Introduction

Abstract Introduction Methods Results Discussion References

Angiotensin-converting enzyme is a membrane-bound metalloexopeptidase which cleaves angiotensin I to angiotensin II (Antonaccio and Wright, 1990). The major actions of angiotensin II include increased vascular tone, enhanced sympathetic nerve activation and modulation in the activity of other neurohormonal systems (Antonaccio and Wright, 1990). In addition to systemic production of Ang-II, studies have demonstrated that Ang-II can be produced within the myocardium through a local ACE system as well as by serine proteases (Baker et al., 1992; Dzau, 1988; Ehring et al., 1994; Gavras, 1994; Gohlke et al. 1994; Hirsch et al., 1991; Lindpaintner and Ganten, 1991; Maisel et al., 1989; Nolly et al., 1994; Schunkert et al., 1993; Urata et al., 1990, 1993; Weber et al., 1994). Although an area of investigation, it appears that a major mode of action of angiotensin II is through a specific receptor subtype, the AT₁ Ang-II receptor (Dudley et al., 1990; Lopez et al., 1994; Sechi et al., 1992; Urata et al., 1989). Clinical trials have demonstrated that chronic ACE inhibition improved symptoms and survival in patients with LV dysfunction because of a wide range of etiologies (The CONCENSUS Trial Study Group, 1987; The SOLVD Investigators, 1991). However, despite these past clinical reports it is still unclear whether the mechanisms of action of ACE inhibition in the setting of LV dysfunction are caused by global hemodynamic effects (changes in loading conditions), reduced production of angiotensin II with subsequent diminished AT₁ Ang-II receptor activation or modulation in the activity of alternative neurohormonal systems and enzymatic pathways. To begin addressing this issue, several studies have been performed in which ACE inhibition was instituted in experimental models of chronic LV dysfunction (McDonald et al., 1994; Sabbah et al., 1994; Spinale et al., 1995). These studies clearly demonstrated that ACE inhibition had direct and beneficial effects on LV geometry and function. In addition, McDonald et al. (1994) demonstrated that, in a model of LV dysfunction caused by myocardial injury, chronic therapy with an alpha-1 receptor antagonist did not provide similar beneficial effects on LV geometry and function when compared with ACE inhibition. Thus, the findings of these past reports suggest that the mechanisms for the beneficial effects of ACE inhibition in the setting of LV dysfunction are the results of direct myocardial effects, rather than changes in systemic loading conditions. However, whether chronic ACE inhibition in the setting of developing LV dysfunction has direct effects on myocyte contractile processes, and whether these effects are mediated specifically through the AT₁ Ang-II receptor remains unknown. Accordingly, the overall goal of the present study was to determine the direct and potentially differential effects of chronic ACE inhibition or specific AT₁ Ang-II receptor blockade on LV function and geometry, and LV myocyte contractile processes in a model of progressive LV dysfunction.

Past reports from this laboratory and others have demonstrated that chronic pacing-induced tachycardia in animals caused LV dilation and dysfunction and activation of several neurohormonal and sarcolemmal systems (Armstrong et al., 1986; Cory et al., 1993; Eble and Spinale, 1995; Finckh et al., 1991; Kim et al., 1994; Margulies et al., 1991; Perreault et al., 1992; Ping and Hammond, 1994; Roth et al., 1993; Spinale et al., 1990, 1992a,b, 1994, 1995; Travill et al., 1992; Williams et al., 1994). Specifically, this laboratory has previously demonstrated that chronic pacing-induced tachycardia resulted in decreased isolated myocyte contractile function (Spinale et al., 1992a, b, 1994). In addition, the development of tachycardia-induced LV dysfunction is associated with down-regulation of beta adrenergic receptors, blunted beta adrenergic responsiveness and alterations in the content and mRNA expression of components of the beta adrenergic receptor system (Ping and Hammond, 1994; Roth et al., 1993; Spinale et al., 1994). This animal model of chronic rapid pacing produces similar functional and neurohormonal alterations which have been observed previously in patients with severe LV dysfunction (Benedict et al., 1993; Bristow et al., 1986; Eschenhagen et al., 1992). In a recent report from this laboratory, concomitant ACE inhibition with chronic rapid pacing improved LV function and geometry (Spinale et al., 1995). Thus, chronic ACE inhibition in this model of pacing-induced LV dysfunction appears to result in effects similar to those observed in clinical studies (Antonaccio and Wright, 1990; Lindpaintner and Ganten, 1991; The SOLVD Investigators, 1991). Accordingly, this model of pacing-induced LV dysfunction was used to test the central hypothesis that concomitant ACE inhibition or AT₁ Ang-II receptor blockade in the setting of progressive LV dysfunction will have significant and differential effects on myocyte contractile processes and the beta receptor transduction system.

	Methods

Abstract Introduction Methods Results Discussion References

This study directly examined the effects of chronic ACE inhibition and AT₁ Ang-II receptor blockade on myocyte contractile processes with the development of LV dysfunction caused by chronic rapid pacing. Accordingly, ACE inhibition or AT₁ Ang-II receptor blockade was begun at the initiation of chronic rapid pacing and continued throughout the pacing period. LV function, geometry and neurohormonal profiles were serially monitored during the entire pacing protocol. At the termination of the study, isolated myocyte contractile function, beta adrenergic receptor transduction and function were examined.

