Adverse Effects of Chronic Endogenous Sympathetic Drive Induced by Cardiac Gsα Overexpression
Abstract To study the physiological effect of the overexpression of myocardial Gsα (protein levels increased by approximately threefold in transgenic mice), we examined the responsiveness to sympathomimetic amines by echocardiography (9 MHz) in five transgenic mice and five control mice (both 10.3±0.2 months old). Myocardial contractility in transgenic mice, as assessed by left ventricular (LV) fractional shortening (LVFS) and LV ejection fraction (LVEF), was not different from that of control mice at baseline (LVFS, 40±3% versus 36±2%; LVEF, 78±3% versus 74±3%). LVFS and LVEF values in transgenic mice during isoproterenol (ISO, 0.02 μg/kg per minute) infusion were higher than the values in control mice (LVFS, 68±4% versus 48±3%; LVEF, 96±1% versus 86±3%; P<.05). Norepinephrine (NE, 0.2 μg/kg per minute) infusion also increased LVFS and LVEF in transgenic mice more than in control mice (LVFS, 59±4% versus 47±3%; LVEF, 93±2% versus 85±3%; P<.05). Heart rates of transgenic mice were higher than those of control mice during ISO and NE infusion. In three transgenic mice with heart rates held constant, LV dP/dt rose by 33±2% with ISO (0.02 μg/kg per minute) and by only 13±2% in three wild-type control mice (P<.01). NE (0.1 μg/kg per minute) also induced a greater effect on LV dP/dt in the three transgenic mice with heart rates held constant compared with three wild-type control mice (65±8% versus 28±4%, P<.05). Pathological and histological analyses of older transgenic mouse hearts (16.0±0.8 months old) revealed hypertrophy, degeneration, atrophy of cells, and replacement fibrosis reflected by significant increases in collagen volume in the subendocardium (5.2±1.4% versus 1.2±0.3%, P<.05) and in the cross-sectional area of myocytes (298±29 versus 187±12 μm2, P<.05) compared with control mouse hearts. These results suggest that Gsα overexpression enhances the efficacy of the β-adrenergic receptor–Gs–adenylyl cyclase signaling pathway. This in turn leads to augmented inotropic and chronotropic responses to endogenous sympathetic stimulation. This action over the life of the animal results in myocardial damage characterized by cellular degeneration, necrosis, and replacement fibrosis, with the remaining cells undergoing compensatory hypertrophy. As a model, this transgenic mouse offers new insights into the mechanisms of cardiomyopathy and heart failure and provides a new tool for their study.
Activation of the sympathetic nervous system is an important compensatory mechanism to a variety of stresses, eg, hypotension, exercise, and the “fight-or-flight” syndrome. The sympathetic nerves are poised to regulate cardiovascular function via autonomic control and, when signaled, to enhance cardiac performance by releasing catecholamines, which in turn activate myocardial beta adrenergic receptors (β-ARs). Whether this activation is beneficial or deleterious over the long term, particularly in the pathogenesis of human heart failure, remains enigmatic. Although it seems clear that the acute role of β-AR activation over a time frame of seconds to hours evolved as an evolutionary advantage, for predators and prey alike, a major clinical puzzle revolves around whether a persistent enhancement of sympathetic drive to the heart, as can occur after major insults such as myocardial infarction or cardiac failure, might paradoxically lead to gradual deterioration of healthy cardiocytes and thereby to global organ dysfunction.
