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Cardiovascular Research 1998 40(2):297-306; doi:10.1016/S0008-6363(98)00181-3
© 1998 by European Society of Cardiology
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Copyright © 1998, European Society of Cardiology

Differential effects of growth hormone on cardiomyocyte and extracellular matrix protein remodeling following experimental myocardial infarction

Daniela Grimm, Daniela Cameron, Daniel P. Griese, Günter A.J. Riegger and Eckhard P. Kromer*

Klinik und Poliklinik für Innere Medizin II, University of Regensburg, D-93042 Regensburg, Germany

* Corresponding author. Tel.: +49-941-944-7211; Fax: +49-941-944-7213; E-mail: eckhard.kromer@klinik.uni-regensburg.de

Received 18 November 1997; accepted 8 April 1998


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objectives: Growth hormone (GH) causes cardiomyocyte hypertrophy without development of fibrosis in the normal rat heart. The aim of this study was to evaluate the effects of GH on cardiac remodeling following experimental myocardial infarction (MI). Methods: Following ligation of the left coronary artery or sham operation, rats were randomized to receive 2 IU GH/kg/day or vehicle for four weeks (n=140). Extracellular matrix proteins were assessed in the non-infarcted myocardium of the posterior wall using immunohistochemistry and automatic image analysis. In addition, cardiomyocyte size was measured. Results: Compared to sham, vehicle-treated rats with moderate (20–40%) and large (>40%) infarct size showed left ventricular (LV)-dilatation, reduced fractional shortening as well as increases in LV end-diastolic and right atrial pressures, LV/body weight (BW) ratio and LV posterior wall thickness. Compared to vehicle-treated MI-rats, treatment with GH considerably increased fractional shortening and attenuated LV-dilatation. Vehicle-treated MI-rats displayed progressive increases in cardiomyocyte width and deposition of collagen I, compared to sham rats. Treatment with GH nearly doubled the increase in cardiomyocyte width and reduced collagen I accumulation by 50%. Conclusions: Our study demonstrates that GH, given early after large MI, elicits a unique pattern of structural effects characterized by enhanced cardiomyocyte hypertrophy and reduced adaptive fibrosis. This attenuation of pathological remodeling translates into a significant improvement in systolic and diastolic LV-function.

KEYWORDS Myocardial hypertrophy; Experimental myocardial infarction; Growth hormone; Cardiac function; Extracellular matrix; Fibrosis


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
Coronary artery disease and congestive heart failure are major clinical problems and coronary artery disease is the leading cause of death [1–3]. Myocardial infarction (MI), as the typical initial insult to the myocardium, induces alterations of infarcted and non-infarcted ventricular regions. The combination of dilatation and, potentially inadequate, hypertrophy subsequently results in changes of left ventricular (LV) volume and geometry. As may be delineated from the SAVE Study, the development of LV-dilatation following MI is associated with an increased incidence of all major cardiac events, including death, irrespective of therapeutic interventions [4].

Chronic cardiomyocyte remodeling following experimental anterior MI in rats was characterized by Olivetti et al. [5]. Heart size increased and systolic LV-function decreased in relation to the size of the MI. Morphometry revealed significant cardiomyocyte hypertrophy. In parallel, significant alterations of the extracellular matrix (ECM) proteins, predominantly due to collagen accumulation, have been reported [6]. The course of remodeling is substantially influenced by loading conditions and neurohormonal stimulation. Emerging experimental and clinical evidence indicates that growth hormone (GH) may be an important regulator of cardiac growth and performance under physiological and pathological conditions. Penney et al. [7]followed rats for eight weeks after implantation of GH-secreting tumors and observed significant elevations of myocardial contractility and cardiac output. Castagnino et al. [8]showed that administration of GH following experimental MI in the rat significantly reduced the incidence of LV aneurysms. Recently, a small number of patients with idiopathic dilated cardiomyopathy were treated with GH for three months and beneficial effects were noted, in particular, a significant increase in LV-mass and an improvement of hemodynamics [9]. Fazio et al. [10]showed that GH induced cardiomyocyte hypertrophy under normal conditions. In a second study, Cittadini et al. [11]found that early treatment of large MI in rats with GH could attenuate pathologic LV remodeling and improve cardiac function. Thus, GH has been demonstrated to provide beneficial effects on cardiac remodeling and function under distinct experimental and clinical conditions. However, detailed effects of GH on cardiomyocyte and ECM protein remodeling in the important setting of remodeling following anterior MI have not yet been reported.