Model of pacing-induced LV dysfunction. Twenty-four adult mongrel dogs of either sex (9-16 months of age, 15-25 kg, Hazelton, Kalamazoo, MI) were used in this study. The animals were instrumented chronically to serially measure LV and arterial pressures as well as obtain plasma samples. In addition, a pacemaker and stimulating electrode were implanted to produce rapid right ventricular pacing. The animals were induced with thiopental (2 mg/kg, Pentothal, Abbot Labs, Chicago, IL), intubated and ventilated with 100% oxygen. Maintaining a surgical plane of anesthesia with 1% to 3% isoflurane (Aurthan, Anaquest, Madison, WI), a left thoracotomy was performed and a shielded stimulating electrode was sutured onto the right ventricular outflow tract, connected to a programmable pacemaker modified for programming heart rates up to 300 beats/min (Spectrax 5985, Medtronic, Inc., Minneapolis, MN) and buried in a subcutaneous pocket. A previously calibrated microtipped transducer (model p5-X4, Konigsberg Instruments, Pasadena, CA) was placed into the LV chamber through a small incision at the apex. The connection of the LV transducer was tunneled and externalized in the suprascapular region of each animal. The pericardium was left open, the incision closed and the pleural space evacuated of air. Next, the right carotid artery was exposed and a vascular access port (model GPV, 9F, Access Technologies, Skokie, IL) was placed in the artery, advanced to the aortic arch and sutured in place for subsequent arterial blood pressure measurements and blood sampling. The animals were allowed a 14-day recovery period at which time proper operation of all implanted instrumentation was confirmed. All animals used in this study were treated and cared for in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (National Research Council. 1985: NIH publication no. 86-23).

Experimental design. After recovery from the surgical procedure, baseline LV pressure and dimensions and arterial pressure were measured, and plasma samples were obtained for each dog as described in the following sections. The pacemakers were activated for rapid ventricular pacing (216 ± 2 bpm), and 1:1 capture confirmed by electrocardiography. The dogs were then randomly assigned to one of four treatment protocols: (1) ACE Inhibition: Dogs were administered the ACE inhibitor, fosinopril (30 mg/kg p.o. b.i.d.), during the pacing period (n = 6). (2) AT₁ Ang-II Receptor Blockade: Dogs were administered the AT₁ Ang-II receptor antagonist, Irbesartan [SR 47436(BMS-186295) Sanofi-Recherche, France/Bristol Myers Squibb, NJ] at a dose of 30 mg/kg p.o. b.i.d. during the pacing period (n = 6). (3) Rapid Pacing Only: Dogs were given gelatin capsules during the pacing period (n = 6). (4) Sham Control: These dogs were instrumented and cared for in a fashion identical with the groups described with the exception of activation of the pacemaker and drug treatment (n = 6). Simultaneous electrocardiograms and ventricular pressure recordings were performed frequently during the 28-day pacing protocol to ensure proper operation of the pacemaker and the presence of 1:1 conduction. At weekly intervals, the dogs were brought to the laboratory and the pacemaker was deactivated. After a 30-min stabilization period, LV pressures were recorded and echocardiographic measurements were obtained as described in the next section. To determine changes in neurohormonal status with the progression of pacing-induced LV dysfunction, plasma samples were obtained immediately after the LV function measurements. After the LV function studies and plasma collection, the pacemaker was reactivated (with the exception of the sham controls). At the conclusion of the 28-day pacing protocol, the dogs were returned to the laboratory for terminal study as described in the next section.

Dosage rationale. The AT₁ Ang-II receptor antagonist chosen for the present study is a selective nonpeptide AT₁ Ang-II receptor antagonist and has been characterized previously (Cazaubon et al., 1993; van den Meiracker et al., 1995). The pharmacological activity of the specific dosage schedule used in the present study was fully characterized in preliminary Ang-I and Ang-II dose-response studies. Dogs (n = 3) were administered the AT₁ Ang-II receptor antagonist (30 mg/kg b.i.d.) for 72 h to achieve steady-state plasma levels, and Ang-II pressor studies were then performed. The pressor response to intravenous infusion of Ang-II (100 ng/kg) was reduced by 90% at 2 h after the morning dose on the fourth day compared with untreated baseline values. At 12 h after the morning dose, Ang-II pressor response was reduced by 33% from untreated base-line values. In four dogs, oral administration of the ACE inhibitor fosinopril at 30 mg/kg reduced the Ang-I pressor response by 80%. Thus, the dosage regimen used in the present study (30 mg/kg b.i.d.) provided a pharmacological profile consistent with specific effects of AT₁ Ang-II receptor blockade (McDonald et al., 1994; van den Meiracker et al., 1995) and ACE inhibition. More importantly, this dosing regimen had no effects on resting mean arterial blood pressure. Thus, the confounding influences of differences in systemic hemodynamics could be minimized and provide for more meaningful comparisons of the direct effects of the different treatments on LV and myocyte function in the setting of developing LV failure.

LV function measurements. Indices of LV systolic and diastolic function were obtained at base line and at weekly intervals during the 28-day pacing period using simultaneously recorded pressure and echocardiographic measurements described previously (Laurenceau and Malergue, 1981; Tomita et al., 1991; Zile et al., 1992). All measurements were performed in a darkened room with the animal resting quietly in a sling. The arterial access port was punctured with a 22-gauge Huber point needle (Access Technologies, Skokie, IL) connected to a fluid-filled catheter. Pressures from the fluid-filled aortic catheter were obtained with use of an externally calibrated transducer (Statham P23ID, Gould, Oxnard, CA). The electrocardiogram and pressure waveforms were recorded by use of a multichannel recorder (Gould, TA4000, Irvine, CA) as well as digitized on computer for subsequent analysis at a sampling frequency of 250 Hz (PO-NE-MAH, Storrs, CT). Two-dimensional and M-mode echocardiographic studies (ATL Ultramark 7, 3.5 MHz transducer, Bothell, WA) were used to image the LV from a right parasternal approach. LV volumes and ejection fractions were computed from the two-dimensional and M-mode echocardiographic recordings (Tomita et al., 1991; Zile et al., 1992). Peak positive and negative (dP/dt) and peak systolic wall stress were computed using methods described previously (Tomita et al., 1991). Finally, LV mass was computed from the two-dimensional targeted echocardiographic recordings using previously validated methods (Zile et al., 1992).