Sympathetically mediated increases in myocardial contractility depend upon the density of the β-ARs, the effector, adenylyl cyclase, and signal transduction regulatory proteins, eg, the GTP stimulatory protein Gs. The goal of the present investigation was to determine the physiological and pathological consequences of overexpression of myocardial Gsα.1 In examining the postreceptor β-adrenergic signal transduction pathway in the heart, we reasoned that manipulation of its various components might yield insight not only into the role of component stoichiometry in signal transduction but also into the intimate role that the sympathetic nerves play in regulating cardiac physiology. A transgenic mouse was developed wherein Gsα is selectively overexpressed approximately threefold in the heart. Although steady state adenylyl cyclase activities were not altered, both the percent of agonist high-affinity or “coupled” β-ARs and the rate of catalyst activation are increased.1 However, it is not possible to predict the physiological outcome of increased Gsα expression in response to β-adrenergic stimulation, since it is widely held that Gsα is in considerable excess relative to the catalytic unit of adenylyl cyclase.2 Once it was determined in the present investigation that the excess Gsα did indeed result in enhanced responsiveness to sympathomimetic amines, a second goal was to determine whether chronically facilitated sympathetic stimulation was deleterious or beneficial. To address this hypothesis, older mouse hearts were analyzed histologically.
Materials and Methods
Five transgenic mice (10.3±0.2 months old) and five wild-type littermates (10.3±0.2 months old) of either sex from the same genetic background as the transgenic mice were bred as described previously.1 The transgene consists of a rat α-myosin heavy chain promoter linked to a Gsα cDNA coding for the short isoform of Gsα from exon 1 to exon 12, which is ligated to a portion of the Gsα gene containing intron 12, exon 13, and the polyadenylation signal. At least 1 day before the day of the study, PE-10 tubing was inserted into the right jugular vein under a dissecting microscope, and the catheter was tunneled subcutaneously to the back. After determination of body weight, mice were anesthetized with ketamine (0.065 mg/g), acepromazine (0.002 mg/g), and xylazine (0.013 mg/g) injected intraperitoneally and were allowed to breathe spontaneously according to the method of Hoit et al.3 The chest was shaved, and mice were positioned prone on a warmed saline bag as support. The saline bag was attached to a warming pad (Deltaphase isothermal pad) to keep temperature constant at 37°C. Electrocardiographic leads were attached to each limb using needle electrodes (Grass Instruments). Echocardiography was performed by using an Interspec Apogee X-200 ultrasonograph (Interspec-ATL). A dynamically focused 9-MHz annular array transducer was applied from below, using the saline bag as a standoff. The heart was first imaged using the two-dimensional mode in the parasternal long-axis and short-axis views. The short-axis views, including papillary muscles, were used to position the M-mode cursor perpendicular to the ventricular septum and LV posterior wall.
Studies were recorded on 1/2-in S-VHS videotape (Sony Corp). Freeze frames were printed on a Sony color printer (UP-5200, Sony Corp). The ECG was printed on the ultrasonograph for heart rate measurement. The images were scanned into a Macintosh IICi computer and digitized at 300 pixels per inch. Gray scale equalization was made using the Photoshop program (Adobe Photoshop, Adobe Systems Corp), and the images were imported into the NIH-Image program (National Institutes of Health) for measurement. M-mode measurements of LVID were made from more than three beats and averaged, using the leading edge–to–leading edge convention adopted by the American Society of Echocardiography.4 End-diastolic measurements were taken at the time of the apparent maximal LV diastolic dimension. End-systolic measurements were made at the time of the most anterior systolic excursion of the posterior wall. LV percent fractional shortening (LVFS) was calculated as follows: LVFS=[(LVIDd−LVIDs)/LVIDd]×100, where d indicates diastolic and s indicates systolic. LVEF was calculated by the cubed method as follows: LVEF=[(LVIDd)3−(LVIDs)3]/LVIDd3.
