This study describes the coronary angiographic imaging of young and aged rabbits. We developed and improved a novel minimally invasive approach for coil embolization that targets a specific area of myocardium and yielded a consistent scar encompassing ~30% of the left ventricular free wall of young and aged rabbit hearts.
Keywords: myocardial infarction, ventricular fibrillation, arrhythmia, aging, coronary angiography, coronary anatomy
Abstract
The incidence of both myocardial infarction (MI) and sudden cardiac death increases with age. Here, we describe the development of a minimally invasive large animal model of MI that can be applied to young or aged animals. We demonstrate that rabbit coronary anatomy is highly variable, more so than described in previous literature. In this work, we categorize the coronary pattern of 37 young rabbits and 64 aged rabbits. Aged rabbits had a higher degree of branching from the left main coronary artery. Standardizing the model across age cohorts required a new approach, targeting an area of myocardium rather than a specific vessel. Here, we present a method for achieving a reproducible infarct size, one that yielded a consistent scar encompassing ~30% of the apical left ventricular free wall. The model’s consistency allowed for more valid comparisons of MI sequelae between age cohorts.
NEW & NOTEWORTHY This study describes the coronary angiographic imaging of young and aged rabbits. We developed and improved a novel minimally invasive approach for coil embolization that targets a specific area of myocardium and yielded a consistent scar encompassing ~30% of the left ventricular free wall of young and aged rabbit hearts.
sudden cardiac death (SCD) is a major public health issue responsible for approximately one in five deaths in industrialized countries (4, 22). The risk of SCD increases 10-fold for people over the age of 65 yr (4, 22). Nearly half of SCD victims show healed myocardial infarctions (MI) at autopsy (1). The mechanisms that make MI more deadly in the aging heart are largely unknown (23). To probe these mechanisms, age-appropriate models of cardiac disease are necessary. One such model is the aged rabbit.
The rabbit has similar action potential morphology and cardiomyocyte Ca2+ homeostasis as humans (17, 19). Accordingly, the rabbit heart has long been used as a model for studying myocardial ischemia and infarction (6, 11, 14, 18, 21). In all models of cardiac disease, understanding coronary anatomy is crucial to standardize surgical techniques and increase animal-to-animal reproducibility. The rabbit heart, like other mammalian hearts, displays a high degree of coronary anatomy variability (15).
Current infarction models rely on ligation or occlusion of the same artery in each surgical case, regardless of coronary anatomy (5, 7, 12, 24). In rabbits, the most commonly ligated vessels are the left anterior descending (LAD) and left circumflex (LCX) (13, 14, 16). Experience with coronary angiography reveals that these vessels vary greatly in length and diameter between hearts. As the literature suggests, targeting one specific vessel, whether it be LAD or LCX, leads to highly unpredictable infarctions (8). We have identified no ischemia or infarct studies to date that have accounted for the complexity of coronary anatomy, and no surgical solution has directly addressed this problem.
The literature on rabbit coronary anatomy suggests a simple dichotomy: half of all hearts exhibit bifurcation and half exhibit trifurcation of the left main (LM) coronary artery (3, 15). This is an oversimplification that fails to account for other common variants. We have observed right coronary dominance in both young and aged rabbits as well as quadrifurcation of the LM in aged rabbits. Thus, coronary variability makes the one-size-fits-all infarct methodology susceptible to unpredictable outcomes (8).
Using our knowledge of rabbit coronary anatomy and optimizing our infarction procedure accordingly, we developed a novel methodology to target an area of myocardium in each rabbit as opposed to a specific artery. This technique standardizes the size and location of the infarction between rabbits. This level of model consistency allows for direct comparisons between study groups, without adjustment for infarct size, across a wide array of assays at the organ, tissue, and cellular levels. The most reproducible area to target in the rabbit heart is the apical, anterolateral left ventricular (LV) wall. This area is almost always served by a single vessel with relatively little anastomosis. We provide a rubric for creating a reproducible infarct of 30% of the LV free wall.
