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Charles A. Dana Research Institute and the Harvard-Thorndike Laboratory, Cardiovascular Division, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School (A.C., H.S., J.D.G., S.E.K., J.P.M., P.S.D.), Boston, Massachusetts 02215; New England Regional Primate Research Center (D.E.V.), Southborough, Massachusetts 01772; and Genentech (R.C.), South San Francisco, California 94080
Address all correspondence and requests for reprints to: Pamela S. Douglas, M.D., Cardiovascular Division, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, Massachusetts 02215. E-mail: pdouglas{at}bidmc.harvard.edu
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In summary, myocyte size, cardiac structure, myocardial contractility, and distensibility are abnormal in GH deficiency. The effects of GH are not mediated by the ß-adrenergic pathway, which, in turn, is unaffected by changes in the GH-insulin-like growth factor I axis. Thus, GH plays a regulatory role in normal cardiac physiology that is independent of the ß-adrenergic system.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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We (7, 8) and other groups (9) have recently demonstrated that both cellular hypertrophy and increased calcium responsiveness of myofilaments may contribute to the positive inotropic effects of GH. The possibility of other mechanisms involved in GH action has been poorly explored. Among these is an interrelationship between GH and the ß-adrenergic system that is suggested by the low heart rate response to exercise observed in GH-deficient humans (10), the increased heart rate and the occurrence of palpitations during IGF-I infusions (11), and the histological appearance of the myocardium in acromegaly, which is similar to that occurring after chronic sympathetic overactivity (12). Furthermore, long term GH administration increases epinephrine sensitivity in adipose tissue (13, 14). Two studies have confirmed such an interrelationship recently in animal models of GH and IGF-I deficiency. An initial study by Brown et al. has shown that ß-adrenoceptor density is significantly increased in GH-deficient rats (15). More recently, Lembo et al. documented enhanced adenylyl cyclase activity with unchanged ß-adrenergic receptor density in a transgenic mouse model of IGF-I deficiency with elevated serum GH levels (16). Considering the well known, long term, negative effects of activation of the adrenergic system in heart failure, a further clarification of this issue appears particularly relevant in view of the proposed therapeutic use of GH in dilated cardiomyopathy (6).
To address these issues, we investigated rats with a specific isolated pituitary GH deficiency, which resembles human GH deficiency, a disease in which cardiac atrophy and impaired left ventricular (LV) dysfunction have been documented consistently (10, 17, 18). Dwarf rats represent a unique model to study the physiological role of GH in the absence of other pituitary abnormalities (19), which has yielded important information concerning the roles of GH and IGF-I in somatic growth (20). Nevertheless, cardiac structure and function have not been investigated systematically in dwarf rats. Accordingly, we undertook a complete assessment of cardiac changes associated with GH deficiency and its replacement. Such studies have been hampered in human GH deficiency due to changing loading conditions associated with GH treatment.
Therefore, the aims of the present study were 2-fold: 1) to assess the regulatory role of GH by evaluating the effects on cardiac structure and function of GH deprivation and replacement, and 2) to investigate the interrelationships between GH and the ß-adrenergic system, specifically to evaluate the activity and the responsiveness of the ß-adrenergic pathway in GH-deficient rats before and after GH replacement therapy and to determine the effects of ß-blockade in modulating the effects of GH replacement on cardiac growth and restoration of LV function.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Hemodynamic studies