M-mode echocardiographic measurements of the LV were performed at baseline and during intravenous infusion of isoproterenol (0.01, 0.02, and 0.04 μg/kg per minute for 5 minutes each) using a microliter syringe (Hamilton Co) and an infusion pump (Harvard Apparatus, Inc). A lower dose of isoproterenol (0.005 μg/kg per minute) was infused in the transgenic mice because of the enhanced response. The total amount of the infusion volume was <100 μL in each mouse. On a separate occasion, each mouse received an infusion of saline as a control to ensure that the volume of infusion alone did not contribute to enhance ventricular performance. On a separate day, a similar protocol was performed during intravenous norepinephrine (0.1, 0.2, and 0.4 μg/kg per minute) in control mice, and a lower dose (0.05 μg/kg per minute) was infused in transgenic mice. These studies were conducted in four transgenic and four control mice for both isoproterenol and norepinephrine. Three animals were common to both isoproterenol and norepinephrine studies.
In addition, three transgenic mice (10.8±1.8 months old) and three wild-type control mice (10.7±1.7 months old) were studied with an acutely implanted 1.8F micromanometer (Millar Instruments) and with the heart rate held constant. After the anesthetic regimen as noted above, a 25-gauge needle was inserted into the LV through the chest wall. The needle was used for electrical pacing (stimulator, model SD9, Grass Instruments), and a 1.8F Millar micromanometer was connected for measurement of LV pressure. The LV pressure signal was differentiated (frequency response of 700 Hz) for calculation of LV dP/dt. Measurements of LV pressure, LV dP/dt, and heart rate were recorded on a multichannel tape recorder and played back on a multichannel oscillograph.
Separate groups of nine transgenic mice (16.0±0.8 months old) and eight wild-type mice (16.4±0.8 months old) from the same genetic background as the transgenic mice were used for this part of the study. The hearts of four animals from each group were fixed by immersion in 10% phosphate-buffered formalin; the remaining, by perfusion fixation with 2% phosphate-buffered glutaraldehyde. All animals were anesthetized with intraperitoneal sodium pentobarbital, the chest was opened, and the heart was either removed and dissected fresh followed by immersion fixation or perfused at 90 mm Hg with a brief saline wash followed by glutaraldehyde via a 21-gauge trocar inserted directly into the LV apex. The heart was dissected to remove the atria and right ventricular free wall, and each portion was weighed. Fixed tissues were dehydrated, embedded in paraffin, sectioned at 6 μm thickness, and stained with hematoxylin and eosin and Gomori’s aldehyde fuchsin trichrome. Heart sections were also stained with picric acid sirius red. Glutaraldehyde-fixed tissues were dehydrated and embedded in Spurr epoxy resin and in glycol methacrylate, sectioned at 1 μm thickness, and stained with toluidine blue and methylene blue–basic fuchsin, respectively, for light microscopic examination. Methacrylate sections were also stained with silver-gold (Accustain silver stain, Sigma Diagnostics) for basement membrane to outline cardiac myocytes for cross-sectional area measurement.
Myocyte cross-sectional area was measured from video prints of silver-stained 1 μm thick methacrylate sections of subendocardial and subepicardial regions of the LV. Suitable cross sections were defined as having nearly circular capillary profiles and circular to oval myocyte cross sections. No correction for oblique sectioning was made. Video prints (×1100 final magnification) were used to trace the outline of 40 to 130 myocytes in each region, using a sonic digitizer (Graf/Bar, Science Accessories). Myocyte cross-sectional area was determined using computer programs developed in our laboratory. The mean area was calculated for each region in each animal, and the group mean was calculated for each region and group.
Myocardial connective tissue was quantitatively analyzed on a single cross section of LV and septum obtained mid-distance from base to apex, embedded in paraffin, and stained with picric acid sirius red. Images were obtained from a video monitor and CCD72 video camera attached to an Olympus AHT microscope, using a ×1 objective and a green (550 nm) filter at a final video screen magnification of ×30, and were analyzed with Image-1 image analysis software (Universal Imaging Corp). The entire inner (subendocardial) and outer (subepicardial) halves of the LV were traced and analyzed separately for volume percent collagen. Areas measured in each region ranged from 2.5 to 10 mm2.