METHODS
This investigation conformed with the current Guide for Care and Use of Laboratory Animals published by the National Institutes of Health (NIH Publication No. 85-23, Revised 1996) as well as the standards recently delineated by the American Physiological Society (“Guiding Principles for Research Involving Animals and Human Beings”) and was approved by the Institutional Animal Care and Use Committee of Rhode Island Hospital. Adult female rabbits (3.5–5.5 kg) aged 6–12 mo were used for the young group and ages of 4.0–5.5 yr were used for the aged group (20). The prophylactic drug regimen included 3 days of oral amiodarone, 400 mg given twice daily 6−8 h apart before and 400 mg the day after the experimental myocardial infarction (MI). Rabbits were anesthetized with intramuscular ketamine-xylazine (25 and 3.75 mg/kg, respectively) and buprenorphine (0.03 mg/kg), intubated, and ventilated with isoflurane (1–2%, fraction of inspired O2: 0.5). Blood pressure and peripheral O2 saturation were measured continuously. A continuous 12-lead ECG was recorded during the induction of MI. Procedurally, a bolus of intravenous amiodarone (5 mg/kg) was given during a 20-min period before and again 40 min after coronary coil placement, with a continuous infusion (12.5 µg·kg−1·min−1) between the boluses. Additionally, premature ventricular contractions were suppressed with intravenous lidocaine using a bolus of 1 mg/kg and a 40 µg·kg−1·min−1 infusion.
Echocardiography.
Transthoracic echocardiography was performed in sedated animals at baseline (pre-MI) and 3 wk post-MI. Two-dimensional echo images (Hewlett Packard 5500) were obtained with a 7.5-mHz probe, and long-axis and short-axis views were performed mimicking those used in human echocardiography. Analysis included dimensions of the LV, fractional shortening, and LV ejection fraction (calculated by simple Quinone’s method) and was performed by an experienced echocardiographer. Functional outcomes were compared between bifurcated and trifurcated cohorts.
Coronary angiography.
Coronary angiography was performed by injection of contrast (3.5 mg/ml, OmniPaque, GE Healthcare) through a 4-Fr guide catheter proximal to the ostia of the LM and right main coronary arteries. Direct insertion into the main coronaries is not possible as a 4-Fr catheter is needed for optimal contrast levels, and the ostia are smaller in diameter than the catheter. Cine acquisitions were taken from the left anterior oblique (+35°), anterior posterior (0°), right anterior oblique (−35°), and right anterior oblique caudal view (−35° and 30° tilt). Angiograms were reviewed with Encompass software (Heartlab, Agfa HealthCare, Westerly, RI) to determine the configuration of the coronary anatomy (Table 1).
Table 1.
Left coronary anatomy
| Branching of the Coil Artery | Young | Aged* | Total | Total | |
|---|---|---|---|---|---|
| Bifurcation | Distal | 23 | 23 | 46 | 64 |
| Proximal | 7 | 11 | 18 | ||
| Trifurcation | Distal | 6 | 16 | 22 | 34 |
| Proximal | 1 | 11 | 12 | ||
| Quadrifurcation | Distal | 0 | 3 | 3 | 3 |
| Total | 37 | 64 | 101 | 101 |
Overview of left coronary anatomy patterns in young and aged rabbit hearts. Since arterial branching affects coil placement, we focused on the branching of the artery perfusing the anterior, apical left ventricle or what we refer to as the “coil artery.” One hundred and one rabbits were studied and characterized by coronary angiography. Aged rabbit hearts displayed a higher degree of left main coronary branching, as assessed by 2 × 3 Fisher’s exact test (P < 0.05).
Two-tailed, 2 × 3 Fisher’s exact contingency table.
MI and coil placement.
Our minimally invasive endovascular site-specific infarct methodology has been previously described (24). In brief, a neck incision was performed to isolate the right carotid, and a 4-Fr guide catheter was advanced to the ostia of the desired coronary artery. The coronary anatomy was defined by angiography, and coil location was decided on. Coil placement was most often performed in the right anterior oblique view for optimal visualization of the LV free wall. A 0.014-in. guidewire was advanced into the selected coronary to the coil deployment site, and a 2.5-fr microcatheter was advanced over the wire. The guidewire was removed, and a 0.018-in. platinum coil, cut to a length of 2 mm, was advanced through the microcatheter. A standardized algorithm was used to determine the location of coil placement (Table 2).