Rats were anesthetized with ketamine HCl (50 mg/kg) and xylazine
(10 mg/kg). A calibrated 2 French micromanometer-tipped catheter
(Millar Instruments, Houston, TX) was passed into the carotid artery,
and aortic pressure was recorded on a strip-chart recorder (model 2400,
Gould, Cleveland, OH), with the high frequency filter cut-off set at
300 Hz.
Perfusion technique
At 3 weeks, 10 rats from each group were killed, and the
isolated hearts were placed in a isovolumic, buffer-perfused rat heart
preparation according to the Langendorff technique, which allows
assessment of intrinsic ventricular performance independent of systemic
influences (22). The person performing the experiments was blinded as
to the groups in which the rats belonged. Briefly, the rats were
anesthetized with ether, followed by an injection of 200 IU heparin in
the femoral vein. One minute later, the thorax was opened, and the
heart was quickly removed, put into ice-cold Krebs-Henseleit solution
(see below), weighed, and mounted on a cannula inserted into the
ascending aorta. Retrograde aortic perfusion of the coronary arteries
was performed within 30 sec after thoracotomy via a constant flow of 10
ml/min·g heart wt, and the pressure was monitored by a Statham P23Db
transducer (Gould, Cleveland, OH). This flow rate was chosen because
preliminary experiments with graded ischemia yielded an aerobic pattern
of lactate consumption measured according to the method of Apstein
et al. (23). Cardiac temperature was set at 25 C, measured
by a temperature probe inserted into the right ventricle (RV). The
composition of the perfusate was as follows: 118 mM NaCl,
4.7 mM KCl, 1.2 mM
KH2PO4, 1.5 mM CaCl2,
1.2 mM MgCl2, 23 mM
NaHCO3, and 5.5 mM dextrose, saturated with a
95% O2-5% CO2 gas mixture to a pH of 7.4
± 0.2. LV pressure was measured using a fluid-filled latex balloon
inserted into the left ventricle (LV) via the mitral valve. After an
equilibration period of 15–30 min at 25 C, temperature was gradually
increased to 30 C, and the hearts were finally paced at 2.5 Hz.
Measurements of LV function were obtained when the preparation achieved
a steady state after instrumentation (
15 min); the balloon volume
was then set to the lowest possible volume at which minimal LV pressure
tracings (<1 mm Hg) could be recorded. This volume was defined as the
zero volume (V0). The LV volume was then further increased
in steps of 10–20 µl. LV functional parameters were obtained 1–2
min after each increment of volume when a new steady state was reached.
The LV volume was increased up to a value (Volmax) at which
maximal peak-developed pressure was reached, and a further increase in
balloon volume led to a decrease in developed pressure. To achieve
comparable loading conditions (i.e. balloon volumes) in
hearts of different sizes, the LV parameter of interest was plotted
vs. Vol/Volmax. This method of normalization has
recently been validated in our laboratory by showing that control
hearts of different sizes reveal superimposable pressure volume curves
as well as superimposable relaxation and contractility curves once the
intracardiac balloon volume is normalized by Volmax (24).
Differences in LV functional parameters at balloon volume normalized by
Volmax can, therefore, be ascribed to intrinsic changes in
cardiac properties. As studying the whole heart function at 100% of
Volmax would represent a situation of unphysiological
preload, we compared function at 50% of Volmax. These
volumes are comparable to balloon volumes used by other researchers
working with the intact rat heart model (25, 26).
The digital signal of the LV pressure tracing was further analyzed
using customized software to obtain the following parameters: peak LV
systolic pressure, LV end-diastolic pressure, LV developed pressure,
time from peak systolic pressure to 90% of relaxation (T90), time
constant of exponential pressure decay (
) using the variable
asymptote method, and maximum and minimum values of the first pressure
derivative with respect to time. Wall thickness (h), relative wall
thickness (hr = h/r), peak systolic (
s), and
end-diastolic (d) circumferential wall stress were
derived from ventricular pressure measurements, balloon volume
(VB), and weight of the LV, as described by Brooks et
al. (25). Briefly, a spherical model was assumed in which the
radius of the LV cavity (Ri) can be calculated by the cubic
formula: Ri = 3
[VB/(4/3 x
)].
The total volume of the LV is the sum of VB (including V0) and the volume of the LV wall (VWall = LV weight/1.05, the specific gravity of myocardium).
Therefore, VB + VWall = (4/3) x x
(Ri + h)3, and h and hr can be
derived by h = 3{(VB +
VWall)/[(4/3) x ]} - Ri and
hr = h/Ri. LV circumferential wall stress is
then derived from LaPlace’s law using the relation described by Mirsky
(27): = P[(Ri2/h)/(2Ri + h)].