Neurohormonal measurements. To examine the relationship between changes in neurohormonal status which accompany changes in LV function with chronic rapid pacing, blood samples were drawn at the conclusion of each LV function study. With the animal resting quietly, 35 cc of blood was drawn from the arterial access port into tubes containing ethylenediaminetetraacetic acid (1.5 mg/ml), sodium azide (0.2 mg/ml) and aprotinin (1.15 trypsin-inhibiting units/ml). The blood samples were immediately centrifuged (2000 × g, 10 min, 4°C), the plasma decanted into separate tubes, frozen in a dry ice/methanol bath and stored at 80°C until the time of assay. Norepinephrine concentration, atrial natriuretic peptide levels, cyclic GMP content and plasma renin activity were determined from these plasma samples. Plasma norepinephrine was measured by high-performance liquid chromatography and normalized to picograms per milliliter of plasma (Goldstein et al., 1986). For the atrial natriuretic peptide and cyclic GMP assays, the plasma was first eluted over a cation-exchange column (C-18 Sep-Pak, Waters Associates, Milford, MA). Standardized radioimmunoassay procedures were performed to determine atrial natriuretic peptide concentrations, cyclic GMP levels and plasma renin activity (Peninsula Laboratories, Belmont, CA). All plasma assays were performed in duplicate.

Myocyte isolation and myocardial sample preparation. Four weeks after the institution of the protocols described above, the dogs were brought to the laboratory, and a final series of LV function measurements and plasma samples were obtained. The animals were then anesthetized as described under "Neurohormonal Measurements," a sternotomy performed and the heart quickly extirpated and placed in a phosphate-buffered ice slush. The great vessels, atria and right ventricle were carefully trimmed away, and the LV weighed. The region of the LV free wall incorporating the circumflex artery (5 × 5 cm) was excised and prepared for myocyte isolation. The posterior region of the LV free wall (4 × 4 cm) was snap frozen in liquid nitrogen for subsequent sarcolemmal preparation. The region of the left ventricular free wall comprising the left anterior descending artery (3 × 5 cm) was cannulated and prepared for perfusion fixation.

Myocyte contractile function measurements. Isolated myocytes were placed in a thermostatically controlled chamber (37°C) fitted with a coverslip on the bottom for imaging on an inverted microscope (Sedival, Jena, Germany). The volume of the chamber was 2.5 ml and contained two stimulating platinum electrodes. The myocytes were imaged using a 20× long working distance objective. Myocyte contractions were elicited by field stimulating the tissue chamber at 1 Hz (S11, Grass Instruments, Quincy, MA) by current pulses of 5-ms duration and voltages 10% above the contraction threshold. The polarity of the stimulating electrodes was alternated at every pulse to prevent the build-up of electrochemical byproducts. Myocyte contractions were imaged by use of a charge-coupled device with noninterlaced scan rate of 240 Hz (GPCD60, Panasonic, Secaucus, NJ). Myocyte motion signals were captured with the cell parallel to the video raster lines, and this video signal was input through an edge detector system (Crescent Electronics, Sandy, UT). The changes in light intensity at the myocyte edges were used to track myocyte motion (Mukherjee et al., 1993). The distance between the left and right myocyte edges was converted into a voltage signal, digitized and entered into a computer (80386, Zenith Data Systems, St. Joseph, MI) for subsequent analysis. Stimulated myocytes were allowed a 5-min stabilization period after which contraction data for each myocyte were recorded from a minimum of 20 consecutive contractions. Parameters computed from the digitized contraction profiles included: percentage shortening (%), peak velocity of shortening (µm/s), peak velocity of relengthening (µm/s), total contraction duration (ms) and time to peak contraction (ms). After collection of baseline indices of myocyte function, measurements were then performed in the presence of 25 nM ()-isoproterenol (Spinale et al., 1994).

Beta adrenergic receptor system. To determine whether changes occurred in the beta adrenergic receptor system or the Na⁺,K⁺-ATPase system with concomitant ACE inhibition or AT₁ Ang-II receptor blockade with chronic rapid pacing, membrane preparations were made by ultracentrifugation methods described previously (Bristow et al., 1986; Ping and Hammond, 1994; Roth et al., 1993; Spinale et al., 1994). After sarcolemmal membrane preparation, protein content was determined by use of a standardized colorimetric assay (Bio-Rad Protein Assay, Bio-Rad, Richmond, CA). The samples were then quick frozen and stored at 80°C until the time of biochemical assay. Beta adrenergic receptor antagonist binding studies were performed in the presence of six concentrations of [¹²⁵I]cyanopindolol (ICYP;74 Bq/mmol, Amersham Corp., Arlington Heights, IL) from 0.015 to 0.75 nM (Bristow et al., 1986; Spinale et al., 1994). A standard Scatchard linear regression analysis was performed on the amount of bound/free ligand with an r² > 0.90 as the criterion for acceptability of the data. With this analysis, the maximal number of binding sites B_max expressed as femtomoles per milligram of protein, and the equilibrium dissociation constant K_D (pM) were computed (Bristow et al., 1986; Roth et al., 1993; Spinale et al., 1994). Adenylate cyclase activity was determined by timed cyclic AMP production in aliquots of 30 to 50 µg/100 µl of membrane preparation by previously described methods (Spinale et al., 1994). Reactions were terminated by placing the tubes in an ice-cold bath followed by centrifugation at 6,500 × g for 5 min. Pellets were resuspended in 0.5 ml buffer (50 mM Tris-HCl, 10 mM MgCl₂, 10 µM EGTA, 10 µM phenylmethylsulfonyl fluoride, 2.8 µM EGTA), boiled for 5 min and then centrifuged at 6,500 × g for 10 min. The supernatant was assayed for cyclic AMP content by a competitive radiolabeled assay (RIA Kit, Advanced Magnetics Inc., Cambridge, MA). Adenylate cyclase activity was determined at baseline as well as in the presence of either 10³ M () isoproterenol or 100 µM forskolin. Results were expressed as picomoles of cyclic AMP produced per milligram of sarcolemmal protein per minute. All measurements were performed in duplicate.