Table 2.
Coil placement rubric
| Anatomy | Major Branching | Coil Location |
|---|---|---|
| Bifurcation | Distal | Proximal to branching |
| Proximal | Anterior branch | |
| Trifurcation | Distal | Proximal to branching |
| Proximal | Anterior branch | |
| Quadrifurcation | Proximal ramus 1 |
Shown is a coil placement rubric to standardize infarct size and location across all coronary branching patterns. With distal branching of the first obtuse marginal artery (OM1) or ramus, a coil is placed proximal to the branch point. In the instance of proximal branching of the OM1 or ramus, a coil is directed into the anterior branch. To account for quadrifurcation, a coil is placed in the proximal ramus 1, ideally at the middle ventricle.
Histochemistry.
LV free wall tissues from young and aged rabbits 3 wk post-MI were embedded in OTC tissue freezing medium and cryosectioned at 10-μm intervals. To assess scar size, hematoxylin and eosin, Masson’s trichrome, and sirius red staining were performed. Histopathology images were collected using a Nikon TE2000 microscope with Retiga EXi camera. A previously published ImageJ macro was used to analyze the blue fibrosis area in the uncompressed image files acquired (9).
RESULTS
We have created an angiographic atlas that includes coronary artery anatomy from 37 young (<1 yr) and 64 aged (>4 yr) New Zealand White female rabbits. The average age of the young animals was 10 mo and 4.6 yr for the aged animals. The New Zealand White rabbit exhibits a wide range of coronary anatomy but the variability is not sex dependent. Comparison of anatomic configurations between 28 young male and 37 young female rabbits revealed no difference (data not shown). No aged male rabbits were studied.
The LM coronary artery supplies the posterior descending artery in 84% (32/38) of cases in which we have studied both the LM and right main coronary arteries. The LM coronary artery can bifurcate into two, three, or four arteries. We refer to each case, respectively, as bifurcation, trifurcation, or quadrifurcation (Table 1). In bifurcated rabbits, the LM coronary artery divides into the LAD and LCX arteries. In these cases, the first obtuse marginal artery (OM1) is the dominant vessel in the left coronary system, perfusing the entire inferior lateral wall of the LV. In some bifurcated cases, the OM1 also perfuses the septum and anterior right ventricle (RV) (Fig. 1). In trifurcated rabbits, the LM coronary artery divides into the LAD, ramus, and LCX (Fig. 2). The ramus stretches from the LM coronary artery to the apex, directly perfusing the inferior anterolateral LV. The circumflex perfuses the posterior aspect of the LV as well as the RV. Quadrifurcated rabbits present with two ramus arteries, with one perfusing the inferior anterior LV, whereas the other perfuses the posterolateral wall of the LV and the posterior RV (Fig. 3).
Fig. 1.
Bifurcation of the left main coronary anatomy. A: young rabbit displaying proximal origin of the first obtuse marginal artery (OM1) with a minor circumflex artery. The major bifurcation of the OM1 is proximal. The left anterior descending (LAD) is minor. B: aged rabbit displaying proximal origin of the OM1 from the circumflex. The circumflex is well developed with significant branching. The OM1 shows proximal branching with a more substantial anterior branch. The posterolateral and posterior perfusion have many collaterals. The white arrow denotes the coil target location.
Fig. 2.
Trifurcation of the left main coronary anatomy. A: young rabbit heart with distal branching of the ramus and a well-developed circumflex system. The OM1 is relatively minor. The LAD is minor. B: aged rabbit heart with the ramus showing distal branching. The circumflex artery is well developed with a significant OM1. The LAD is minor, and the first septal is pronounced. The white arrow denotes the coil target location.
Fig. 3.
Quadrifurcation of the left main coronary artery in an aged rabbit. The aged quadrifurcated rabbit exhibited a substantial ramus 1 with distal branching and a smaller ramus 2 with proximal branching. The LAD is minor, and the circumflex exhibits distal branching. The LAD is not visible from the right anterior oblique view. The white arrow denotes the coil target location.