Developed wall stress (Dev ) was defined as: Dev =
s - d = DevP
[(Ri2/h)/(2Ri + h)].
In this manuscript, we use the term diastolic chamber distensibility to refer to the LV diastolic pressure relative to simultaneous volume, as some researchers restrict use of the terms stiffness and compliance to refer to the slope of the pressure-volume or stress-strain curve (28).
Isoproterenol response
After recording baseline mechanical parameters, isoproterenol
was added to the perfusate in four steps, achieving concentrations of
10-8, 10-7, 10-6, and
10-5 M; each step lasted approximately 5–7
min. Inotropic and lusitropic parameters of function [peak positive,
peak negative rate of change of pressure (dP/dt) normalized to
developed pressure, and time to 90% of relaxation] were recorded
before and during continuous perfusion with isoproterenol.
Isoproterenol-induced peak responses were expressed as percent changes
related to baseline. EC50 of the dose-response curve was
obtained for each parameter (see below).
After in vivo or in vitro hemodynamic evaluation, hearts were rapidly removed, and the free walls of the RV and LV were dissected, blotted, and weighed.
Blood analysis
Blood samples were obtained at the time of death, 8–12 h after
the last GH injection. Duplicate hematocrit samples were prepared in
microhematocrit tubes. Serum was prepared from the remainder of the
sample and frozen at -20 C for subsequent analysis. Human GH was
measured in rat serum by enzyme-linked immunosorbent assay, and total
serum IGF-I was measured by RIA, according to previously described
methods (29, 30).
Histology
The tissues were immersion fixed in 10% buffered formalin.
Specimens for histological examination were obtained from each heart,
which was cut in cross-section at four levels from apex to base. The
samples were embedded in paraffin and stained with hematoxylin-eosin
for measurements of muscle fiber diameter and with Masson Trichrome for
assessment of interstitial fibrosis. Quantitative evaluation was
carried out by morphometry, according to previously described methods
(31). Muscle fiber diameter was evaluated by direct measurements at a
magnification of x400, using only cross-sections that included a
nuclear profile. A total of 100 cells/animal were evaluated.
Quantitative assessment of interstitial tissue as a measure of fibrosis
was accomplished using a special grid with horizontal and vertical
lines, providing 100 intersection points at x400 magnification. Four
fields at each of the four sample sites were examined for each animal,
yielding a total of 16 fields in each rat. Reproducibility studies
showed a good correlation between data obtained at 2 studies in 4 rats
for both techniques, with an r value of 0.90.
Tibial length
At the end of the experiment, a right hind leg was removed from
the rat by disarticulating the femur from the acetabulum at the hip.
The tibias were dissected free of soft tissue and frozen at -20 C.
Four radiographic films (X-Omat XTL2, Eastman Kodak, Rochester, NY) of
the tibias were then obtained, and the tibial length of each animal was
assessed with a caliper from the radiograph.
ß-Adrenergic receptor studies
Membrane preparation. Five rats from each treatment group
were studied. After the rats were anesthetized with sodium
pentobarbital, their hearts were immediately excised and placed in iced
saline. The LV and septum were minced and homogenized in 4 ml Tris
buffer/g tissue (100 mM Tris and 1 mM EGTA, pH
7.2), using a Polytron S-20 for 15 sec at a setting of 6. The
suspension was centrifuged at 14,500 x g for 15 min.
The supernatant was discarded, and the pellet was resuspended in Tris
buffer. The tissue was homogenized and centrifuged two more times as
described above. Then the pellet was resuspended in Tris buffer. The
suspension was filtered through one layer of silk screen (size 14),
then the pellet was centrifuged at 14,500 x g for 15
min and stored at -70 C.
ß-Adrenergic receptor antagonist binding studies.