Data analysis. Indices of LV and myocyte function were compared among the treatment groups by analysis of variance. Analysis of the morphological data was performed with the average measurements obtained for each animal, and the groups were compared by analysis of variance. If the analysis of variance revealed significant differences, pairwise tests of individual group means were compared with Bonferroni probabilities (Steel and Torrie, 1980). The critical values obtained from the Bonferonni probabilities were adjusted for the multiple comparisons performed with respect to the LV and myocyte function data. For comparisons in neurohormonal values between groups, the Mann-Whitney rank-sum test was used (Steel and Torrie, 1980). All statistical procedures were performed with the BMDP statistical software package (BMDP Statistical Software Inc., Los Angeles, CA). Results are presented as mean ± S.E.M. Values of P < .05 were considered to be statistically significant.

	Results

Abstract Introduction Methods Results Discussion References

In the present study, six dogs were successfully studied in each of the following treatment groups: (1) 28 days of rapid ventricular pacing and concomitant ACE inhibition, (2) 28 days of rapid pacing with concomitant AT₁ Ang-II receptor blockade, (3) 28 days of rapid pacing with no drug (gelatin capsule only) and (4) sham-operated controls (no pacing or drug administration). Myocytes were successfully harvested from all animals at terminal study with no differences in the yield of viable myocytes among groups (P > .75).

LV function with chronic rapid pacing; effects of ACE inhibition or AT₁ Ang-II blockade. The weekly changes in LV end-diastolic volume, ejection fraction and peak wall stress with chronic rapid pacing are shown in figure 1. LV end-diastolic volume significantly increased in a time-dependent fashion with each week of rapid pacing when compared with sham controls or base-line values (P < .05). In the rapid pacing only group, LV ejection fraction significantly decreased from baseline values after 1 week of pacing (P < .05) and continued to decline with each week of pacing. After 2 weeks of rapid pacing, LV end-diastolic volume had increased from baseline values in the ACE inhibition and AT₁ Ang-II receptor blockade group. However, with concomitant ACE inhibition and rapid pacing, LV end-diastolic volume was significantly lower than with rapid pacing alone values for the entire pacing protocol (P < .05). After 1 week of pacing, LV ejection fraction was significantly lower in the ACE inhibition and AT₁ Ang-II receptor blockade groups than in baseline value or sham control groups (P < .05) and declined with each week of pacing. After 4 weeks of rapid pacing, the LV ejection fraction was higher in the ACE inhibition group than in the rapid pacing alone group (P = .038). LV peak wall stress significantly increased with each week in the rapid pacing only group when compared with the sham control group or base-line values (P < .05). With concomitant ACE inhibition and rapid pacing, LV wall stress was not significantly different from base-line or sham control values after 1 week of pacing (P = .417). In the ACE inhibitor and rapid pacing group, LV peak wall stress remained significantly lower than in the rapid pacing only group for the entire pacing protocol (P < .05). A summary of LV function and hemodynamics obtained in sham controls, after 28 days of rapid pacing and 28 days of rapid pacing with concomitant ACE inhibition or AT₁ Ang-II receptor blockade is presented in table 1. Resting heart rate was increased and mean arterial pressure was reduced in the rapid pacing only group when compared with sham controls. Concomitant ACE inhibition or AT₁ Ang-II receptor blockade and rapid pacing resulted in a significant reduction in mean arterial pressure when compared with sham controls, but was not significantly different from the rapid pacing only group (P = .261). After 4 weeks of rapid pacing, LV peak systolic pressure and peak +dP/dt were significantly lower with rapid pacing than with the control group, irrespective of drug treatment.

View larger version (20K): [in this window] [in a new window]   Fig. 1.   Changes in LV end-diastolic volume (EDV), ejection fraction (EF) and peak wall stress plotted with respect to week 0 (baseline) values in controls, with chronic rapid pacing, with chronic pacing and concomitant ACE inhibition and with chronic rapid pacing and concomitant AT1 Ang-II receptor blockade (AII blockade). (top panel) The LV end-diastolic volume significantly increased with each week of rapid pacing and appeared to plateau by week 4. LV end-diastolic volume was significantly lower in the concomitant ACE inhibition group at each week of pacing when compared with the rapid pacing only group (P < .05). LV end-diastolic volume was lower with AT1 Ang-II receptor blockade at weeks 1 and 2 when compared with rapid pacing only values (P = .05). However by weeks 3 and 4, LV end-diastolic volumes were similar in the concomitant AT1 Ang-II receptor blockade group and the rapid pacing only group (P > .30). (middle panel) The LV ejection fraction decreased in a time-dependent manner with each week of pacing irrespective of drug treatment (P < .05). However, at weeks 3 and 4, a higher LV ejection fraction was observed in the rapid pacing and ACE inhibition group when compared with rapid pacing alone values (P < .05). There was no significant difference in the LV ejection fraction with concomitant AT1 Ang-II receptor blockade when compared with the rapid pacing only values at any time point (P > .50). (bottom panel) In untreated dogs, LV wall stress increased significantly with each week of pacing (P < .05). In contrast, a significant reduction in LV wall stress was observed with either concomitant ACE inhibition or AT1 Ang-II receptor blockade at weeks 2 to 4 of chronic rapid pacing (P < .05). See table 1 for week 4 summary results.