Therefore, the purpose of our investigation was twofold: (1) to examine, in normal rats, the effects of ascending doses of GH on cardiac structure, including cardiomyocyte hypertrophy and ECM proteins, and (2) to investigate the effects of GH on cardiac remodeling and function following experimental MI. In particular, we studied cardiomyocyte width, along with laminin, a marker of cell activation and signal transduction with growth-modulating properties, and of collagen I and fibronectin, to differentiate cardiomyocyte from ECM remodeling.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
2.1 Animals
Normotensive, male Wistar rats (body weight (BW), 180–200 g; age, six weeks) were obtained from Charles River Wiga (Sulzfeld, Germany). They were maintained on standard rat chow (H1003, Alma KG, Kempten, Germany) with water ad libitum. All animals were individually housed in a 12-h dark/light cycle controlled room. The protocols had been approved by the local standing committee on animal research. The investigation conforms with the ‘Guide for the Care and Use of Laboratory Animals’, published by the US National Institutes of Health (NIH Publication No. 85-23; revised 1985).

2.2 Dose establishment
In order to determine the most efficacious dosage to induce significant cardiomyocyte hypertrophy, three different dosages of recombinant human GH were tested: 2, 5 and 10 IU/kg/day (n=15 for each group). Fifteen animals received vehicle (VEH). GH solutions (1U=0.4 mg; Serono Pharma, Munich, Germany) were prepared using a provided vehicle immediately before subcutaneous injections under sterile conditions. As will be described in detail, 2 IU/kg/day was found to be the optimal dosage.

2.3 Experimental myocardial infarction
Rats were intubated under general anaesthesia (methohexital, 80 mg/kg, i.p.) and placed on a respirator. The heart was exposed via a left-sided thoracotomy, and the anterior descending branch of the left coronary artery was ligated between the pulmonary outflow tract and the left atrium [12]. Sham rats underwent the same procedure except that the suture was passed under the coronary artery and then removed. Immediately after the surgical procedure, the animals were randomly assigned to receive daily subcutaneous injections of VEH or 2 IU/GH/kg/day.

2.4 Follow-up
After 28 days, the rats were anaesthetized with thiopental sodium (100 mg/kg, i.p.) prior to performing echocardiography and hemodynamic measurements. Finally, rats were killed and the hearts were removed for biochemical and histological examinations.

2.5 Echocardiography
LV dimensions were assessed in vivo by transthoracic echocardiographic examinations using a 7.5-MHz electronic probe (Hewlett Packard Sonos 2500). Left longitudinal imaging was performed at approximately 45° through the left parasternal rib space, with a maximum imaging depth of 40 mm. Two-dimensionally guided M-mode tracings were recorded on strip-chart paper at a paper speed of 100 mm/s. LV end-systolic and end-diastolic dimension (LVESD, LVEDD) and LV end-diastolic posterior wall thickness (PWTd) were measured by the leading-edge method from at least three consecutive cardiac cycles on the M-mode tracing at a precision of 0.1 mm, as proposed by the American Society for Echocardiography [13]. Fractional shortening was calculated as FS=(LVEDD–LVESD)/LVEDD times 100 and relative left ventricular end-diastolic dimension as REDD=LVEDD/BW. In our laboratory, variance of the presented echocardiographic parameters is <3%.

2.6 Hemodynamics
Central hemodynamic parameters were measured under light anaesthesia via the right carotid artery (aortic pressure, LV pressure) and right jugular vein (right atrial pressure) using a 2-French catheter pressure transducer (Millar Instruments, Houston, TX, USA). Pressure signals were digitized at a frequency of 2 kHz. At least five consecutive beats were averaged. The time constant of LV isovolumic pressure decline was calculated using the exponential method p(t)=aebt+c, with the time constant {tau}=–1/b (p=pressure; t=time; a and b characterize the slope of the curve; c=cutpoint of the curve and the y-axis; [14, 15]).

2.7 Tissue preparation
Hearts were excised, rinsed with saline, and blotted dry. The atria were dissected free from the remaining heart tissue at the atrio-ventricular ring. The right ventricle was dissected along its septal insertion. Specimens (free posterior wall), for determination of LV cardiac angiotensin converting enzyme (ACE) activity, were snap-frozen in liquid nitrogen within 3 min and stored at –80°C until analyzed.