ß-Adrenergic receptor antagonist binding studies were performed using
[125I]cyanopindolol (0.01–0.40 nM),
isoproterenol (100 µM), or Tris buffer and membrane
protein (15 µg/assay). Assays were performed in a 150-µl final
volume in triplicate, incubated at 37 C for 45 min, terminated by rapid
filtration on Whatman GF/C filters (Clifton, NJ), and counted in a
Tracor (TM Analytic, Inc., Elk Grove Village, IL) 
-counter for 1
min. The binding data were analyzed by the interactive Ligand program
of Munson and Rodbard (32). A linear regression was performed on the
amount bound vs. bound/free ligand. An r2 value
of 0.85 or greater was the criterion used for acceptability of the
data.
Adelylyl cyclase activity. Adenylyl cyclase activity was
assayed according to the method of Salomon et al. (33), as
previously described (34). Cardiac membranes (50 µg protein) were
added to a solution containing 3 mM ATP (1–2 x
106 cpm [
-32P]ATP), 20 mM
creatine phosphate, creatine phosphokinase (1 U), 1 mM cAMP
(4000–5000 cpm [3H]cAMP as an internal standard), 100
mM Tris, 15 mM MgCl2, 1
mM EGTA, and the test substance to measure adelylyl cyclase
activity [GTP (0.05 mM), isoproterenol (0.1
mM), 5'-guanylylimidodiphosphate (0.05 mM), NaF
(10 mM), and forskolin (0.5 mM)]. Ten
microliters of stopping solution (2% SDS) were added to each tube to
terminate the reaction (33, 34). cAMP was separated as previously
described, using Dowex (Bio-Rad Laboratories, Hercules, CA) and then
alumina columns (33). The recovery of added cAMP was 70–80%.
Na+,K+-adenosine triphosphatase (Na+,K+-ATPase) activity. Na,K-ATPase activity was determined according to the method of Jones and Besch (35). The protein concentration for each sample was determined by the method of Lowry et al. (36).
In summary, after 3 weeks of treatment, 10 rats of 20 in each group were randomly chosen for in vivo hemodynamic studies, whereas the other 10 rats from each group underwent perfusion studies. Subsequent histological examination was performed on 7 animals/group, and studies of the ß-adrenergic pathway was carried out on hearts from 5 rats/group. Blood work, including hematocrit, was carried out in each animal.
Statistics
All values are shown as the mean ± SE.
Comparisons were performed using one-way ANOVA. Where appropriate,
comparisons to determine the significance of changes within the same
group over time and between groups at each time point were performed
with the Newman-Keuls test after testing the samples for normal
distribution. To determine the sensitivity (EC50) and the
maximal isoproterenol response (
%max), the percent
change in the respective parameter, %, was plotted against the
negative logarithm of the isoproterenol concentration in the buffer
(pIso) and fitted to the function: % = %max/(1 +
10(a - b x pIso)) with EC50 = a/b,
using nonlinear regression. A probability of P < 0.05
was considered significant.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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). GH treatment restored both indexes to
control values. Human GH levels, undetectable in the untreated control
and dwarf groups, were 60 and 80 ng/ml in both the dwarf-GH and
dwarf-GH-propranolol groups, respectively. Single determinations of rat
GH levels were not different among the four groups, a finding not
unexpected because of large daily variations in GH levels. Plasma IGF-I
levels, a better index of GH activity, were significantly decreased in
the dwarf group compared with those in normal controls. GH replacement
therapy raised IGF-I to values similar to those in the control group.
Propranolol treatment did not significantly affect either GH or IGF-I
plasma levels. Hematocrit did not differ among the four study
groups.
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). GH treatment restored both LV
and RV weight normalized to tibial length and myocyte area to control
values. The percentage of interstitial tissue as a measure of fibrosis
was not different among the study groups. Propranolol did not affect RV
or LV growth or the increase in cell size due to GH treatment.
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In the isolated whole heart at 50% of maximum velocity
(Vmax), systolic function was significantly impaired in the
dwarf group, as demonstrated by lower values of both developed pressure
and developed wall stress compared with those in the control group
(Table 3). The differences tended to be
more significant at balloon volumes higher than 50% of
Vmax (Fig. 1, A and B).