Neurohormonal changes with rapid pacing and ACE inhibition or AT₁ Ang-II blockade. A summary of weekly changes in plasma norepinephrine, ANF and cyclic GMP are presented in figure 2. Plasma norepinephrine significantly increased from baseline values after 1 week in all of the dogs undergoing chronic rapid pacing when compared with sham controls or baseline values (P < .05). However, these 1-week plasma norepinephrine values were lower in both the ACE inhibition and AT₁ Ang-II receptor blockade groups than in the rapid pacing only group (P < .05). With longer durations of pacing, plasma norepinephrine values appeared to plateau, but remained significantly elevated from baseline values. In the ACE inhibition and AT₁ Ang-II receptor blockade groups, plasma norepinephrine remained higher than base-line values throughout the pacing protocol, but were consistently lower than rapid pacing only values (P < .05). After 1 week of rapid pacing, plasma ANF and cyclic GMP concentrations significantly increased from baseline values and remained elevated throughout the pacing protocol. With rapid pacing and concomitant ACE inhibition or AT₁ Ang-II receptor blockade, plasma ANF values were variable during the pacing protocol. Plasma ANF increased from baseline values with both ACE inhibition or AT₁ Ang-II receptor blockade after 1 and 2 weeks of pacing, but remained significantly lower than with rapid pacing only values (P < .05). With either concomitant ACE inhibition or AT₁ Ang-II receptor blockade and chronic rapid pacing, cyclic GMP was not significantly increased from baseline values. In the rapid pacing only group, plasma renin activity remained unchanged from sham control values after 28 days of rapid pacing (3.2 ± 0.9 vs. 3.1 ± 0.9 pmol/ml/h, respectively). With rapid pacing and concomitant ACE inhibition or AT₁ Ang-II receptor blockade, plasma renin activity was higher than with the sham control and rapid pacing only groups (6.9 ± 1.6 and 7.27 ± 1.9 pmol/ml/h, respectively, P < .05).

View larger version (22K): [in this window] [in a new window]   Fig. 2.   Serial changes in plasma norepinephrine, ANF and cyclic GMP in controls, with chronic rapid pacing, with chronic pacing and concomitant ACE inhibition and chronic rapid pacing with concomitant AT1 Ang-II receptor blockade. (top panel) Plasma norepinephrine significantly increased from baseline values in the rapid pacing only group (P < .05) and appeared to plateau with longer durations of pacing. Plasma norepinephrine concentrations were significantly lower with ACE inhibition or with AT1 Ang-II receptor blockade when compared with rapid pacing only values (P < .05). (middle panel) Plasma levels of ANF were significantly increased after 1 week of rapid pacing (P < .05) and remained elevated for the entire 4-week pacing protocol. With concomitant ACE inhibition or AT1 Ang-II receptor blockade, plasma ANF values were somewhat variable during the pacing protocol. ANF was increased from baseline values after 2 and 4 weeks of pacing in both the ACE inhibition and AT1 Ang-II receptor blockade groups (P < .05). After 4 weeks of pacing, plasma ANF were lower in both drug treatment groups than with pacing alone values (P < .05) (bottom panel) Plasma cyclic GMP levels increased after 1 week in the rapid pacing only group (P < .05) and remained elevated for the remainder of the rapid pacing protocol. In contrast, there was no significant increase in plasma cyclic GMP levels with either concomitant ACE inhibition or AT1 Ang-II receptor blockade during the pacing period (P > .50).

LV myocardial structure with rapid pacing and ACE inhibition or AT₁ Ang-II receptor blockade. LV mass obtained at autopsy for the four groups of dogs is summarized in table 1. There was no significant change in LV mass in the chronic rapid pacing group when compared with the sham control group. The LV mass/body weight ratios obtained in the present study for the control and rapid pacing groups were all within normal limits for dogs of this size and were not significantly different between groups (Bienvenu and Drolet, 1991). Absolute LV mass was lower in the group with rapid pacing and concomitant ACE inhibition than in the sham control group (P = .025). When LV mass was normalized to tibial length, LV mass/tibial length was lower in the rapid pacing and ACE inhibitor group than in the sham control group (P = .041). In the rapid pacing and concomitant AT₁ Ang-II receptor blockade group, LV mass did not change from control or rapid pacing only values (P > .50). LV myocardial structure and composition was examined by morphometric analysis of perfusion fixed myocardial sections. Myocyte cross-sectional area was computed from a minimum of 300 myocyte profiles from each group. The frequency distribution for this parameter is shown in figure 3. Myocyte cross-sectional area was 297 ± 6 µm² in the sham control group and decreased to 249 ± 5 µm² with chronic rapid pacing (P < .05). With concomitant ACE inhibition and rapid pacing, myocyte cross-sectional area was decreased from both sham control and rapid pacing only values (207 ± 4 µm², P < .05). Concomitant AT₁ Ang-II receptor blockade caused changes in myocyte cross-sectional area similar to rapid pacing only values (256 ± 5 µm², P = .37).

View larger version (40K): [in this window] [in a new window]   Fig. 3.   Frequency distribution of the myocyte cross-sectional area from perfusion-fixed LV myocardial sections taken from sham controls after 28 days of rapid ventricular pacing, rapid pacing and concomitant ACE inhibition and rapid pacing with concomitant AT1 Ang-II receptor blockade. Myocyte cross-sectional area values were fitted to a Gaussian distribution (solid lines). Chronic rapid pacing resulted in a significant decline in the myocyte cross-sectional area compared with controls (P < .05). A further decline from rapid pacing alone values was observed with concomitant ACE inhibition and rapid pacing (P < .05). There was no significant difference in the myocyte cross-sectional area between the rapid pacing only group and the concomitant AT1 Ang-II receptor blockade group (P > .65). Summary statistics for this index of myocyte geometry are presented under "Results."

Beta adrenergic receptor system: effects of ACE inhibition or AT₁ Ang-II receptor blockade. Beta receptor density decreased significantly with chronic rapid pacing with no change in affinity (table 2). In contrast, beta adrenergic receptor density remained unchanged with chronic rapid pacing and concomitant ACE inhibition or AT₁ Ang-II receptor blockade. Although beta receptor affinity remained unchanged in the ACE inhibition group, beta receptor affinity was significantly increased in the AT₁ Ang-II receptor blockade group. In the control group, cyclic AMP production significantly increased from basal levels after beta receptor stimulation and with direct adenylate cyclase activation with forskolin. Basal cyclic AMP production was reduced in the chronic rapid pacing only group when compared with controls. In addition, cyclic AMP production was reduced by approximately 50% in the rapid pacing group either after beta receptor stimulation or by adenylate cyclase activation. Concomitant ACE inhibition during rapid pacing resulted in a normalization of cyclic AMP production both after beta receptor stimulation and by direct activation of adenylate cyclase. In contrast, cyclic AMP production remained significantly reduced in the AT₁ Ang-II receptor blockade group.