2.8 Infarct size measurements
The whole left ventricles were cut serially (n=10–15) from apex to base. Each one slice was restored for cryocut sections and one for immunohistochemical studies. Representative sections were used for infarct size measurement. Sections (5 µm) were cut, stained with Masson trichrome and mounted. The lengths of the infarcted surfaces, involving both epicardial and endocardial regions, were measured with a planimeter digital image analyzer and expressed as a percentage of the total circumferencex100. Final infarct size was calculated as the average of all slices from each heart [12].

2.9 Biochemical studies
Ten rats from each group were randomly selected and killed by decapitation. Trunk blood was collected for determination of atrial natriuretic peptide (ANP) and insulin-like growth factor-1 (IGF-1). The methods used have been described in detail previously [16, 17]. IGF-1 was measured in ten sham rats and in each of ten rats with large MI with and without treatment with 2 IU GH/kg/day using a commercially available radioimmunoassay [18].

2.10 Morphometry, immunohistochemical staining and automatic image analysis
Morphometry, including automatic image analysis (AIA), was applied to quantitatively assess the structural changes of the non-infarcted LV posterior wall using a computer-assisted image analysis system device (Olympus Optical, Hamburg, Germany). Frozen specimens were sectioned at 5 µm and fixed with acetone (–20°C) for 10 min. Sections were stained with hematoxylin and eosin, for measurement of cardiomyocyte width. Other sections were selected to visualize antigen–antibody complexes using the indirect peroxidase technique. Incubation with the first antibody (collagen I, Serva; fibronectin, Boehringer Mannheim; laminin, Sigma) was followed by incubation with the second antibody, which was peroxidase-labelled. After repeated washing with phosphate-buffered saline (PBS), the slices were exposed to diaminobenzidine and H2O2 (Sigma), generating a brown color. Finally, the specimens were dehydrated and embedded with entellan (Merck) [17]. Controls for the antibody staining were negative. All sections were visualized by light microscopy using an oil immersion objective with a calibrated magnification of x400. Visual fields had 757x506 square-pixels, with a resolution of 0.2053 µm/pixel (area=0.0161 mm2). Automatic analysis used an 8-bit-color system, that translates colors to 256 grey levels for automatic border detection. Slices were accepted for quantitative analysis (1) if cross-sections of cardiomyocytes with a centrally located nucleus were present and (2) if their cellular membranes were unbroken. Cardiomyocyte width was assessed by marking their borders according to Anversa et al. [19, 20]. The hearts were not arrested in diastole prior to their harvesting for further morphometric assessments, because we previously had shown, using normal rats, that cardiomyocyte width was unaffected by diastolic arrest vs. random arrest [n=20; 11.14±0.2 vs. 11.46±0.2 µm; not significant (n.s.)]. The thickness of the laminin layers that surrounded cardiomyocytes was similarly determined by marking its borders. For each parameter, 50 visual fields were analyzed to calculate averages (variance <2%). These measurements were performed by two independent investigators (D.C., D.G.), who were blinded for modality of treatment. Variability was assessed by performing repeated analyses and was calculated as 1% (intraobserver) and 3% (interobserver). Areas positive for fibronectin and collagen I have a brown color, which is translated by an 8-bit-color-depth system to 256 grey levels. Differences are used to identify borders. Volume fractions were calculated as the sum of all positive areas related to the area of the entire visual fieldx100. Twenty randomly selected visual fields were analyzed to calculate the average of the respective volume fractions (variance <2%).

2.11 Statistical analysis
Statistical analysis was performed using SPSS 7.5 (SPSS Inc.) [21]. Results are expressed as mean±SEM. Comparisons between multiple groups were assessed by one-way analysis of variance, including a modified least-significant difference (Bonferroni) multiple range test to detect significant differences between two distinct groups, which were further analyzed using the Mann-Whitney-U-test. The strength of the relationship between two variables was assessed by calculation of the product–moment correlation coefficient, r, using CORRELATIONS. Statistical significance was accepted at P<0.05 [22].


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Dose establishment (Table 1)
The effects of three dosages of GH were studied in 60 normal rats that received subcutaneous injections of GH (2, 5 or 10 IU/kg/day) or VEH for 28 days. We observed no deaths. There were small, but significant, increases of body weight and LV weight (corrected for BW). Both right ventricular and kidney weight-to-body weight ratios (data not shown) increased in GH-treated rats (P<0.01 vs. VEH). No dose-dependency could be observed. Echocardiography displayed a small increase in the relative LV end-diastolic dimension and a significant increase of end-diastolic posterior wall thickness, without changes in fractional shortening and left ventricular end-systolic dimension (data not shown) following treatment with GH (2 IU/kg/day). Administration of higher doses resulted in significant LV dilatation. Central hemodynamics showed elevated loading conditions following administration of 5 and 10 IU/kg/day of GH, but not after 2 IU/kg/day. There were no significant changes in cardiac ACE activity and plasma ANP levels following treatment with GH (data not shown). We found a significant (45%) increase in cardiomyocyte width, which was present following treatment with 2 IU GH/kg/day. The amount of collagen I and fibronectin was unaltered (data not shown), however, there was a dose-dependent increase in laminin.