Diastolic pressure at 50% of Vmax was significantly higher
in the dwarf group than in Lewis control rats, demonstrating a reduced
LV diastolic distensibility. This is shown in Fig. 2, in which the entire normalized
pressure-volume curve of the dwarf group is shifted upward and
leftward. Time to 90% relaxation and (Table 3) were not different
among the study groups.
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). Similarly, the
maximal increments of increase produced by isoproterenol in peak
positive and peak negative dP/dt normalized to developed pressure were
significantly lower in the dwarf rats compared with those in controls.
Treatment with GH, with or without propranolol, normalized both
responses to isoproterenol. Thus, the positive inotropic and lusitropic
(i.e. enhancement of diastolic relaxation) responses to
isoproterenol were compromised in GH-deficient hearts.
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). Therefore, neither down-regulation of
ß-adrenergic receptor density nor alterations at the level of
receptor coupling and/or adenylyl cyclase activity were present in GH
deficiency.
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Discussion |
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GH’s modulation of cardiac structure and function
GH-deficient rats demonstrate reduced myocyte area and LV weight
normalized to tibial length, both pointing to the presence of relative
cardiac atrophy in this condition. These data substantiate our previous
findings of reduced LV mass index in GH-deficient humans (18, 19), and
for the first time show that the cardiac atrophy, already demonstrated
at the organ level, is at least partly due to reduced myocyte size. The
increase in LV mass observed in conditions of GH excess supports the
concept of GH’s trophic effects on the mammalian myocardium (7, 36, 37). A signaling pathway potentially responsible for these actions is
provided by the demonstration of cardiac GH and IGF-I receptors (38, 39) and by exogenous GH and IGF-I stimulation of protein synthesis in
the isolated whole heart (40) and the isolated cardiomyocyte (41).
Several studies performed under conditions of GH excess (7, 37, 42) and deficiency (10) have suggested that GH affects systolic and diastolic performance, although GH’s effects on loading conditions (reduced afterload and increased preload) qualify these findings. This study shows that GH has distinct effects on LV contractility and diastolic distensibility, congruent with previous reports from our and other groups that GH excess increases contractility in the isolated whole heart and in papillary muscle (7, 8, 9). Changes in calcium handling, in particular in the responsiveness of the myofilament apparatus to calcium, may be likely mechanisms by which GH mediates its effects on contractility (8, 9). Another plausible mechanism is the cardiac trophic effects of GH, with atrophy caused by GH per se being responsible for the functional impairment, and there being less contractile tissue available for force development.
Although diastolic distensibility is decreased in GH-deficient rats
(leftward shift of pressure-volume relationship), relaxation parameters
such as time to 90% relaxation and especially were not
significantly impaired in the dwarf group. Therefore, considering
that changes in the pressure-volume relationship reflect late diastolic
events (43), we can speculate that the diastolic abnormalities of the
dwarf rats are not due to impairment of active processes, such as
calcium uptake by the sarcoplasmic reticulum and calcium dissociation
from troponin (probably reflected by increased values of ), but are
caused by changes in the passive properties of the ventricle and/or
pericardium. The morphometric histology excludes an increase in
fibrotic tissue as a significant component of such altered diastolic
distensibility.
Taken together, our data provide strong evidence to support the concept that GH modulates cardiac structure and function and suggest the existence of a dynamic continuum from GH deficiency, characterized by cardiac atrophy and systolic and diastolic dysfunction to conditions of GH excess that involve cardiac hypertrophy and enhanced function.
Reversibility
It is important that the cardiac structural and functional
abnormalities observed in GH-deficient rats were fully reversible after
GH treatment. These alterations are, therefore, specifically related to
GH absence, as the secretion of the other pituitary hormones is normal
in dwarf rats, and the abnormalities were normalized by replacement of
GH alone. Secondly, even though in this animal model GH deficiency
occurs early in development (19), the cardiac abnormalities are still
reversible in adult life. This observation is in concert with previous
evidence obtained in childhood-onset human GH deficiency, where GH
replacement therapy in adulthood reversed the cardiac structural and
functional abnormalities. Therefore, although GH appears to have a
fundamental regulatory role in maintaining normal cardiac structure and
function throughout the life span, the abnormalities caused by its
absence appear plastic and reversible. Such a continued ability to
affect cardiac growth and performance throughout adulthood increases
the potential usefulness of GH as a therapeutic agent.
