and studied cardiac function. This timing and model were chosen because we knew already from our own work as well as that from other groups that cardiac function is decreased in this model and that plasma levels of ANG II are not elevated at this time.2 7 8 9 As it is also known that peripheral vascular alterations do occur after induction of a myocardial infarction and that ANG II has strong effects on vascular structure,10 11 we also studied the effects of ANG II on vascular structural parameters.
The results show that infusion of ANG II in MI rats had similar effects on cardiac function in MI and sham-operated rats. The increases in mean arterial pressure and vessel wall mass were, however, smaller in MI rats than in sham rats. We suggest that the activation of inhibitory neurohumoral systems such as ANP and NO may be responsible for the suppression of the hypertensive and growth stimulatory effects of ANG II in MI rats.
Male Wistar rats (n=161, 200-250g at the start of training on day −14, Winkelmann, Borchen, Germany) were housed under standard conditions and fed standard rat chow (RMH-TM, Hope Farms, Woerden, The Netherlands) and water ad libitum. The experiments were performed in accordance with local institutional guidelines. The randomization scheme is shown in Fig 1⇓. Of the 161 rats used, 26 died after coronary artery ligation and 2 died after sham surgery. Of the 133 remaining rats, 39 were used for measurement of cardiac function and hemodynamics (experiment 1), 46 were used in the final evaluations of experiment 2 (tail cuff plethysmography and morphometric measurements), and 39 rats were used for experiment 3 (blood sampling and excision of the thoracic aorta). The remaining nine rats had infarct sizes less than 20% of the left ventricular circumference or showed nontransmural infarcts and were not used in the final evaluations, as previous hemodynamic measurements in our laboratory indicated that these rats exhibit no signs of cardiac failure.12
Coronary Artery Ligation
Myocardial infarction was induced by ligation of the left anterior descending coronary artery according to the method of Fishbein et al13 as described in detail previously.7 After induction of anesthesia with sodium pentobarbital (60 mg/kg intraperitoneally), positive pressure respiration was started through an endotracheal tube. The thorax was opened in the fourth left intercostal space, and the left anterior descending coronary artery was occluded by placing a 6-0 silk ligature near the origin of the pulmonary artery. In the sham procedure, a superficial suture was placed in the epicardium of the left ventricle, near the left anterior descending coronary artery. Surgery was performed on day 0.
Rats were randomly assigned to the protocol for measurement of hemodynamics and cardiac function (experiment 1, n=39), tail cuff plethysmography and morphometric measurements (experiment 2, n=46), or blood sampling (experiment 3, n=39) as outlined in Fig 1⇑. For each study, rats were assigned to one of the following four groups: SH-NaCl underwent a sham operation and were infused with saline (0.9% NaCl). MI-NaCl were infused with saline after infarct induction. SH-ANG and MI-ANG were subjected to sham surgery and myocardial infarction, respectively, and infused with ANG II. The number of animals in each group is presented in Fig 1⇑.
Infusions. ANG II (human [5Val]ANG II, 250 ng/kg/min, Brunschwig, Amsterdam, The Netherlands) dissolved in saline or saline alone were infused subcutaneously for 2 weeks by osmotic minipumps (Alzet model 2002, Alza Corporation, Palo Alto, Calif) and implanted subcutaneously between the shoulder blades. Minipumps were inserted at day 0.
In this study, hemodynamic measurements were performed on saline- and ANG II-infused sham and MI rats. These rats were implanted with an electromagnetic probe and various catheters to determine cardiac and peripheral hemodynamics.
Implantation of measuring equipment. Seven days after coronary artery ligation or sham surgery and the implantation of the osmotic minipump, all rats were equipped with an electromagnetic flow probe (2.7 mm diameter; Skalar, Delft, The Netherlands) on the ascending aorta according to previously described methods.7 Rats were anesthetized with sodium pentobarbital (60 mg/kg intraperitoneally). After the rat underwent endotracheal intubation and the start of positive pressure respiration, the thorax was opened in the third right intercostal space and the ascending aorta was dissected from surrounding tissue. An electromagnetic flow probe was placed around the aorta at 1 to 2 mm above the heart. The thorax was closed in layers, the cable was fixed to the ribs, and the connector was exteriorized in the neck where it was sutured to the skin.
