Introduction
Besides well-recognized effects such as promoting longitudinal bone growth in childhood and exerting a number of metabolic effects, growth hormone (GH) has an important role for the development of a normal heart [
1]. GH also has a major impact on maintaining the structure and function of the normal adult heart [
1,
2]. Apart from stimulating cardiac growth and possibly also contractility, GH/IGF-I interacts with the vascular system and has a role in the regulation of vascular tone and thereby peripheral resistance. Also central effects including modulation of sympathetic outflow contribute to regulation of peripheral resistance [
3].
The myocardium and vessels express IGF-I [
4‐
6] and functional receptors for both GH [
7‐
9] and IGF-I [
10,
11], and IGF-I production is up-regulated in response to GH [
5]. Thus, there are possibilities of direct actions of GH as well as endocrine or autocrine/paracrine effects of IGF-I on the cardiovascular system. However, although interaction of the GH/IGF-I axis and the cardiovascular system has been extensively studied, the relative importance of direct effects of GH and local and endocrine IGF-I remains unclear.
GH/IGF-I and contractility
The notion of GH and IGF-I as molecules endowed with stimulatory properties on myocardial contractility is interesting but so far, only demonstrated in experimental studies. In line with reports on cardiac hypertrophy, there are several in vitro studies demonstrating direct effects of IGF-I on intrinsic cardiac contractility [
77‐
79], while there is still so far no evidence of direct, IGF-I-independent effects of GH on cardiac contractility. However, if animals are treated with GH in vivo, allowing stimulation of IGF-I synthesis, subsequent in vitro assessment shows improved contractility [
80,
81]. Accordingly, decreased contractility has been demonstrated in dwarf rats with GH/IGF-I deficiency [
82‐
84]. In contrast, a paradoxical enhancement of cardiac contractility was observed in the IGF-I mutant mouse [
18].
At least three different mechanisms have been suggested for the GH/IGF-I to induce increased cardiac contractility: 1. altered intracellular Ca2+ transients, 2. increased sensitivity of myofilaments to Ca2+, and 3. a shift in myosin isoforms.
Regarding intracellular Ca-transients, IGF-I has been shown to acutely affect Ca
2+ currents within the cardiomyocyte, with increased peak Ca
2+ levels [
78,
85] and an altered time course of the current [
78] in association with increased contractility. Specifically, the activity of L-type Ca
2+-channels was acutely increased by IGF-I in vitro [
86]. In cardiomyocytes from acromegalic rats, the action potential duration was increased due to a decrease in density of a transient outward current carried by K
+, which, in turn prolongs the Ca
2+-influx through L-type Ca
2+-channels [
87]. In contrast to other in vitro studies, the acute increase of inotropy by IGF-I was associated with decreased peak Ca
2+ levels but increased Ca
2+ sensitivity of the contractile elements in isolated whole heart preparations [
79]. No influence of GH on Ca
2+ currents has been seen in acute settings [
79,
85], while after more long-term treatment in vivo, GH has been suggested to increase peak intracellular Ca
2+ levels measured ex vivo [
80,
81]. Accordingly, reduced peak intracellular Ca
2+ levels as well as slowed intracellular Ca
2+-clearing have been demonstrated in GH/IGF-I deficiency [
84], while others report peak intracellular Ca
2+ levels to be unchanged in GH/IGF-I deficiency [
83].
To date, little has been known about possible gene regulations involved in the action of GH/IGF-I in altering Ca
2+ handling. An up-regulation of sarcoplasmic reticulum ATPase (SERCA) levels has been suggested to contribute to the increased contractile function elicited by GH after myocardial infarction [
81] and in rapid pacing heart failure [
88], while another study [
89] has not been able to detect any change in SERCA expression. SERCA may increase contractility by enhancement of the so-called contractile reserve, i.e., the Ca
2+ storage within the sarcoplasmic reticulum, allowing higher peak Ca
2+ levels upon stimulation. Ueyama et al. [
90] also suggested that GH treatment in cardiomyopathic, but not normal, hamsters preserved cardiac ryanodine receptor density.
Myofilament Ca
2+-
sensitivity and myosin isoform shift. GH/IGF-I has been suggested to increase myofilament Ca
2+ sensitivity [
79,
84,
91] and maximum Ca
2+ activated force [
79,
80,
91]. However, data are conflicting, and others report unchanged [
85] or even decreased [
80] myofilament Ca
2+ sensitivity by GH/IGF-I. In dwarf rats, unchanged myofilament Ca
2+ sensitivity has been reported [
83,
84], although maximum Ca
2+ activated tension was less [
83]. In animal models of GH excess, a shift toward a myosin isoform with lower ATPase activity has been demonstrated, which may decrease the energy demand of the contractile process [
91,
92].
Taken together, available data suggest that GH/IGF-I may increase cardiac contractility through modulations of intracellular Ca2+ transients, myofilament Ca2+ sensitivity, and myosin isoform expression, although the findings depend upon different experimental settings and among studies. Besides regulation of ion channel activity, GH/IGF-I may also regulate the expression of ion channels. Solid evidence for increased contractility by GH and IGF-I is still lacking.