In general, the LAD is a relatively minor artery in the rabbit, perfusing the superior anterior aspect of the heart. The LAD does not have prominent diagonal branches and therefore is not a major contributor to the perfusion of the LV wall. The LAD does exhibit a single prominent septal branch (Fig. 2).
The LCX perfuses the posterior LV as well as the posterolateral RV. The LCX artery varies in size and prominence from rabbit to rabbit. In general, if the origin of the OM1 is proximal to the midsegment of the LV, the LCX is prominent and shows two or more branches. If the origin of the OM1 is distal to the midsegment of the LV, the artery is minor and does not have branches.
Thirty young animals exhibited bifurcation of the LM coronary artery, whereas only seven exhibited trifurcation (Table 1). This result was significantly different than previously published results (15). Thirty-four aged animals exhibited bifurcation, whereas 27 showed trifurcation. Additionally, three aged animals exhibited quadrifurcation of the LM coronary artery. The coronary anatomy of the aged rabbit has not been previously described. We found that the anatomy of our aged animals was significantly different than our young animals (two-tailed, 2 × 3 Fisher’s exact contingency table, P < 0.05).
Bifurcation and trifurcation.
Eighty-one percent (30/37) of our young rabbits exhibited bifurcation of the LM coronary artery. The OM1 exhibited proximal branching in 7 (23%) bifurcated young rabbits and distal branching in 23 (77%; Table 2).
Fifty-three percent (34/64) of aged rabbits exhibited bifurcation of the LM coronary artery. The OM1 exhibited proximal branching in 11 (32%) cases and distal branching in 23 (68%) cases.
Trifurcation.
Nineteen percent (7/37) of young rabbits exhibited trifurcation of the LM coronary artery. One (14%) rabbit exhibited proximal branching of the ramus, and six (86%) exhibited distal branching of the ramus (Fig. 3).
Forty-two percent (27/64) of aged rabbits exhibited trifurcation of the LM coronary artery. In these rabbits, the ramus exhibited proximal branching in 11 (41%) cases and distal branching in 16 (59%) cases.
Quadrifurcation in aged rabbits.
Three (5%) of our 64 aged rabbits exhibited quadrifurcation of the LM coronary artery. All of these rabbits exhibited distal branching of the ramus (Fig. 4 and Table 1). These rabbits had a LAD, ramus 1, ramus 2, and LCX. The LAD and LCX were usually normal in these rabbits. Ramus 1 extends to the apex and perfuses the anterior inferior portion of the LV. Ramus 2 is a more lateral artery perfusing the lateral and posterior LV.
Fig. 4.
Instance of right coronary dominance in an aged rabbit. A: the left coronary system of an aged right dominant rabbit. The circumflex is minor, and the OM1 is by far the major artery on the left side. The LAD is paralleled by a proximal branch. B: the right coronary system provides the posterior perfusion with a clearly defined posterior descending artery and a substantial right marginal.
Right coronary artery.
We have studied the right and left coronary systems in 38 rabbits, and the right was relatively diminutive in 32 (84%) of these rabbits. We did find, however, that the right coronary artery (RCA) uniformly stretched to at least the midpoint of the heart. This was contradictory to previous studies (15). The RCA perfused the lateral wall of the RV. Three of 19 young rabbits exhibited right dominance, and 3 of 19 aged rabbits exhibited right dominance. Right dominance has never before been documented in rabbits. Right dominant rabbits have right marginal and posterior descending arteries. The LCX was diminutive in these rabbits and the OM1 was unaffected. In general, the RCA was more developed in the aged rabbit heart.
Third coronary artery.
We found that the majority of rabbits had a third coronary artery, which originated at a minuscule ostium within millimeters of the RCA. This result matches similar studies of the RCA (2).
Standardized infarct.