Data Analysis and Statistics
All data were reported as mean±SE. Comparisons between transgenic and control mice were made by using Student’s t test for group data. The dose-response curves were analyzed by one-way ANOVA for repeated measurements. If the ANOVA demonstrated significant overall differences, individual comparisons between baseline and the responses to each dose were made by contrast analysis. A value of P<.05 was taken as the minimal level of significance.
Preliminary experiments indicated that the transgenic mice responded with a positive effect on LV function at a lower dose than the control mice, which did not respond at all to the lowest doses, and that the transgenic mice reached the top of the dose-response curve, ie, nearly 100% ejection fraction, at a lower dose than the control mice. Accordingly, the lowest dose was only examined in the transgenic mice, and the highest dose was only examined in the control mice.
Response to Isoproterenol
Representative responses to isoproterenol in a control mouse and a transgenic mouse are illustrated in Fig 1A⇓ and summarized in Table 1⇓. Both the baseline heart rate and the chronotropic responses to increasing doses of isoproterenol were greater in transgenic mice than in control mice. LV end-diastolic dimensions decreased dose-dependently in both groups. Although baseline levels of fractional shortening and ejection fraction were not different, isoproterenol infusion produced significantly greater increases in both fractional shortening and ejection fraction in transgenic mice given isoproterenol (Fig 1B⇓ and 1C⇓). In three wild-type control mice with heart rates held constant (8.0±0.0 Hz), isoproterenol (0.02 μg/kg per minute) increased LV dP/dt by 13±2% from a baseline of 6923±281 mm Hg/s, which was significantly less (P<.01) than in three transgenic mice with heart rates held constant (9.0±0.6 Hz), in which isoproterenol increased LV dP/dt by 33±2% from a baseline similar to that observed with wild-type control mice (Fig 2⇓).
Response to Norepinephrine
Responses to norepinephrine in control and transgenic mice are summarized in Table 2⇓. Although the baseline heart rate remained significantly greater in transgenic mice, there was no significant chronotropic effect of norepinephrine in either group, which was most likely due to secondary reflex cardiac slowing induced by the rise in blood pressure. In contrast to isoproterenol, norepinephrine infusion had no effect on LV end-diastolic dimensions in either group. However, similar to isoproterenol, norepinephrine produced significantly greater (P<.05) increases both in LVFS and in LVEF in transgenic mice. In three wild-type control mice with heart rates held constant (6.0±0.0 Hz), norepinephrine (0.1 μg/kg per minute) increased LV dP/dt by 28±4% from a baseline of 7292±461 mm Hg/s, which was significantly less (P<.05) than in three transgenic mice with heart rates held constant (8.3±1.2 Hz), whereas norepinephrine increased LV dP/dt by 65±8% from a baseline similar to that observed in wild-type control mice.
Six of the nine transgenic mice had moderate to severe multifocal areas of mature replacement type fibrosis and interstitial fibrosis throughout the LV and septum but most severe in the subendocardium (Fig 3⇓). There was a marked increase in the cross-sectional area of cardiac myocytes, especially in the subendocardial half of the LV myocardium (Fig 3⇓). In the subendocardial regions, particularly in areas adjacent to focal fibrosis, there was extreme variability in myocyte cross-sectional area; some myocytes were smaller than normal, suggesting atrophy, and others had very large cross-sectional areas, with some exceeding 500 μm2. Quantitative evaluation of myocardial fibrosis revealed a significant increase in volume percent collagen for the entire group of transgenic mice compared with control mice; this increase was most prominent in the subendocardium (Fig 3⇓). Apparently, the myocyte hypertrophy was offset by the cellular degeneration and necrosis, since heart weight, even normalized to body weight, was not significantly elevated in the transgenic mice (5.07±0.34 mg/g) compared with control mice (4.66±0.30 mg/g) (Table 3⇓). Electron microscopy revealed individual cells with myofibrillar disorganization and loss, small mitochondria, increased lipofuscin, and bizarre nuclei, indicative of myocyte degeneration and atrophy in the transgenic mice. Qualitative histological data indicated the presence of hypertrophied myocytes in 7-month-old transgenic mice (n=3) and hypertrophied myocytes plus fibrosis in 10-month-old (n=2) and 12-month-old (n=5) transgenic mice compared with age-matched control mice.