Four days later (day 11), rats were anesthetized with ether and implanted with a PE-10 catheter in the abdominal aorta through the right femoral artery to measure arterial blood pressure. Furthermore, through the right femoral vein, a PE-10 catheter was implanted into the abdominal vena cava for infusions. A Silastic (602-175, Dow Corning, Midland, Mich) catheter was placed in the thoracic vena cava for measurement of central venous pressure. All catheters were exteriorized in the neck, filled with saline, and closed with metal plugs. After these operations, rats were allowed to recover for 3 days before hemodynamic measurements.
Measurements and protocol. On day 14, the electromagnetic flow probe was connected to a sinewave flowmeter (model MDL 401, Skalar) to measure blood flow through the ascending aorta. Although this flow comprises cardiac output minus coronary blood flow, we refer to it as cardiac output. The baseline was established by taking late diastolic blood flow as zero. The arterial and central venous catheters were connected to low-volume displacement pressure transducers (CP01; Century Technology, Inglewood, Calif). Mean values for arterial blood pressure and central venous pressure were obtained by digital integration. Stroke volume was calculated from the flow signal by integration of each beat. Total peripheral resistance was calculated as (mean arterial pressure minus central venous pressure)/cardiac output. Stroke work was estimated by multiplying stroke volume with the difference between mean arterial pressure and central venous pressure. All derivations were made on-line and stored on disk for later processing.
After 45 to 60 minutes, baseline recordings were made for 15 minutes. Then a rapid infusion of 12 mL of a warm (37°C) Ringer’s solution was given in 1 minute through the abdominal caval vein catheter. This has been shown to increase cardiac output to a plateau level, which can be used as an indicator of maximal cardiac function.7 During this period, hemodynamics were monitored continuously. The plateau cardiac output was obtained during the final 10 to 15 seconds of the volume loading and is termed “maximal cardiac output during volume loading.” The values for cardiac output and stroke volume (at baseline and during volume loading) were normalized for body weight and termed cardiac index and stroke volume index.
After the measurements, rats were killed using an overdose of pentobarbital, after which the heart was arrested in diastole by injecting CdCl2 (0.1 M) into the inferior caval vein. The heart was excised and weighed after removal of the atria. Lungs were also excised and weighed. The heart was then fixed overnight in 10% phosphate-buffered formalin and processed and embedded in paraplast via routine histologic procedures. Infarct size was determined on Azan-stained sections (4 μm) on a computerized morphometric system (Quantimet 570, Leica, Cambridge, United Kingdom) of a slice of the heart taken at the level of the papillary muscle. Infarct size was expressed in percent of left ventricular circumference.
In this study, systolic blood pressures were measured by tail cuff plethysmography throughout the protocol. Before the rats were killed on day 14, mean arterial blood pressures were measured through an arterial catheter, and some rats were used for assessment for a cumulative ANG II pressor dose-response curve.
Tail cuff plethysmography. Systolic blood pressures were measured by tail-cuff plethysmography (IITC Inc, Life Science Instruments, Woodland Hills, Calif) in conscious rats. During 7 days (day −14 to day −8), rats were trained for the procedure. Measurements started 1 week before surgery (day −7 to day 0) and continued during infusions. Blood pressures and heart rates were measured three times per week. The mean of four to five measurements per animal in one session was used for calculations.