GH and ß-adrenergic system
A link between GH and the ß-adrenergic system is suggested by
the following observations: 1) GH-deficient humans have a low resting
heart rate that shows a reduced response to physical exercise (10); 2)
acute IGF-I infusions cause tachycardia, a side-effect of treatment in
diabetic patients (11); and 3) experimental studies performed in
lactating cows and humans demonstrate that GH excess increases the
sensitivity of adipose tissue to ß-adrenergic agonists in
vivo and in vitro (13, 14). It is possible that
GH-deficient states reduce ß-adrenergic activation or decrease tissue
sensitivity to ß-adrenergic stimulation, whereas GH excess may have
the opposite effect. Nevertheless, few studies have examined whether
changes in the degree of activation of the GH-IGF-I axis affect the
ß-adrenergic pathway, or whether the effects of GH on the heart might
be partially mediated by ß-adrenergic activation. These issues are
particularly relevant considering the pivotal role of the adrenergic
system in the regulation of cardiac function and the recent
demonstration of GH’s beneficial therapeutic effects in humans and
animals with heart failure (2, 3, 4, 5, 6).
Our data show that GH deficiency does not alter ß-receptor density, affinity, or coupling with the adenylyl cyclase system. Therefore, GH does not appear to play a regulatory role in the ß-adrenergic pathway. Further, GH replacement therapy did not affect ß-adrenergic receptor density or affinity, or the activity of the adenylyl cyclase system. Therefore, the suggestion that some of the effects of GH on cardiac structure, in particular its positive inotropic actions, are mediated by the ß-adrenergic receptors appears unlikely, as ß-blockade with propranolol did not prevent or modify any of GH’s cardiovascular effects. The decreased responsiveness to isoproterenol observed in the present study in the whole heart could be due to a nonspecific decreased response of the myofilament contractile apparatus to an inotropic stimulus. In other words, the site of the defect appears to be beyond the ß-adrenergic receptor-effector complex. This possibility is in concert with the previous demonstration of changes in the myofilament sensitivity to Ca2+ occurring in states of GH excess together with a possible change in the phenotype of the myocardium induced by long term GH/IGF-I excess (8, 9) and with our recent demonstration of reduced myofilament responsiveness in the same model of GH deficiency (44).
Our findings are not congruent with those of two previous studies addressing the same topic. A recent investigation by Lembo et al. (16) found unchanged ß-adrenergic receptor density but enhanced adenylyl cyclase activity in mice with severe IGF-I deficiency; this is thought to be a likely explanation for the increased in vivo cardiac function documented in the same model. The researchers speculated that conditions of GH deficiency may enhance ß-adrenergic signaling, whereas GH/IGF-I excess may inhibit the ß-adrenergic pathway. The most likely explanation for this discrepancy is that the model of IGF-I deficiency used by Lembo et al. is substantially different from that used in the current investigation, including the animal studied, and also may not represent a pure IGF-I-deficient state. Although serum IGF-I levels were reduced by 69%, circulating GH levels were increased by 375%, whereas IGF-I tissue levels were not provided. As it is well established that GH has its own receptors in the heart and has differential effects in several target tissues, including the heart (7), it is reasonable to speculate that the differences observed in the phenotype may be due to either direct GH effects or paracrine and/or autocrine IGF-I actions.