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  		Table 1 Dose establishment of growth hormone in the rat

 
3.2 Experimental myocardial infarction
Mortality within the first 48 h following ligation of the anterior branch of the left coronary artery was 30%, irrespective of whether the animal was treated with GH or not. One hundred and forty rats entered follow-up. Three rats on VEH and one rat on GH died (n.s.), all displaying large MIs. After 28 days, the surviving rats were killed for further studies. According to the size of the MI, rats were divided into 3 groups: small MI (<20% of LV circumference), n=21 (VEH) and n=19 (GH); moderate MI (20%<x<40%): n=18 (VEH) and n=21 (GH) and large MI (40%<x<60%): n=21 (VEH) and n=20 (GH). With respect to clinical importance, only rats with moderate and large MI were selected for detailed investigation. Mean moderate infarct sizes were 31±2% (VEH) vs. 32±4% (GH; n.s.) and mean large infarct sizes were 51±5% (VEH) vs. 52±4% (GH; n.s.).

3.3 Somatic and cardiac weights, echocardiography (Table 2)
Rats with MI had a significantly lower BW compared to sham-operated rates, and treatment with GH attenuated these findings. MI was associated with an increase in LV mass, which was further increased following treatment with GH, in particular in the case of a large MI. Relative kidney weight was unaltered (data not shown). LV dilatation, as assessed by in vivo echocardiography, occurred in all rats with MI. GH treatment significantly reduced the LV end-systolic dimensions and attenuated LV end-diastolic enlargement. LV fractional shortening as a measure of contractility significantly improved subsequent treatment with GH (Fig. 1).


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  		Table 2 Growth hormone in experimental myocardial infarction. Somatic and cardiac growth, echocardiography

 

Figure 1
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  		Fig. 1 Echocardiographically determined left ventricular fractional shortening in sham rats and rats with moderately sized and large anterior myocardial infarctions that were treated with vehicle or growth hormone; m, moderate sized; l, large; MI, myocardial infarction; V, vehicle; GH, growth hormone; #, no significance versus rats with moderately sized MI and with large MI treated with GH; *P<0.001 vs. sham.

 
3.4 Hemodynamics (Table 3)
In vivo hemodynamic data, as assessed under mild general anaesthesia, substantiated the mild beneficial effects of GH on diastolic LV function in experimental anterior MI. Increases in mean right atrial pressure were reduced in both MI groups, and the LV end-diastolic pressure increase was reduced in rats with large MI. The isovolumic relaxation time increased in parallel with the size of the MI and these changes were blunted by GH. There was no significant changes in peak systolic LV pressure.


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  		Table 3 Growth hormone in experimental myocardial infarction. Hemodynamic data

 
3.5 Biochemical findings (Table 4)
Cardiac ACE activity (1.5-fold) and plasma-ANP levels (threefold) significantly increased in MI rats irrespective of treatment with GH or not (data not shown). Total serum IGF-1 levels were significantly increased in GH-treated rats with moderate or large myocardial infarction.


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  		Table 4 Growth hormone in experimental myocardial infarction. Biochemical findings and morphometry

 
3.6 Morphometry (Table 4)
Compared with sham-operated rats, cardiomyocyte width was significantly increased in vehicle-treated MI rats and GH application resulted in a further significant increase (Fig. 2). Laminin was significantly enhanced following MI without only mild additional effects (n.s.) of GH. There was an excellent positive correlation between laminin and cardiomyocyte width (r=+0.81; n=103; p=0.001). The considerably increased depositions of fibronectin and collagen I were substantially reduced by GH application, in particular, in cases where the MI was large (Fig. 3).


Figure 2
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  		Fig. 2 Cardiomyocyte width of sham rats and rats with moderately sized and large anterior myocardial infarctions treated with vehicle or growth hormone; m, moderately sized; l, large; MI, myocardial infarction; V, vehicle; GH, growth hormone; *2P<0.001 versus all other groups.