Assessment of mean arterial pressures and heart rates on day 14. On day 13, rats were anesthetized with sodium pentobarbital (60 mg/kg intraperitoneally) and provided with a polyethylene (PE10) catheter in the abdominal aorta through the left femoral artery to measure mean arterial blood pressure. The catheter was exteriorized in the neck, filled with saline, and closed with a metal plug. On day 14, the arterial catheter was connected to low-volume displacement pressure transducers (CP01; Century Technology). All signals were fed into a microcomputer to sample all signals at 500-Hz each. Mean values for arterial blood pressure and heart rates were obtained by digital integration.
Assessment of cumulative ANG II pressor response curve. On day 14, some rats assigned to this study protocol (SH-NaCl, n=7; MI-NaCl, n=7; SH-ANG, n=6; MI-ANG, n=6) were anesthetized with pentobarbital (60 mg/kg intraperitoneally), and a PE10 catheter was implanted in the right jugular vein. The previously implanted arterial catheter was connected to the same system as described above. An ANG II dose-blood pressure response curve in a dose ranging from 0.3 ng to 30 μg of ANG II was determined, while body temperature was maintained at 37°C by using heating pads. To minimize interference of tachyphylaxis, doses were given cumulatively, with each successive injection given immediately after the maximum effect of the preceding dose was achieved (10 to 20 seconds). The dose at the half-maximal effect (ED50) was computed by fitting the mean arterial pressure response to ANG II to a sigmoidal curve using the equation where ΔMAPmax=maximal increase in mean arterial pressure, D=dose of ANG II, and n=Hill coefficient.
Tissue processing. The heart was arrested in diastole under pentobarbital anesthesia by injecting 1 to 2 mL of CdCl2 (0.1 M) into the inferior caval vein. Rats were perfused with phosphate-buffered saline (pH 7.0), followed by perfusion with 5% phosphate-buffered formalin (pH 7.0, 10 minutes each) at a pressure of 100 mm Hg via a catheter in the right carotid artery. To ensure maximal vasodilation, sodium nitroprusside (5 mg/mL, Sigma, St Louis, Mo) was added to both perfusion solutions.
After excision of the heart and removal of the atria, the ventricles were blotted dry and weighed. For determination of the wet:dry weight ratio, the apex was removed and weighed separately. Apex dry weight was determined after freeze drying. The percentage water in the heart (apex) was calculated as 100×(wet weight-dry weight)/wet weight. Lungs were also excised and weighed.
Vessel segments (2 to 3 mm) were obtained using anatomic landmarks. The superior mesenteric artery was taken from its origin at the aorta to its first branching point. Three mesenteric resistance arteries supplying the jejunum were excised. The mesenteric circulation was chosen because it has receptors for ANG II,14 and ANG II induces hypertrophy of the vascular wall of this artery.15 To investigate a second type of resistance artery, the right lung was taken to study the pulmonary resistance arteries, because in an earlier study,10 MI induction caused changes in vessel dimensions of the pulmonary resistance arteries. All tissues were fixed overnight in 10% phosphate-buffered formalin. Fixed tissues were processed and embedded in paraplast via routine histologic procedures.
Morphometric measurements. For measurement of vessel dimensions, rehydrated 4-μm sections were incubated for 30 minutes in Lawson’s solution (Klinipath, Zevenaar, The Netherlands), differentiated in 70% alcohol, and dehydrated in graded series of alcohol and 100% xylene. Medial cross-sectional areas (defined as the area between the internal and external lamina elastica) and lumen area of 2 to 3 cross-sections of each vessel were measured on a computerized morphometric system (Quantimet 570, Leica) by one experienced investigator (SH), who was blinded to the experimental groups. Intraobserver variations of this method are less than 2%. Media:lumen ratio was calculated as medial cross-sectional area/lumen area×100. Infarct size was determined as described in experiment 1.
In this protocol, blood samples were taken for the determination of plasma ANG II and atrial natriuretic peptide in ANG II- and saline-infused MI and sham rats. Also, the thoracic aorta was excised for the determination of cyclic GMP (cGMP) concentrations.16 In these groups of rats, mean arterial pressures were measured on day 15.