Myocyte contractile function and chronic pacing: ACE inhibition or AT₁ Ang-II receptor blockade. A summary of isolated myocyte resting length and baseline contractile function is presented in table 3. Representative contraction profiles of isolated myocytes taken from sham controls, with chronic rapid pacing, and chronic rapid pacing with concomitant ACE inhibition or AT₁ Ang-II receptor blockade are shown in figure 4. Isolated myocyte resting length significantly increased from control values in all three groups of dogs with rapid pacing. Isolated myocyte length was lower in the groups with concomitant ACE inhibition or AT₁ Ang-II receptor blockade than in rapid pacing alone values. Myocyte percent and velocity of shortening significantly decreased from control values in all of the rapid pacing groups. However, in the rapid pacing and concomitant ACE inhibition group, myocyte percent and velocity of shortening were higher than in the rapid pacing only group or the group with rapid pacing and AT₁ Ang-II receptor blockade. The velocity of myocyte lengthening was lower in all three groups of dogs with chronic rapid pacing. In the rapid pacing and concomitant ACE inhibition group, the velocity of myocyte lengthening was significantly higher than in the rapid pacing group or the rapid pacing group with concomitant AT₁ Ang-II receptor blockade. The time to peak myocyte contraction and total duration of contraction were prolonged in the rapid pacing group without drug treatment. The time to peak myocyte contraction was more prolonged in all rapid pacing groups, irrespective of drug treatment. In the rapid pacing and AT₁ Ang-II blockade group, the total duration of contraction was similar to control values. Myocyte contractile function was examined in the presence of the beta adrenergic agonist, isoproterenol (table 3). Isolated myocyte contractile function in the presence of isoproterenol was significantly lower in all three rapid pacing groups. However, beta adrenergic responsiveness was significantly greater in the group with rapid pacing and ACE inhibition than in the group with rapid pacing alone or with concomitant AT₁ Ang-II receptor blockade. In fact, myocyte function after beta adrenergic stimulation in the AT₁ Ang-II receptor blockade and rapid pacing group was lower than the rapid pacing alone values.

View larger version (13K): [in this window] [in a new window]   Fig. 4.   Representative isolated myocyte contraction profiles taken from sham controls after 28 days of rapid ventricular pacing, rapid pacing with concomitant ACE inhibition and rapid pacing with AT1 Ang-II receptor blockade. Isolated myocyte length significantly increased with chronic rapid pacing. With concomitant ACE inhibition or AT1 Ang-II receptor blockade, myocyte length was reduced from rapid pacing alone values. Statistics for indices of myocyte contractile performance obtained at baseline and after beta adrenergic receptor stimulation are summarized in table 3.

	Discussion

Abstract Introduction Methods Results Discussion References

ACE inhibition has provided beneficial effects on symptoms and survival in patients with LV dysfunction (Spinale et al., 1995; The CONCENSUS Study Group, 1987; The SOLVD Investigators, 1991). However, the cellular and molecular events that occur within the LV myocardium with chronic ACE inhibition and LV dysfunction are not fully understood. ACE is an important determinant of Ang-II production both systemically and at the myocardial level (Antonaccio and Wright, 1990; Baker et al., 1992; Dzau, 1988; Gavras, 1994; Lindpaintner and Ganten, 1991). Although the distribution, types and function of Ang-II receptors is an area of active research, AT₁ Ang-II has been studied the most intensively and appears to mediate numerous physiological responses (Dudley et al., 1990; Lopez et al., 1994; Sechi et al., 1992; Urata et al., 1989). In addition, the AT₁ Ang-II receptor has been prevalent within the myocardium (Dudley et al., 1990; Lopez et al., 1994; Sechi et al., 1992; Urata et al., 1989). However, it remains unclear whether the effects of ACE inhibition in the setting of LV dysfunction are caused primarily by diminished activation of the AT₁ Ang-II receptor or by alternative mechanisms. To address these issues, the present study quantified changes in myocyte function and the beta adrenergic system after either ACE inhibition or AT₁ Ang-II receptor blockade during the development of LV dysfunction caused by chronic rapid pacing. The important findings from the present study were 2-fold. First, ACE inhibition reduced the degree of LV dilation associated with chronic rapid pacing, improved myocyte function and normalized beta receptor density and cyclic AMP production. Second, AT₁ Ang-II receptor blockade did not prevent the development of LV dilation and dysfunction which invariably occurs with chronic rapid pacing. Moreover, concomitant AT₁ Ang-II receptor blockade did not result in significant improvement in myocyte contractile function or beta adrenergic responsiveness. Therefore, the results from this study demonstrated that contributory mechanisms for the beneficial effects of ACE inhibition in a model of LV dysfunction with respect to myocyte contractile processes are not mediated solely through the AT₁ Ang-II receptor subtype.