The stimulatory guanine nucleotide binding protein, Gsα, plays an important role in β-adrenergic signal transduction. It is not known whether increases in the levels of Gsα would facilitate adrenergic signal transduction, particularly since Gsα is thought to be in excess of the effector, adenylyl cyclase.2 The logical extension of that concept suggests that increasing the expression and amount of myocardial Gsα might have little effect on maximal adenylyl cyclase activity and potentially on responses to sympathomimetic amines in vivo. Indeed, this is one potential interpretation from the initial study by Gaudin et al1 on the transgenic murine model in which Gsα was overexpressed. Protein levels of Gsα as measured in the hearts of these transgenic mice by Western blotting rose 2.8-fold, and Gsα activity as assessed by S49 lymphoma cell mutant cyc– reconstitution assay rose by only 88%. Effector activity at steady state (ie, adenylyl cyclase, either basal or stimulated) failed to increase, which is consistent with the concept that Gsα is present in excess. Only the findings that the fraction of β-ARs in the high-affinity state was increased and that the activation of adenylyl cyclase occurred more rapidly in sarcolemma from mice with overexpressed Gsα presaged possible augmented β-adrenergic function.
One of the most important findings in the present investigation is that responsiveness to sympathomimetic amines is clearly augmented in mice with Gsα overexpression. With intravenous infusion of either isoproterenol or norepinephrine, ventricular systolic performance, as reflected by LVEF and LVFS as well as by LV dP/dt, was enhanced in the transgenic mice. Because these sympathomimetic amines exert opposing effects on preload and afterload, the changes in indices of ventricular performance cannot be ascribed to loading conditions but rather to a true change in the inotropic state.
An additional concern is that heart rates were higher in the mice with overexpressed Gsα. To address this concern, we examined the effects of isoproterenol and norepinephrine in three transgenic mice and three wild-type control mice with heart rates held constant. With heart rates constant, a greater effect of the sympathomimetic amines was observed in the transgenic mice compared with the control mice (Fig 2⇑). In further support of the concept that the inotropic action of sympathomimetic amines can be dissociated from their chronotropic actions, note that norepinephrine (0.1 μg/kg per minute) increased LV function significantly (both LVEF and LVFS) but did not increase heart rate in the transgenic mice (Table 2⇑).
These observations raise an interesting conundrum when relating the in vivo physiological consequences of cardiac Gsα overexpression to the biochemical alterations in vitro. We have documented in younger animals that at steady state, adenylyl cyclase is not altered whether activated with agonist plus GTP, Gpp(NH)p, NaF, or forskolin, a finding that is consistent with a scenario in which the sarcolemmal content of the catalyst itself has not changed.1 Nevertheless, in vivo, the hearts of these mice hyperrespond to catecholamines compared with the hearts of their wild-type littermates. Moreover, previous observations indicate that the activation of more distal effector pathways such as the L-type Ca2+ channel, known to be regulated by cAMP-dependent protein kinase A, was markedly enhanced in transgenic cardiocytes.5 How can these apparently contradictory observations be reconciled? Several hypotheses are suggested. First, it is possible that these enhanced physiological responses are transduced via Gsα but not through adenylyl cyclase activation. There has been preliminary evidence that Gsα can directly modulate the activity of other effector pathways.6 7 8 Nevertheless, the enhanced inotropic and chronotropic responses seen in the present study are characteristic, if not classic, responses to enhanced cAMP generation. A more attractive hypothesis would argue that steady state cAMP in vitro measurements do not accurately reflect the activity of the β-adrenergic signaling pathway in the heart. This organ responds on a second-to-second basis to changing levels of norepinephrine released and removed at the synaptic terminal as the organism moves, changes posture, eats, or becomes excited. In responding to these continuously changing demands, the heart operates on the steep portion of the receptor occupancy curve with this parameter continuously changing, thereby not permitting steady state measurements in vitro to accurately mirror in vivo signaling activity. Biochemical measurements