Blood sampling. On day 13, rats were provided with a PE10 catheter in the abdominal aorta through the left femoral artery, as described above. On day 14, blood (2 mL) was sampled from the arterial catheter in nondisturbed rats and collected in chilled heparinized tubes containing 1.4 μM enalaprilat. Tubes were centrifuged at 3000 revolutions per minute at 4°C for 15 minutes and stored at −70°C until plasma ANG II and ANP measurements.
Because of the blood sampling, measurement of mean arterial pressures in these rats was postponed to day 15. After these measurements (see experiment 2), rats were killed using an overdose of pentobarbital. As in experiment 1, the heart was arrested in diastole by injecting 1 to 2 mL of CdCl2 (0.1 M) into the inferior caval vein. The thoracic aorta was rapidly excised, rinsed in cold buffered saline, frozen in liquid nitrogen, and stored at −70°C. Heart and lungs were excised and weighed. The heart was then prepared for infarct size measurement as described in experiment 1.
Detection of plasma angiotensin II and ANP concentrations. Plasma ANG II and ANP concentrations were measured using a radioimmunoassay. For ANG II, plasma samples (0.5 mL) were extracted using ethanol. For ANP, plasma samples (0.5 mL) were acidified with 1.5 mL of 4% acetic acid and ANP was eluted from a C18 column (Millipore, Waters Chromotography, Etten-Leur, The Netherlands) by applying 3×1 mL of 4% acetic acid in 86% ethanol at the top of the column. Both collected eluates were evaporated to dryness in a vacuum evaporator. The dried extracts were reconstituted by adding assay buffer to each tube and then stored at −15°C.
Radiolabeled ANG II (Dupont, NEN products, Dordrecht, the Netherlands) and ANP (Nichols Institute, Diagnostics B.V., Wijchen, The Netherlands) competed with unlabeled ANG II and ANP in the test samples and standards for a limited number of specific antibody binding sites. At the end of the incubation period (42 hours), antibody-bound ANG II and ANP were separated from unbound ANG II and ANP using anti-rabbit-coated cellulose (Nichols Institute, Diagnostics B.V.) in suspension as a solid phase. After a brief incubation and centrifugation, the unbound ANG II and ANP were decanted and the antibody-bound radiolabeled ANG II and ANP were measured in a gamma counter. A standard curve was prepared using a dose-response relationship, and the test sample concentrations were read from the curve. Intra- and interassay variations are 4.9% and 7.9% for ANG II and 6.9% and 12.7% for ANP.
Tissue sampling. Thoracic aortas were powered with a mortar and pestle and placed in liquid nitrogen. Tissues were further homogenized in 6% trichloroacetic acid with an ultraturrax at 4°C and centrifuged at 4000g for 10 minutes at 4°C, and the supernatant was transferred to a clean test tube. Trichloroacetic acid was removed from the supernatant by extracting three times with three volumes of water-saturated diethyl ether. Samples were then dried under nitrogen and stored at −20°C until assayed.
Determination of cGMP concentration. The cGMP content of thoracic aorta segments was determined with a commercially available kit (125I-cGMP-RIA, IBL, Hamburg, Germany). The residues were dissolved in 300 μL of assay buffer, of which 100 μL was used for the assay. A total of 100 μL of 125I tracer and 200 μL antiserum were added. Samples for the standard curve and nonspecific binding were prepared according to the same procedure. After an incubation of 24 hours at 4°C, cooled separation reagent was added, after which the tubes were centrifuged. The supernatant was discarded, and residual radioactivity was counted in a gamma counter. The protein content was measured using a commercially available assay (Biorad protein assay, Biorad Lab. München, Germany), and cGMP values are presented as femtomoles per milligram of protein.
Data are expressed as means±SEM. Statistical significance was defined at P<.05. The Friedman procedure was used to examine the interaction between the infusion of ANG II and the induction of a MI. Intergroup differences were evaluated with a nonparametric Mann-Whitney test with a Bonferroni correction for multiple group comparison. For the evaluation of the systolic blood pressure measurements during the experimental period, areas under the curve were determined and used for subsequent Friedman and Mann-Whitney procedures.