Standardizing the size and location of the infarct in an experimental model is necessary for comparisons between groups. In the rabbit heart, the supply of blood to the inferior, anterolateral portion of the LV is not redundant. In bifurcated, trifurcated, and quadrifurcated rabbits, there was a single vessel that supplies this region, encompassing 30% of the LV. We targeted this region via an endovascular coil embolization technique in 31 young and 61 aged rabbits using the target locations defined in Table 2 (24). Eighty-one percent (25/31) of young and 72% (44/61) of the aged rabbits survived the procedure [P = not significant (NS)]. The remaining rabbits included in the construction of our coronary atlas were unsuitable for infarction because of the size of coronary artery being small or instance of vasospasm that would not allow the advancement of the catheter system to the target location. For these animals, angiography was collected, and the animal was excluded from the infarction cohort. Figure 5A shows a typical case in a bifurcated heart, and Fig. 5D shows a typical trifurcated heart. We have also included relevant leads and gross histology (Fig. 5, B, E, C, and F, respectively) to illustrate the similarity of MI despite anatomic differences. Using coronary angiography to identify the arterial supply to the anterolateral LV, we were able to achieve consistent size and location of the infarct. Specific target locations are shown in Fig. 6. To quantify the reproducibility of the scar size, we calculated the percentage of the LV free wall infarcted using two methodologies: Fig. 7A quantifies the size of the scar in eight young and nine aged rabbits using photos of the dissected LV free wall and Fig. 7B quantifies the scar size in histological studies done in five young and eight aged rabbits (the rest of the infarcted young and aged hearts were devoted to other ongoing functional and molecular studies). Despite the variations in coronary anatomy, two-tailed tests revealed no difference in infarct size between young and aged infarcted hearts (P = NS) regardless of coronary anatomy (Fig. 7). The presence of scar was confirmed in histological studies performed in serial frozen sections. Figure 7D shows the heart stained with hematoxylin and eosin. Figure 7E shows a serial section stained with Masson’s trichrome fibrosis staining. Finally, Fig. 7F shows a serial section stained with sirius red collagen stain imaged with polarized light to visualize the birefringence of collagen fibers.
Fig. 5.
Location-specific coil embolization yields anatomy-independent scars. A: aged bifurcated rabbit with a very high take off of the OM1 and distal branching of the OM1. AP, anterior posterior view; RAO, right anterior oblique view. We occluded the first obtuse marginal above the branch point. B: the baseline ECG was normal. At 3 wk, the ECG showed pathological Q waves and ST inversion. C: the epicardial surface of the excised heart showing an infarct of 30% of the left ventricle (LV). These findings were confirmed by the endocardial view. D: aged trifurcated rabbit heart with a significant circumflex system and distal branching of the ramus artery. The OM1 is quite large, perfusing the posterior aspect of the heart. This rabbit also had a relative diminutive first septal artery and a large LAD. We occluded the ramus above the distal branch point. E: the baseline ECG was normal. At 3 wk, the ECG showed pathological Q waves and T wave inversion. F: the epicardial surface of the excised heart showing an infarct of 30% of the LV. These findings were confirmed by the endocardial view.
Fig. 6.
Target location of the coil. The target location for each type of anatomy we encountered in the rabbit is shown. A: bifurcation. The coil should be placed down the OM1 distal to the proximal branch point. B: trifurcation. The coil should be placed down the ramus, proximal to the distal branch point. C: quadrifurcation. The coil should be placed down ramus 1, proximal to the branch point. D: right dominant. The coil should be placed down the OM1, similar to the bifurcation case. The gray bar in each diagram notates the coil location, and the gray shading denotes of the region of myocardium to be infarcted.
Fig. 7.
Comparison of scar size across age groups with minimally invasive procedures produces infarcts of the same size. A: infarct size as assessed by epicardial and endocardial area measurements of the scar and total LV free wall in eight young and nine aged infarcted hearts [not significant (NS), P = 0.63]. B: infarct size as assessed by Masson’s trichrome fibrosis staining in histology sections in five young and eight aged infarcted hearts (NS, P = 0.84). C: gross anatomy of the LV free wall confirming the presence of an apical infarct. D: hematoxylin-eosin staining was performed to visualize myocytes in red/pink and fibrosis in white. E: fibrosis in both the infarct border zone and scar zone were assessed using Mason’s trichrome staining, where red/purple color indicates myocytes and blue indicates fibrosis. F: sirius red collagen stain. All histology was performed on 10-µM serial frozen sections. Error bars represent means ± SD.