It appears more difficult to reconcile our results with those obtained by Brown et al. in males (not females) of the same model of GH deficiency employed in the current study (15). In fact, they found increased ventricular weight normalized to body weight, enhanced basal force of contraction in vitro, unchanged chronotropic responses in vivo to isoprenaline, and increased ß-adrenergic receptor density in dwarf rats compared with Lewis controls. The differences in animal gender and experimental conditions used in the two studies may in part account for the discrepancies observed. For instance, Brown et al. used papillary muscle to assess in vitro function and tested isoproterenol responsiveness in vivo. However, our results are more congruent with previous animal and human studies of GH’s cardiac effects (1, 2, 3, 4, 5, 6, 7, 8, 10, 17, 18). Brown et al. also evaluated ß-receptor density using receptor agonist competition curves rather than a full Scatchard analysis as we did. Further, it is difficult to compare the two studies, as a detailed description of the experimental conditions is not provided (15). Finally, the conclusions of Brown et al. were similar to ours, in that they believed that ß-adrenergic signal transduction changes were not important in GH deficiency.
Two additional considerations suggest that the ß-adrenergic system is not affected by GH: 1) our findings describing cardiac function in dwarf rats are consistent with those reported in human childhood-onset GH deficiency (17); and 2) the lack of changes in cardiac morphological and functional parameters in dwarf rats treated with ß-blockers parallels the absence of differences in ß-adrenergic receptor density, affinity, and coupling to the adenylyl cyclase activity in this group.
Limitations of the study
Structural and functional measures of cardiac parameters were
obtained in rats with different degrees of somatic growth due to GH
deficiency and GH replacement. The ideal method to normalize measures
of dimension and volume in rats of different sizes is presently
unknown; we presented unnormalized data and data normalized to both
tibial length and body weight. Although body weight is a common way to
correct biological parameters, tibial length may be more closely
proportional to lean body mass than to body weight (45), in particular
in GH deficiency where fat tissue distribution is altered (1), so we
(7, 8) and others (2, 4, 5) have used this approach.
The use of hypothermic perfusion conditions in the isolated whole heart experiments (30 C) can be viewed as a potential limitation of the study, in particular for the assessment of relaxation parameters, because hypothermia per se prolongs relaxation and may bring out disturbances not present under normothermic conditions. However, the absence of significant differences between the groups is reassuring. Formalin fixation is known to cause cell shrinkage; however, all groups were consistently handled, thus confirming the validity of intergroup differences.
Clinical implications
In addition to the delineation of cardiovascular physiology, our
findings support the alternative view (46), that long term GH
replacement therapy may be beneficial in adult patients. This idea is
in concert with the demonstrated increase in cardiovascular mortality
of hypopituitary humans ascribed to lack of GH replacement therapy
(47). In addition, some components of weakness and fatigue noted by
GH-deficient patients (10, 18, 19, 48, 49) may be secondary to the
systolic and diastolic abnormalities described and, therefore, may be
reversible.
The observation that GH improves cardiac performance and ventricular efficiency not only in rats with postinfarction heart failure (2, 3, 4, 5) but also in humans with dilated cardiomyopathy (6) suggests the future clinical use of GH outside of GH deficiency. Therefore, a better understanding of the effects of GH on the cardiovascular system is essential to delineating and justifying such therapeutic uses. In this respect, we have demonstrated that GH modulates normal cardiac growth as well as systolic and diastolic function; furthermore, changes in the GH-IGF-I axis do not influence the ß-adrenergic receptor pathway, which, in turn, does not appear to influence any of the cardiac effects of GH/IGF-I. Considering the well known, long term, negative effects of ß-adrenergic activation in heart failure, these latter observations are important indications supporting the potential of GH for treating this disease.
Conclusions
Using an animal model of GH deficiency, we provide evidence for an
ongoing regulatory role of GH in the modulation of cardiac structure,
systolic function, and diastolic distensibility. In addition, we show
that the cardiac effects of GH are not mediated by the ß-adrenergic
pathway, which, in turn, is unaffected by changes in GH-IGF-I axis
activation. These findings may provide mechanistic support for the
recent reports of beneficial effects of GH in heart failure. Future
research at the molecular and cellular levels is needed to further
elucidate the effects and mechanisms of action of GH on the heart.
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Footnotes |
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Received May 5, 1997.
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