Effect of a Myocardial Infarction (SH-NaCl versus MI-NaCl)
Ligation of the left anterior descending coronary artery resulted in an infarction of approximately 45% of left ventricular circumference. Infarct sizes were comparable in all experiments (Tables 1⇓ and 2⇓). Body weights at day 0 were comparable in all groups. Induction of a MI decreased the body weight in saline-infused rats (Tables 1⇓ and 2⇓). Of note, the weight loss of saline-infused rats in experiment 1 (measurement of cardiac function, see Table 1⇓) was more excessive compared with the rats used in experiments 2 and 3 (Table 2⇓), presumably as a result of the intensive operation procedures. There were no differences in absolute heart weight or heart/lung-to-body weight ratios between saline infused sham and MI rats (Tables 1⇓ and 2⇓). The lung weight-to-body weight ratio increased in the MI rats (Table 1⇓).
The measurements of cardiac function in experiment 1 demonstrated that MI induction resulted in a decrease in baseline and maximal cardiac index, stroke volume index (Fig 2⇓), and baseline stroke work (Table 1⇑). Total peripheral resistance and central venous pressure did not change in MI-NaCl rats (Table 1⇑).
Systolic blood pressures, measured in experiment 2, were comparable at day 0 in all groups. Although at the end of the experimental period systolic blood pressures had decreased by 25 mm Hg in MI-NaCl rats compared with SH-NaCl rats (Fig 3⇓), this did not reach statistical significance (P=.09). However, the direct intra-arterial mean arterial blood pressure measurement on day 14 demonstrated a significant difference between saline-infused sham and MI rats (Tables 1⇑ and 2⇑). No differences between saline-infused sham and MI rats were found in the cumulative ANG II pressor response curve as presented in Table 3⇓.
The vessel dimensions of the superior mesenteric artery and mesenteric and pulmonary resistance arteries are presented in Tables 4⇓ and 5⇓. Compared with SH-NaCl rats, MI induced no changes in the vessel dimensions of these three arteries.
Finally, plasma ANP concentrations were increased in the saline-infused MI rats, whereas plasma ANG II concentrations did not differ between groups. Also, aortic cGMP concentrations did not differ between the two groups (experiment 3, Table 6⇓).
Effect of ANG II Infusion in Sham-Operated Rats (SH-NaCl Versus SH-ANG)
ANG II infusion in sham rats resulted in a substantial weight loss (Tables 1⇑ and 2⇑). Although heart weight did not differ between groups, ANG II infusion increases heart-to-body weight ratios in SH-ANG rats. There was no evidence of edema in the heart, as mass fraction of water was comparable in all groups (SH-NaCl 76±2%, remaining data not shown). Lung weight-to-body weight ratio increased during ANG II infusion in sham rats (Tables 2).
Compared with SH-NaCl rats, ANG II infusions in sham rats decreased both baseline and maximal cardiac index and stroke volume index (experiment 1, Fig 2⇑). Stroke work, however, did not change. The total periphe-ral resistance significantly increased in these rats, accounting for the increase in mean arterial pressure (Table 1⇑).
ANG II treatment gradually increased systolic blood pressures in SH-ANG rats by approximately 70 mm Hg at day 12 (experiment 2, Fig 3⇑). The same increase was seen in mean arterial pressures measured at day 14 (Tables 1⇑ and 2⇑). The characteristics for the acute ANG II dose-blood pressure responses were comparable in ANG II and saline-infused sham rats (Table 3⇑).
ANG II infusion resulted in a 33% increase in medial cross-sectional area of the superior mesenteric artery in sham-operated rats (Table 4⇑). Vessel dimensions of both the mesenteric and pulmonary resistance arteries were similar in saline- and ANG II-infused sham-operated rats.