Echocardiography results were analyzed before and after MI in both bifurcated (n = 7) and trifurcated rabbits (n = 5; Table 3). Significantly, lower LV fractional shortening and LV ejection fraction were observed in all rabbits 3 wk after MI was performed compared with the pre-MI values (P < 0.05). Importantly, after MI, there were no significant differences found in LV internal diameter at end diastole and end systole, LV ejection fraction, and LV fractional shortening between bifurcated and trifurcated rabbits either before or after MI (P = NS).
Table 3.
Comparison of derived echocardiographic functional parameters
| LVIDs, mm | LVIDd, mm | LVEF, % | LVFS, % | |
|---|---|---|---|---|
| Bifurcation | ||||
| Pre-MI | 11.04 ± 2.62 | 14.80 ± 2.55 | 53.94 ± 13.69 | 25.72 ± 9.91 |
| Post-MI | 13.22 ± 1.95 | 15.82 ± 2.18 | 39.37 ± 10.84* | 16.84 ± 3.73* |
| Trifurcation | ||||
| Pre-MI | 11.11 ± 2.04 | 14.29 ± 2.69 | 49.35 ± 3.76 | 22.13 ± 2.43 |
| Post-MI | 13.53 ± 1.98 | 16.00 ± 2.00 | 36.54 ± 4.28* | 15.51 ± 4.28* |
| P value | 0.80 (NS) | 0.88 (NS) | 0.55 (NS) | 0.056 (NS) |
Echocardiography-derived parameters [left ventricular (LV) internal diameter at systole (LVIDs) and diastole (LVIDd)], LV ejection fraction (LVEF), as well as LV fractional shortening (LVFS) in bifurcated (n = 7) and trifurcated (n = 5) female rabbits. P values were determined by Student’s t-test. MI, myocardial infarction.
P < 0.05 comparing pre-MI with post-MI within the same anatomic configuration. All comparisons between bifurcation and trifurcation were not significant (NS).
DISCUSSION
Our understanding of rabbit coronary anatomy has enabled us to develop a method of site-specific occlusion that yields a highly reproducible infarct model. Using echocardiography, we report no difference in functional outcomes between anatomy cohorts. Both bi- and trifurcated hearts displayed the same degree of reduction in LV ejection fraction and LV fractional shortening 3 wk after MI.
Angiographic data demonstrated that the standard experimental technique of induced MI in rabbits, LCX ligation, is unsuitable to create a reproducible infarct model. The coronary anatomy of the rabbit varies considerably from case to case. Not only does the left coronary system display different and complex branching patterns but the right coronary varies in size and importance.
The LCX artery ligation model cannot produce a consistent infarct for several reasons. First, the LCX varies greatly in size from animal to animal. In most animals, the OM1 is the dominant artery in the left coronary system and the LCX proper is relatively minor. In some cases, however, the LCX is a dominant artery. Ligating or occluding this type of artery almost always results in periprocedural death. Finally, the occasional occurrence of right coronary dominance is a confounding variable. In right coronary dominant cases, the LCX is minuscule after branching from the OM1. We found that in right dominant cases, occluding the LCX proximal to the origin of the OM1 was fatal and occluding it distal to the OM1 did not result in infarcts of sufficient size. For these reasons, right dominant rabbits were best infarcted by targeting the OM1.
The LAD or LCX ligation model makes physiological assays, such as high-resolution optical mapping of the epicardial surface, impossible because of fibrosis and surgical artifact. Moreover, the LAD is a minor vessel in the rabbit. In our experience, occlusion of this vessel did not guarantee a transmural infarction. Furthermore, since the LAD perfuses the septum, the infarction is more complicated, encompassing the septum and anterior, superior LV. This complicated infarction lessens the investigators ability to use optical mapping to view the entire infarction. For these reasons, we chose to target an area of myocardium rather than a specific vessel.