ANF is a peptide hormone of cardiac origin, and ANF receptor activation results in the generation of cyclic GMP (Margulies et al., 1991). Consistent with past reports (Margulies et al., 1991; Travill et al., 1992), chronic rapid pacing caused an early and persistent elevation in plasma levels of ANF and cyclic GMP. Concomitant ACE inhibition or AT₁ Ang-II receptor blockade caused a significant reduction in plasma ANF or cyclic GMP when compared with untreated dogs by chronic rapid pacing. Although beyond the scope of the present study, potential mechanisms for this reduction in ANF and cyclic GMP levels with chronic ACE inhibition or AT₁ Ang-II receptor blockade include diminished local ANF production caused by modulation of local neuroendocrine function, and enhanced ANF degradation. The development and progression of LV dysfunction is associated with increased plasma catecholamine levels (Armstrong et al., 1986; Benedict et al., 1993; Bristow et al., 1986; Eble and Spinale, 1994; Eschenhagen et al., 1992; Margulies et al., 1991; McDonald et al., 1994; Roth et al., 1993; Sabbah et al., 1994; Spinale et al., 1994; Travill et al., 1992). In the present study, both concomitant ACE inhibition or AT₁ Ang-II receptor blockade caused an equivalent and significant reduction in circulating plasma norepinephrine when compared with chronic rapid pacing only values. Consistent with this observation, Sabbah and colleagues (1994) demonstrated that chronic ACE inhibition attenuated the increase in plasma norepinephrine which occurred in the setting of progressive LV dysfunction caused by coronary embolization. It has been demonstrated previously that a chronic elevation in circulating catecholamines and persistent activation of the beta receptor system causes a reduction in beta receptor density (Bristow et al., 1986). Thus in the present study, the normalization of beta receptor density which was observed with either concomitant ACE inhibition or AT₁ Ang-II receptor blockade and rapid pacing was probably caused, at least in part, by the reduction in plasma norepinephrine levels.

In the present study, chronic rapid pacing in dogs increased myocyte resting length and reduced myocyte cross-sectional area. Concomitant ACE inhibition with chronic rapid pacing was associated with a reduction in myocyte length from rapid pacing only values and a further reduction in cross-sectional area. Although concomitant AT₁ Ang-II receptor blockade reduced myocyte length from rapid pacing only values, there was no significant change in myocyte cross-sectional area. Thus, a contributory mechanism for the reduction in LV end-diastolic volume with ACE inhibition and chronic rapid pacing was the direct and selective effects on myocyte geometry. This laboratory has demonstrated previously that the development of LV dysfunction caused by chronic rapid pacing is associated with a significant reduction in the contractile performance of isolated myocytes (Spinale et al., 1992b). Consistent with a recent report (Spinale et al., 1995), chronic ACE inhibition increased indices of myocyte contractile performance by more than 30% from rapid pacing only values. Thus, in addition to the changes in myocyte geometry, concomitant ACE inhibition with chronic rapid pacing caused a significant improvement in contractile function. To quantitate more carefully the ability of the isolated myocyte to respond to an inotropic stimulus, the present study examined myocyte function in the presence of the beta adrenergic receptor agonist isoproterenol. Consistent with past reports (Spinale et al., 1994), myocyte beta adrenergic responsiveness was significantly reduced with the development of pacing-induced LV dysfunction. Contributory mechanisms for the blunted myocyte beta adrenergic response included a reduction in beta adrenergic receptor density and diminished cyclic AMP production. Concomitant ACE inhibition with chronic pacing improved myocyte beta adrenergic responsiveness. Results from the present study suggest that contributory mechanisms for the improved myocyte beta adrenergic response with ACE inhibition included a normalization of beta adrenergic receptor density and cyclic AMP production. Maisel et al. (1989) demonstrated that the ACE inhibitor captopril normalized beta receptor density and transduction in the setting of cardiac hypertrophy induced by chronic isoproterenol administration. Taken together, the results from these past reports and the present study suggest that a contributory mechanism for the beneficial effects of ACE inhibition in the setting of progressive LV dysfunction is the modulation of beta adrenergic receptor density and transduction. Although concomitant ACE inhibition with chronic rapid pacing prevented the reduction in beta receptor density and cyclic AMP production, myocyte response to beta adrenergic stimulation remained lower than normal myocytes. The persistent defect in myocyte beta adrenergic response with ACE inhibition is probably caused by several factors. First, alterations in the content and activity of the guanine nucleotide-binding regulatory protein complex (G-protein complex) associated with the beta adrenergic receptor transduction system have occurred with the development of LV dysfunction caused by chronic rapid pacing (Ping and Hammond, 1994; Roth et al., 1993; Spinale et al., 1994). Second, findings from the present study as well as past reports have demonstrated that pacing-induced LV dysfunction is associated with abnormalities in Na⁺,K⁺-ATPase density and function (Kim et al., 1994; Spinale et al., 1992a). The present study demonstrated that concomitant ACE inhibition with chronic rapid pacing did not completely prevent these abnormalities in the Na⁺,K⁺-ATPase system. Finally, down-regulation of Ca⁺⁺ transport systems within the sarcoplasmic reticulum and alterations in Ca⁺⁺ homeostasis have occurred with the development of LV dysfunction caused by chronic rapid pacing (Cory et al., 1993; Perreault et al., 1992). Thus, potential mechanisms for the failure of ACE inhibition and chronic rapid pacing to normalize myocyte function and beta adrenergic responsiveness include persistent defects in sarcolemmal function and alterations in Ca⁺⁺ homeostasis. Based on the findings of the present study, future studies which more carefully examine myocyte Ca⁺⁺ homeostasis after chronic ACE inhibition and chronic rapid pacing would help clarify this issue. In the present study, concomitant AT₁ Ang-II receptor blockade prevented the reduction in beta adrenergic receptor density but failed to normalize cyclic AMP production. With concomitant AT₁ Ang-II receptor blockade and chronic rapid pacing, cyclic AMP production could not be returned to normal levels either by beta adrenergic receptor stimulation or by direct activation of adenylate cyclase. These persistent defects in cyclic AMP production with AT₁ Ang-II receptor blockade were associated with a reduction in myocyte beta adrenergic responsiveness from both normal and chronic rapid pacing values. Activation of the AT₁ Ang-II receptor caused a reduction in cyclic AMP production and activation of G-proteins independent of the beta adrenergic receptor system (Allen et al., 1988; Antonaccio and Wright, 1990; Baker and Singer, 1988; Baker et al., 1992). Characterization of AT₁ Ang-II receptor activity and G-protein structure and function with chronic rapid pacing and either ACE inhibition or AT₁ Ang-II receptor blockade were beyond the scope of the present study. However, the important and unique findings from this portion of the study are 2-fold. First, these results suggest that the protective effects of chronic ACE inhibition in this model of LV dysfunction with respect to beta adrenergic contractile responsiveness and transduction are not solely caused by modulation of AT₁ Ang-II receptor activation. Second, as opposed to ACE inhibition, chronic AT₁ Ang-II receptor blockade with pacing-induced LV dysfunction appeared to have differential effects on beta adrenergic receptor transduction.