 

Figure 3
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  		Fig. 3 Collagen I antibody stain of rat myocardium (original magnification x20). (A) A cross-section of a normal rat. Brown (diaminobenzidine), collagen I-positive, extracellular matrix material was present in small amounts throughout the entire interstitium. (B) A cross-section of the non-infarcted posterior wall of a rat with a large anterior myocardial infarction treated with vehicle. Compared to (A), collagen I expression was considerably increased in the interstitium and encapsulated cardiomyocytes. (C) A cross-section of the non-infarcted posterior wall of a rat with a large anterior myocardial infarction treated with growth hormone. Collagen I overexpression is substantially reduced compared to that in (B).

 

   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
This study demonstrates that GH administered to rats at a dose of 2 IU/kg/day induced a prominent effect on cardiomyocyte hypertrophy without a concomitant accumulation of major structural ECM proteins. Furthermore, that particular dose was highly effective in the setting of cardiac remodeling following experimental MI, where both additional cardiomyocyte hypertrophy and a significantly reduced amount of collagen I and fibronectin in non-infarcted areas contributed to less unfavorable cardiac remodeling and to a considerable improvement in the depressed systolic and diastolic LV function.

4.1 Effects of GH in normal rats
To date, few papers have addressed the effects of chronic GH administration on cardiac structure in normal rats and demonstrated mild LV-hypertrophy without significant fibrosis along with a modest increase of systolic and diastolic function [7, 23]. We investigated the effects of 0, 2, 5 and 10 IU GH/kg/day in normal rats and found a nearly 50% increase in cardiomyocyte width following treatment with 2 IU GH/kg/day. Heart size moderately increased and LV function remained unaltered. There was a minor inconsistency regarding the increases of the LV/BW ratio, compared to cardiomyocyte width and echocardiographically determined posterior wall thickness, as indices of LVH. However, our findings, including a 15% increase in LV/BW, are in accordance with a recent study by Cittadini et al. [23], who observed a 17% increase of LV/BW ratio, compared to a 40% increase of LV/tibial length. Additionally, we provide detailed data on ECM proteins. In particular, we observed a significant accumulation of laminin, an ECM protein with important growth-modulating properties, that paralleled cardiomyocyte hypertrophy [17, 24, 25]. Major structural ECM proteins, like fibronectin and collagen I, were only slightly, but not significantly, increased. However, following the administration of doses higher than 2 IU GH/kg/day, the effects on cardiac geometry and function became less favorable. This is in contrast to a recent paper by Cittadini [11], who reported beneficial effects on cardiac structure and function following the administration of 3.5 mg GH/kg/day, which compares to our 10 IU dose. Neither our hemodynamic nor our morphometric findings offer a conclusive explanation for the difference. Further studies are necessary to definitively clarify a dose-dependency. They should also include smaller doses that have not yet been investigated.

Taken together, we were able to identify a dose of GH that induced considerable cardiomyocyte hypertrophy without causing any fibrosis or unfavorable hemodynamic effects under normal conditions.

4.2 Effects of GH on remodeling following experimental MI
Our data document the beneficial effects of GH on the remodeling process following experimental MI. In particular, in the case of a large anterior MI, there was a reduction in LV dilatation and a considerable improvement in LV function. Additional hypertrophy of the non-infarcted areas could be demonstrated by morphometric assessment of cardiomyocyte width. In parallel, laminin was enhanced by GH treatment. The role of laminin as a growth-modulator was further strengthened by the excellent positive correlation with cardiomyocyte width. This finding is in agreement with our previous observations in isoproterenol-induced cardiac injury in the rat [17]. Furthermore, we found a consistent 40–50% reduction in the accumulation of collagen I and fibronectin. The latter colocalizes with fibrillar collagens I and III and shares, with laminin, important interactions with the entactin/nidogen system, providing structural support and potentially complex growth regulatory functions [24, 25]. After MI, the heart consists of infarcted and non-infarcted areas, where structural ECM proteins may have quite different effects. In infarcted regions, collagen and fibronectin are very important for preserving an organ's structural integrity and thereby function, a process usually called reparative fibrosis. On the other hand, adaptive fibrosis may typically occur as a consequence of pressure-overload. Indeed, following a large MI, the remaining non-infarcted myocardium is subjected to increased hemodynamic stress [5]. Accumulation of ECM proteins in these regions, as typically present in our study, however, may further impair diastolic function, resulting in a detrimental course. By applying automatic image analysis to the non-infarcted posterior wall, we demonstrated, in MI rats treated with GH compared to vehicle, a further 22% increase in cardiomyocyte width along with a 40% reduction in fibronectin and a 50% reduction in collagen I. The combination of bigger cardiomyocytes and reduced fibrosis represents an ideal pattern of cardiac hypertrophy, which should improve hemodynamics. Indeed, both diastolic LV function, as assessed by LV end-diastolic pressure and isovolumic relaxation time, and systolic LV function, as assessed by echocardiographically determined fractional shortening, significantly improved following treatment with GH.