of the myocardial β-AR–Gs–adenylyl cyclase pathway made in vitro are thereby limited in that they may not accurately reflect the activity of this pathway in the dynamic range in which it operates in vivo. It is exactly in this framework that the increased Gsα levels in the transgenic cardiocytes are likely exerting their effect by allowing small changes in β-AR occupancy to signal the catalyst to activate more rapidly. Static measurements of second messenger activity, whether cAMP or Ca2+, cannot capture the dynamic nature of these activities, particularly in the intact innervated heart, which responds to changing neural activity on a time frame measured in seconds. Although the present experiments were carried out with exogenously administered sympathomimetic amines, one can readily extrapolate to the in vivo situation, in which sympathetic tone fluctuates on a moment-to-moment basis. If it is assumed that similar augmented inotropic and chronotropic effects would be observed with physiological increases in adrenergic drive, this transgenic model becomes useful for the study of the cardiac effects of chronic adrenergic hyperactivity.
The next most interesting finding of the present investigation involved the histological studies in older transgenic mice. It is possible to predict that the cumulative effects of increased endogenous sympathetic stimulation secondary to the overexpression of Gsα might result in catecholamine-induced myocardial injury. For example, a prior study in rats that received infusions of isoproterenol for several weeks demonstrated myocardial hypertrophy with cellular necrosis and replacement fibrosis.9 Similarly, another prior study in mice with chronic isoproterenol administration demonstrated enhanced hypertrophic responses.10 Indeed, in the present study, there was clear evidence of myocardial cellular degeneration and extensive increases in fibrosis, as reflected by the increased collagen, most prominent in the subendocardium. Significantly increased myocyte size confirmed the existence of hypertrophy in both the subepicardium and subendocardium, but again, hypertrophy was more prominent in the subendocardium, where wall stresses are higher and the impact of reduced coronary reserve is greater.11 Importantly, the hypertrophic process was not evident on gross morphology. Apparently, the cellular degeneration and myocyte loss, evident on light microscopy and corroborated by electron microscopy, offset the expected increase in total heart weight that should have been observed with myocyte hypertrophy. The hypertrophy presumably resulted as a compensatory response to the increases in cardiac work imposed by the chronically enhanced endogenous adrenergic function. With cellular degeneration and myocyte loss, cardiac function might be expected to decline and provide a further stimulus to hypertrophy in the remaining cardiocytes, potentially through local application of the Frank-Starling mechanism. In addition, a direct role of Gsα in myocardial growth cannot be ruled out.
The results of the present investigation have implications for the understanding of the pathogenesis of heart failure. There are currently two opposing views as to the role of adrenergic mechanisms in the pathogenesis and therapy of heart failure. One point of view holds that catecholamine desensitization is a signature of heart failure,12 resulting in a critical defect in the normal compensatory mechanism of increased sympathetic drive, and that by replacing the sympathetic tone (eg, through increasing the expression of β-AR13 14 or treatment with adrenergic agonists), heart failure can be ameliorated.
The studies by Koch et al13 and Milano et al14 examined the effects of overexpressing β2-ARs and found enhanced baseline LV function, with little further effect due to β-AR stimulation, and went on to suggest that this model may be potentially beneficial as a treatment for heart failure. Koch et al15 also found similar results with overexpressing a β-AR kinase inhibitor. However, the later development of myocyte hypertrophy, cellular necrosis, and fibrosis in the present investigation supports the diametrically opposite point of view. The present study is the first to examine the effects of long-term adrenergic stimulation in transgenic animals with overexpression of a component of the β-adrenergic signaling pathway. This is the major difference among the studies and may explain the differences between the present study and those of Koch and colleagues13 15 and Milano et al.14 Bertin et al16 overexpressed β1-ARs in the atrium and found an increased incidence of arrhythmias.