ANG II infusion increased plasma ANG II and ANP in sham rats (experiment 3, Table 6⇑). Aortic cGMP concentrations tended to decrease from 559±87 to 306±59 fmol/mg protein in SH-NaCl and SH-ANG rats (P=.06), respectively (Table 6⇑).
Effect of ANG II in Myocardial-Infarcted Rats (MI-NaCl Versus MI-ANG)
ANG II infusion in MI rats resulted in a substantial weight loss (Table 2⇑). Similar to the SH-ANG rats, absolute heart weights of ANG II-infused MI rats were comparable with those saline-infused MI rats, whereas the heart-to-body weight ratios of ANG II-infused MI rats were increased (Tables 1⇑ and 2⇑). ANG II infusions in MI rats increased the lung weight-to-body weight ratios.
Cardiac function measurements (experiment 1) demon-strated that the baseline cardiac index of ANG II-infused MI rats was not different from SH-ANG rats and also not from MI-NAG II rats (Fig 2⇑). Stroke work was reduced compared with SH-ANG but not different from MI-NaCl rats (Table 1⇑). Cardiac index and stroke volume index during volume loading of MI-ANG rats were significantly reduced compared with SH-ANG rats but not different from MI-NaCl rats (Fig 2⇑). Thus, although the ANG II infusion in sham rats significantly reduced cardiac function compared with saline-infused sham rats, infusion of ANG II in MI rats did not further reduce cardiac function. This is illustrated by calculations of the relative decrease of cardiac index during volume loading. Thus, the cardiac index during volume loading of ANG II-infused MI rats decreased −22±7% compared with SH-ANG and −22±7% compared with MI-NaCl rats. This decrease in cardiac function was fully comparable with the decrease of 25±4% in cardiac index during volume loading in SH-ANG rats compared with SH-NaCl rats. The total peripheral resistance increased by 62±15% in MI-ANG rats and by 146±24% in SH-ANG rats (P<.001), although the absolute values did not differ (Table 1⇑).
Despite the comparable effects on cardiac function, infusion of ANG II in a dose that increased systolic blood pressures in ANG II-infused sham rats resulted in a much smaller increase of 30 mm Hg in MI rats (experiment 2, Fig 3⇑). In fact, blood pressures in ANG II-infused MI rats were not different from SH-NaCl rats during the entire experimental period. The same pattern of changes was seen in mean arterial pressures of the rats used in both experiments 1 and 2, measured at day 14. Although ANG II infusion in sham rats increased mean arterial pressure by approximately 60 mm Hg compared with SH-NaCl rats (relative increase, 47±5%), the mean arterial pressure in the MI-ANG rats was only 20 mm Hg higher than in MI-NaCl rats (relative increase, 21±4%) and not different from SH-NaCl rats (Table 2⇑).
ANG II infusion in MI rats, in contrast to its infusion in sham rats, did not increase the medial cross-sectional area of the superior mesenteric artery (Table 4⇑). Furthermore, the ANG II infusion tended to increase the medial cross-sectional area of the pulmonary resistance arteries, but this was not statistical significant (P=.06; Table 5⇑).
ANG II infusion in MI rats increased the plasma concentrations of ANG II and ANP (experiment 3, Table 6⇑). Plasma ANG II concentrations did not differ in MI-ANG and SH-ANG rats. Plasma ANP levels showed a clear tendency to increase in MI-ANG rats (SH-ANG, 58±13 pg/mL; MI-ANG, 152±48; P=.07). ANG II infusions in MI rats tended to increase cGMP concentrations in ANG II-infused MI rats (from 422±72 to 635±106 fmol/mg protein; P=.15). As a result of the decrease in cGMP concentrations in SH-ANG rats, aortic cGMP concentrations significantly increased in ANG II-infused MI rats compared with SH-ANG (Table 6⇑).