The variability of coronary anatomy between young and aged rabbits does not inhibit reproducibility. Our main challenge in creating a reproducible model was comparing rabbits with different anatomies, i.e., bifurcated versus trifurcated and quadrifurcated. That aged rabbits tend to have more collateral blood supply has not affected our model because we were targeting an area that does not experience blood supply redundancies in either age cohort. Similarly, and for the same reasons, quadrifurcation in aged rabbits does not seem to be confounding variable.
One of the striking differences in epicardial branching patterns between age cohorts is the presence of quadrifurcation in the aged cohort exclusively. The origins of this difference are not clear. Does the second ramus artery result from angiogenesis over time or is it present throughout the animal’s lifetime? The latter is certainly possible as angiography is apt to miss a very small progenitor artery. As for the former, to our knowledge, angiogenesis of this kind has not been previously documented. We believe a long-term study involving multiple angiographic time points across the animal’s lifetime would be necessary to answer this intriguing question.
As shown by others, the use of perioperative amiodarone has reduced our incidence of SCD in the first 48 h post-MI. Indeed, several studies have shown that amiodarone treatment results in decreased incidence of tachyarrhythmias and ventricular fibrillation after MI in rats (1a), sheep (10a), and pigs (21a). Amiodarone may, however, modify ventricular remodeling through reducing matrix remodeling (10b). In rats, administration of amiodarone over 4 wk post-MI prevents the upregulation of matrix metalloproteinases-2 in the LV and preserves fractional shortening compared with vehicle control (6a). Importantly, long-term administration of amiodarone in pigs post-MI was not associated with increased ventricular fibrosis, and there was no significant elevation of brain natriuretic peptide or matrix metalloproteinase-9 compared with vehicle control (21a). In humans, a small study of eight patients after coronary artery bypass grafting surgery showed increased proinflammatory activity via increased fibrinogen but without increased mortality (8a). This study reported that plasma levels of TNF-α, IL-6, IL-10, and monocyte chemoattractant protein-1 all changed over time, but amiodarone treatment did not significantly change the expression of these factors between amiodarone- versus placebo-treated groups. Given our relatively short dosing duration, we do not expect a substantial effect on the long-term outcomes of our study.
Although we have chosen the method of endovascular embolization, we believe our findings should also benefit those using open-chest infarction methodology. The open-chest method can be specific enough to differentiate between bifurcation and trifurcation of the LM coronary artery (10). We posit that open-chest operators would be able to use our atlas not only to predict what LM coronary artery branching to expect but also the branching of more distal epicardial vessels. In this light, use if our infarction rubric is feasible. Additionally, since our methodology does not involve reperfusion, it is possible that an open-chest method could translate our anatomic findings and infarction rubric to the field of myocardial reperfusion.
Here, we present an age-appropriate model of MI in the rabbit. By targeting a specific area of the myocardium with minimally invasive coil embolization, we achieved infarcts in the same location with similar size and similar functional outcomes. We believe this approach solves the long-lasting reproducibility problem associated with coronary artery ligation. This new methodology allows investigators to objectively assess interventions to reduce infarct size, regardless of anatomic configuration.
GRANTS
Support for this study was provided by National Institute on Aging Grant 1-R21-AG-049608-01.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
P.J.M., K.R.M., and G.K. conceived and designed research; P.J.M., K.R.M., J.M.D., L.S., and N.N.T. performed experiments; P.J.M., K.R.M., K.A., and G.K. analyzed data; P.J.M., K.R.M., J.M.D., L.S., K.A., J.D.A., and G.K. interpreted results of experiments; P.J.M., K.R.M., and K.A. prepared figures; P.J.M., K.R.M., and K.A. drafted manuscript; P.J.M., K.R.M., J.M.D., L.S., N.N.T., K.A., J.D.A., and G.K. edited and revised manuscript; P.J.M., K.R.M., J.M.D., L.S., N.N.T., K.A., J.D.A., and G.K. approved final version of manuscript.
ACKNOWLEDGMENTS
We thank Dr. Malcolm Kirk for his valuable insights and recommendations in creating the antiarrhythmic drug regimen.
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