A potential therapeutic role for GH in heart failure?
Despite considerable advances in both medication strategies and use of medical devices in patients with heart failure in the last decades, the prognosis is still poor, and there is a continued interest to develop alternative or additional treatment modalities.
One of the first clinical publications to mention possible beneficial effects of GH in heart failure was the paper by Caidahl et al. [
37] in 1994, describing improvement in systolic function in GH-deficient patients treated with GH.
This triggered several research groups to study effects of GH and/or IGF-I in experimental models with states of impaired cardiac function. In an established rat model of congestive heart failure following ligation of the left coronary artery, GH and IGF-I have been found to increase stroke volume and cardiac output [
111,
112], also in the presence of ACE inhibition [
113]. GH treatment of rats with experimental myocardial infarction has also been found to improve myocardial bioenergetics [
114] and long-term survival [
115].
The first clinical studies regarding GH treatment in heart failure were limited to case reports [
116,
117] where GH administration dramatically improved cardiac function. A subsequent small open study of seven patients with idiopathic dilated cardiomyopathy and CHF without GH deficiency, who received GH treatment for 3 months, demonstrated considerable improvement of left ventricular ejection fraction, cardiac output, and exercise performance [
118]. Further studies have demonstrated beneficial effects in patients with CHF due to both ischemic and idiopathic dilated cardiomyopathy with improvements in hemodynamics when GH was added both as a maintenance therapy and as short-term infusion [
119,
120]. Moreover, another interesting observation was that patients with a low base-line serum IGF-I had less beneficial effects of an acute GH infusion [
121]. A later study showed that GH treatment decreased circulating levels of cytokines such as TNF-α and IL-6 and apoptotic agents such as FAS and its soluble ligand in patients with heart failure [
122].
Even though a more recent placebo-controlled trial on 16 patients with CHF showed correction of endothelial dysfunction and improved non-endothelium dependent vasodilation [
123], other randomized, placebo-controlled studies have failed so far to show any significant GH-mediated improvement of cardiac performance in patients with heart failure, despite significant increases in IGF-I [
124,
125]. However, in a follow-up study of the same patients in the former study, analyzed in more depth, Perrot et al. [
126], found a significant increase in left ventricular mass, which correlated with serum IGF-I. Moreover, there was a significant increase of ejection fraction in those patients that responded with higher serum IGF-I levels during GH treatment. Acevedo et al. [
127] performed a randomized controlled trial of 19 patients with daily GH administration for 8 weeks. However, at the end of treatment, there was no significant effect of GH on LVEF or peak oxygen consumption, although left ventricular mass was reported to be increased.
The prevalence of cardiac cachexia in patients with chronic congestive heart failure has been uncertain. However, in a recent study, it was estimated to be 10.5 % in a population of 238 patients with stable congestive heart failure [
128]. Importantly, cardiac cachexia in severe heart failure could lead to a number of endocrine disturbances [
129], including acquired GH resistance and may explain some of the diverse responses to GH therapy observed in different patients [
130].
The reasons for the lack of convincing positive effects of GH on cardiac function in these randomized trials in all likelihood depend upon several factors. Duration of treatment has been relatively short (2–3 months) compared to conventional heart failure trials, and the studies have so far included only a small number of patients, which may also contribute to lack of power and negative results. Encouragingly, a meta-analysis encompassing 12 clinical trials still suggested that GH treatment had beneficial effects on left ventricular geometry, ejection fraction, and exercise parameters, all correlated to increase in serum IGF-I levels [
131]. Hence, whether GH treatment will finally find a place in the treatment of heart failure remains to be established and need to be studied in larger placebo-controlled clinical trials.
A topic which has received increased attention during the last years is the endocrine status of heart failure patients. It has been suggested that multiple anabolic deficiencies are common in heart failure patients and that this may be associated with worse outcome [
132‐
134]. At present, the literature is inconsistent regarding the GH/IGF-I axis in heart failure describing both low levels of IGF-I [
133,
135,
136] and normal or even higher levels [
137]. Possible explanations for these diverging results may be due to heterogeneous heart failure patients and the use of different IGF-I assays. Even less is known about GH secretion in heart failure, although there are indications of disturbed secretion [
138]. A more recent study suggests that as many as 40 % may fulfill criteria for GH deficiency [
139].
Recently, a novel approach in the selection of heart failure patients to be treated with GH was taken by the group of Saccà and collaborators. Using a GH provocation test, only patients that fulfilled criteria for GH deficiency (
n = 56) were enrolled in a randomized, single blind study where they were treated with GH for 6 months. Here, GH had a number of beneficial effects compared to controls, including improved quality of life score, increased peak oxygen uptake, exercise duration, and flow-mediated vasodilation. Moreover, GH treatment leads to a moderate but significant increase in left ventricular ejection fraction and a reduction in circulating
N-terminal pro-brain natriuretic peptide levels [
139]. Future studies with more robust RCT design are needed to verify the validity of this approach of selecting heart failure patients for GH. However, preliminarily, results so far are very promising.