The results of the present study indicate that infusion of ANG II decreased cardiac function in both sham and MI rats. Thus, although an increased plasma ANG II concentration is correlated with a poor prognosis in humans,5 6 it did not result in the expected adverse effects on cardiac function in MI rats. Other expected findings of the exogenous ANG II infusion, i.e., the increased mean arterial blood pressure and the hypertrophy of the peripheral vascular wall, were also not observed in MI rats. The data further suggest that during the infusion of ANG II in compensated heart failure, other hormonal systems are activated and suppress the hypertensive and hypertrophic effects of ANG II.
Infusion of ANG II in experimental animals is known to reduce cardiac output.17 18 This is confirmed in the study presented herein, as ANG II infusion reduced cardiac function in sham rats. The reduction of baseline cardiac index and stroke volume index in ANG II-infused MI rats was not different from those observed in SH-ANG rats and, more importantly, not different from MI-NaCl rats. As outlined in the results section, the relative reductions in the indexes during volume loading of MI-ANG rats were comparable with the relative reductions observed in ANG II-infused sham rats. Also, MI-ANG animals were able to generate a peak pressor response after injections of ANG II comparable with SH-ANG (Table 3⇑; MI-NaCl, +59±5 mm Hg; MI-ANG, +74±10 mm Hg; P=NS), indicating that the combination of myocardial infarction and ANG II infusion did not result in a further deterioration of cardiac function. Finally, absolute values of total peripheral resistance did not differ in ANG II-infused sham and MI rats, indicating that ANG II infusions did result in similar increases in total peripheral resistance in sham and MI rats.
Although ANG II infusion had comparable negative effects on cardiac function in rats with and without MI, it did not result in the large increase in blood pressure seen in the ANG II-infused sham rats and it did not induce the typical hypertrophic response of the vascular wall of the large conduit arteries. ANG II is well known for its hypertrophic effects on the peripheral vasculature.11 19 20 21
At least three phenomena may be responsible for the suppression of pressor and structural vascular effects of ANG II in MI rats. First, ANG II may deteriorate cardiac function in MI rats, which did not occur. Second, different ANG-receptor subtypes may be involved in the acute and long-term hypertensive and hypertrophic actions of the peptide. The aortic wall of a normal adult rat contains the AT1 and AT2 receptor subtypes in a 4:1 ratio.22 AT1 receptors mediate vasoconstriction and vascular smooth muscle cell growth.23 AT2 receptors are thought to have a functional role during embryonal development24 and may mediate antiproliferative and apoptotic effects.25 26 The subtypes may differ in the extent to which they are regulated by elevated levels of the agonist,23 and a switch from the AT1 to the AT2 receptor in the peripheral vasculature of MI rats might explain the suppression of pressor and structural effects of ANG II infusions. In the heart, a change in the expression of AT1 and AT2 receptors has been shown to occur during heart failure.27 28 29 In the present study, however, both the ED50 and maximal elevation of pressure after acute injections of ANG II were not affected in ANG II-infused MI rats, indicating the presence of functionally active vascular AT1 receptors after a 2-week ANG II infusion. A possible upregulation of AT2 receptors, however, cannot be excluded.
Another possibility is that in MI rats the pressor and mitogenic effects of ANG II may have been counteracted by a concomitant activation of other neurohumoral systems. Candidate vasodilating and growth suppressor systems are ANP and NO, which have increased plasma levels during heart failure.30 31 In cultured vascular smooth muscle cells, both ANP and NO have antimitogenic effects.32 33 In vivo, infusions of nondepressor doses of ANP decrease medial thickness in spontaneously hypertensive rats,34 whereas inhalation of NO inhibits neointimal formation after balloon-induced arterial injury.35 In our experimental model of heart failure, ANG II infusion tended to increase plasma ANP concentrations. Moreover, although ANG II had divergent effects on aortic cGMP concentrations (i.e., decrease in SH-ANG rats and a tendency to increase in MI-ANG rats), aortic cGMP concentrations were significantly higher in ANG II-infused MI rats compared with ANG II-infused sham rats. The in vivo basal aortic cGMP concentration seems to mainly depend on NO synthase,36 because the contribution of endogenous ANP to basal aortic cGMP generation, via stimulation of the particulate guanylate cyclase, appears to be minor compared with that of NO via soluble guanylate cyclase. Thus, it is attractive to hypothesize that during infusion of ANG II in experimental heart failure, the growth-stimulatory and hypertensive effects of ANG II are counteracted by concomitant activation of vasodilating and growth-inhibitory systems, such as ANP and NO.