Whether or not the observed effects of GH on cardiomyocytes were mediated by IGF-1 cannot be delineated from our data. However, our study was not designed to elucidate the underlying mechanisms. IGF-1 levels were significantly elevated after treatment with GH, as was reported by others [11, 26]. GH stimulates IGF-1 production in either a paracrine manner at the target organ or through a distal endocrine mechanism by IGF-1 production at peripheral tissues [27]. However, GH has its own receptors in tissues such as the heart, liver, kidney, lung and testis and may act independently [28]. The deposition of laminin paralleled cardiomyocyte hypertrophy, as was previously reported in another experimental setting [17]. Laminin, the major ECM protein of the basement membrane, is suggested to affect cell growth, differentiation and migration [24, 29, 30]. Whether or not its expression was directly influenced by GH cannot be deferred from our data. Furthermore, our study provides excellent evidence, for the first time, that GH significantly blunts adaptive fibrosis, as may be delineated from a nearly 50% reduction of structural ECM proteins in non-infarcted areas. The potential beneficial effects of that finding on the diastolic function of an already diseased ventricle are obvious. However, we cannot delineate the underlying mechanism from our data. There are at least two possible explanations: (1) GH or IGF-1 directly reduce synthesis or increase the breakdown of collagen and fibronectin. (2) Early treatment with GH improves regional cardiomyocyte hypertrophy, which, in turn, reduces elevated diastolic wall stress [5]. As a consequence, improved local and/or global hemodynamics induce less accumulation of structural ECM proteins. Because the contents of collagen I and fibronectin were unchanged in normal rats, improved hemodynamics in MI rats treated with GH seem to play an important role. Nevertheless, repeated assessments of all involved parameters would be necessary to clarify the mechanism. Although the renin–angiotensin system might be involved in pressure-induced LV remodeling, our finding of an unaltered cardiac ACE activity in the non-infarcted myocardium does not suggest an important contribution [17, 31]. Further studies are necessary to clarify the interplay between GH and angiotensin II, in particular, in the setting of remodeling following MI for several reasons: (1) angiotensin II has been shown to induce apoptosis, (2) apoptosis has been documented in remote non-infarcted areas, (3) cardiomyocyte number was found to be reduced in remote non-infarcted areas following experimental MI and (4) IGF-1 overexpression was able to attenuate myocyte death in non-infarcted areas [5, 32–34].

Plasma levels of ANP showed progressive elevation in parallel with MI size, which was moderately blunted (n.s.) by treatment with GH in the case of a large MI. This was probably due to the observed overall hemodynamic improvement.

4.3 Study limitations
(1) Anaesthesia has at least some influence on arterial, venous and intracardiac pressures. (2) Handling-induced postmortem changes may alter dimensions of tissues. However, because all groups were consistently handled, intergroup differences may be valid. (3) Morphometry assessed cardiomyocyte width but not length. However, diverging changes of cell width and length have not been reported following pharmacological interventions. Therefore, we assumed cell width to be a reliable marker of cardiomyocyte hypertrophy in our study.

4.4 Clinical implications
The particular pattern of beneficial structural effects is unique for GH and may have an important clinical impact. Therapeutical options in patients with impaired LV function include the administration of ACE inhibitors and β-adrenoceptor blockers [35–37]. Their use ameliorates pathological remodeling, improves symptoms and LV function and, most importantly, prolongs survival. Beneficial effects are attributed to a sustained attenuation of neurohormonal stimulation and unloading of the heart. Interestingly, clinical endpoints are effectively reduced in those patients in whom pathological remodeling can be attenuated [6, 38]. In this scenario, GH, with its distinct effects on cardiomyocyte and ECM protein remodeling, might become a, possibly temporary, means of further improving cardiac function.

Time for primary review 21 days.


   Acknowledgements
 
This study was supported by a grant from Serono Pharma GmbH, Munich, Germany (Study-No.: GF 7198). The study was presented in part at the 70th Scientific Sessions of the American Heart Association in November 1997 in Orlando, FL, USA.


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

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