In further support of the concept that chronic β-adrenergic stimulation is not beneficial in heart failure, a variety of therapeutic approaches have been devised to augment the adrenergic support that dissipates with heart failure (eg, approaches involving dobutamine,17 18 prenalterol,19 xamoterol,20 and milrinone21 ). Unfortunately, all of these therapeutic strategies have failed to materially alter the course of heart failure. In contrast, the opposite approach (ie, β-AR blockade) may be potentially more useful.22 23 In further support of the latter opposing point of view is the concept that inhibition of β-adrenergic function is salutary in heart failure, particularly in view of recent beneficial effects of another β-blocker, carvedilol, in heart failure.24 25 Our results support the latter concept; ie, whereas the normal physiological compensatory mechanisms to the fight-or-flight syndrome (including increased sympathetic drive) are important for the acute adjustments required for exercise, excitement, and hypotension, these mechanisms are deleterious on a chronic basis, as evidenced by the histological studies in the older transgenic mice in the present study. The mechanism underlying this deleterious effect remains unclear, but the Gsα transgenic mouse may offer clues as to how enhanced sympathetic nerve drive might trigger cell death via mechanisms such as apoptosis. A further extension of this concept is that catecholamine desensitization mechanisms, which are pathognomonic of heart failure, are actually salutary chronic compensatory mechanisms and that overcoming these compensatory mechanisms by enhancing sympathetic activation is deleterious over the long term.
Interestingly, had the physiological and histological studies not been conducted, the interpretation of the model of increased Gsα would have been entirely different. Since steady state adenylyl cyclase activity was not enhanced and since there was no evidence of hypertrophy on gross examination of the heart (ie, the heart weight–to–body weight ratio was not increased), one might conclude that the increased expression of Gsα did not have profound physiological or pathological consequences. In vivo cAMP levels, which may fluctuate on a minute-to-minute basis or actually be heterogeneously distributed throughout the heart, may not be faithfully recapitulated by in vitro steady state assays.26
In conclusion, this transgenic mouse model demonstrates first that physiological effects may be a more sensitive indicator of β-adrenergic signaling than in vitro assays because of the rapid kinetics of the response, which are hampered by the time required to achieve steady state conditions, particularly under conditions of low levels of receptor occupancy. Second, the unique advantage of examining the effects of overexpression of a component of the β-adrenergic signaling pathway over the life of the animal permitted demonstration of the deleterious effects of chronic sympathetic stimulation, which may not have been apparent if only younger animals had been studied. Thus, the transgenic mouse with overexpressed Gsα sheds insight not only into mechanisms regulating the efficiency of β-AR signal transduction but also into the pathogenesis of heart failure and may provide a tool for investigating the biological mechanisms underlying this process and for evaluating new therapies aimed at arresting its progression.
Selected Abbreviations and Acronyms
|LV||=||left ventricle, left ventricular|
|LVEF||=||LV ejection fraction|
|LVFS||=||LV percent fractional shortening|
|LVID||=||LV internal dimension|
This study was supported in part by National Heart, Lung, and Blood Institute grants HL-38070, HL-33107, HL-33065, HL-37404, and HL-45332. Dr Iwase was a recipient of a fellowship grant from the Uehara Memorial Foundation. We thank Drs Brian D. Hoit and Richard A. Walsh for teaching us the techniques of murine echocardiography. We also thank Thomas Patrick for his assistance in the development of measurements of echocardiographic data.
This manuscript was sent to Howard Morgan, Consulting Editor, for review by expert referees, editorial decision, and final disposition.
- Received September 11, 1995.
- Accepted January 5, 1996.
- © 1996 American Heart Association, Inc.
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