It should be noted that the lack of hypertrophic response of the superior mesenteric artery in MI rats may also be explained by the normal, nonhypertensive blood pressures after 2 weeks of ANG II infusions. However, infusion of ANG II in normal rats induces a hypertrophic response in peripheral vessels,11 19 and this effect has been shown to be independent of the increase in blood pressure.15 Also, ANG II has growth-stimulating effects for vascular smooth muscle cells in culture.19 37 Thus, we assume that the dose of ANG II given in this study was high enough to potentially induce a hypertrophic vascular response and that this effect is independent of blood pressure.
A different response to ANG II was seen in the resistance arteries. MI alone or in combination with ANG II infusion did not induce changes in dimensions of the mesenteric resistance arteries. ANG II has been shown to have different effects on large conduit arteries and resistance arterioles (reviewed in References 38 and 3938 39 ). Although the increase in wall thickness in large vessels is the result of hypertrophy, the changes in wall thickness in small vessels appear to result from hyperplasia. ANG II has been shown to induce increases in vessel wall thickness in the mesenteric circulation of young and adult rats15 40 at similar infusion rates as the ones used in this study. However, in those studies,15 40 medial thickness was measured after myograph experiments (no in situ fixation) and in smaller branches of the mesenteric circulation (lumen diameter of 200 μm versus 300 μm in the present study), which might explain the differences with the results of the present study. Also, mesenteric resistance arteries have been reported to have only weak contractions to ANG II (compared with, e.g., femoral resistance arteries41 ). It is also possible that, in this study, the time span was too short to induce hyperplastic changes in the mesenteric resistance arteries.
Next to its effects on peripheral vascular structure, ANG II also increased the heart-to-body weight ratio in ANG II-infused sham and MI rats. Many studies have shown a development of cardiac hypertrophy during chronic ANG II infusions.42 43 Most studies define “cardiac hypertrophy” as an increase in the heart weight-to-body weight ratios. According to this definition, infused sham and MI rats in the present study would also display cardiac hypertrophy. However, given the fact that the absolute heart weights did not differ and the animals lost a significant amount of body weight, the heart weight-to-body weight ratio does not give a fair representation of the amount of cardiac hypertrophy. Thus, in the present study, a 2-week ANG II infusion of 250 ng/kg/min did not result in the development of a pronounced cardiac hypertrophy.
In conclusion, this study shows that elevation of plasma ANG II infusion 2 weeks after infarction results in a comparable reduction of cardiac function in both sham and MI rats. Interestingly, in contrast to the ANG II-infused sham rats, the ANG II-infused MI rats did not develop hypertension or structural vascular alterations. We suggest that the reason for this divergent response in sham and MI rats is the activation of counterregulatory hormonal systems in the MI rats, which are, at least in the compensated phase of heart failure, responsible for suppression of the hypertensive and hypertrophic effects of ANG II. This suggests that adverse effects of ANG II on the function and structure of the cardiovascular system become evident only when counterregulatory hormonal systems are failing, as in decompensated heart failure.
Selected Abbreviations and Acronyms
|ANG II||=||angiotensin II|
|ANP||=||atrial natriuretic peptide|
|AT1 receptor||=||angiotensin II type-1 receptor|
|AT2 receptor||=||angiotensin II type-2 receptor|
|RAS||=||renin angiotensin system|
This study was supported by a grant from Dutch Heart Foundation and the Dutch Foundation for Medical Research (NWO 902-18-291).
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