A unique finding of the present study was that chronic ACE inhibition or specific AT₁ Ang-II receptor blockade administered at equivalent and subhypotensive doses did not provide equivalent effects on LV and myocyte geometry and function in a model of developing LV dysfunction. Thus, the beneficial effects of concomitant ACE inhibition with chronic rapid pacing were probably at least partly the result of alternative receptor pathways and enzymatic processes other than that of preventing myocardial Ang-II formation. It has been well established that ACE inhibitors have inhibitory effects on other enzyme systems such as bradykinin production, neurotensin, and substance P (Antonaccio and Wright, 1990; Gavras, 1994; Levens et al., 1992). Thus, the beneficial effects of ACE inhibition on LV and myocyte function observed in the present study may be caused by the modulation of these active peptide systems. There is significant evidence to suggest that kallikrein-kinin proteolytic cascade systems exist within the myocardium (Ehring et al., 1994; Gavras, 1994; Gohlke et al., 1994; Nolly et al., 1994; Weber et al., 1994). Bradykinin, a nonapeptide which is produced by the kallikrein cascade, has been implicated to play a direct role in myocardial remodeling and functional recovery from myocardial ischemia (Ehring et al., 1994; Weber et al., 1994). Moreover, ACE inhibition seems to prevent the rapid degradation of bradykinin and thereby potentiates the beneficial effects of this peptide in the environment of myocardial ischemia (Ehring et al., 1994). McDonald and colleagues (1995) demonstrated that in a canine model of myocardial injury the beneficial effects of ACE inhibition could be attenuated by the administration of a bradykinin antagonist. Thus, a contributory mechanism for the beneficial effects of concomitant ACE inhibition which were observed in the present study may be caused by enhanced bradykinin levels within the myocardium. Future studies which use specific bradykinin and AT₁ Ang-II receptor antagonists in this model of chronic LV dysfunction will be necessary to elucidate the interdependence and functional significance of the myocardial Ang-II and bradykinin forming pathways with the progression of LV failure.

A limitation of the present study is that in this model of chronic rapid pacing, a significant increase in plasma renin activity did not occur in the untreated dogs. This is in contrast to previous reports (Eble and Spinale, 1995; Margulies et al., 1991; Travill et al., 1992), and suggests that significant activation of systemic neurohormonal systems such as the renin-angiotensin system had not occurred. Thus, the potential differential effects between ACE inhibition and AT₁ Ang-II receptor blockade in which the progression of LV dysfunction was accompanied by activation of the endocrine-humoral renin angiotensin system could not be determined. Future studies would be appropriate in which the direct effects of ACE inhibition or AT₁ Ang-II receptor blockade are instituted in this model of LV dysfunction and in which more severe hemodynamic compromise with subsequent activation of the systemic renin-angiotensin system occurs. The present study used an identical dosage protocol for ACE inhibition and AT₁ Ang-II receptor blockade. This dose of AT₁ Ang-II receptor antagonist was chosen because inhibition of the Ang-II pressor response was achieved without secondary systemic hemodynamic effects. This dosage strategy was chosen to directly compare potential direct and differential effects of chronic ACE inhibition and AT₁ Ang-II receptor blockade on myocyte contractile processes. It has been recently reported that approximately a 10- fold higher dose of AT₁ Ang-II receptor antagonist is required to achieve an equivalent blood pressure reduction when compared with ACE inhibition (van den Meiracker et al., 1995). Thus, it must be recognized that a much higher dose of AT₁ Ang-II receptor blockade may be necessary to potentially provide additional beneficial effects in the setting of LV dysfunction. Mean arterial pressure in dogs with either ACE inhibition or AT₁ Ang-II receptor blockade was similar to untreated dogs undergoing chronic rapid pacing. Thus, the direct and differential effects of either concomitant ACE inhibition or AT₁ Ang-II receptor blockade with chronic rapid pacing on LV geometry and myocyte contractile processes were probably not caused by differences in systemic loading conditions. However, although not statistically significant, mean arterial pressure was lower in the ACE inhibition group than in the AT₁ Ang-II receptor blockade or rapid pacing group. Thus, the possibility remains that more favorable LV loading conditions were achieved in the ACE inhibition group than in the AT₁ Ang-II receptor antagonist group, which would in turn influence overall LV pump function. In the present study, plasma renin activity was not significantly increased in the dogs undergoing chronic rapid pacing. This was probably because of the duration of rapid pacing and the degree of LV dysfunction which had been induced in this model (Travill et al., 1992). These observations suggest that the differences in LV and myocyte geometry and function and the beta adrenergic receptor system, which were observed with ACE inhibition or AT₁ Ang-II receptor blockade with pacing-induced LV dysfunction, were caused by local myocardial effects. Finally, the present study examined LV and myocyte structure and function with chronic rapid pacing at only one point in time. Thus, serial changes in LV myocardial and myocyte structure and the effects of ACE inhibition or AT₁ Ang-II receptor blockade in this model of LV dysfunction were not addressed. Nevertheless, the present study demonstrated that concomitant ACE inhibition in a model of chronic rapid pacing-induced LV dysfunction improved LV and myocyte geometry and function, and normalized beta receptor density and cyclic AMP production. Concomitant AT₁ Ang-II receptor blockade with chronic rapid pacing did not provide similar effects on LV and myocyte geometry and function. Therefore, the findings from the present study suggest that the beneficial effects of ACE inhibition on LV geometry and myocyte contractile function are not primarily caused by modulation of AT₁ Ang-II receptor activation but rather by alternative mechanisms.