A preliminary study of growth hormone in the treatment of dilated cardiomyopathy

The role of hypertrophy and growth factors in heart failure

The physiologic trophic factors growth hormone (GH) and insulin-like growth factor 1 (IGF-1) generally increase body weight and cardiac mass proportionately, and several studies suggest that both growth factors cause vasodilation and increased myocardial contractility. Established clinical benefits of ACE inhibitors can be explained, at least in part, by inhibition of cell hypertrophy, lowered systemic vascular resistance (SVR) and afterload, leading to reduction of progressive left-ventricular (LV) enlargement. An alternative approach would be to administer IGF-1 or GH to stimulate compensatory hypertrophy and reduce afterload by their vasodilator action, as well as through potential favorable effects on myocardial contractility. In our initial study in the rat myocardial infarction (MI) model, when IGF-1 was administered early (at 2 days) post-MI and continued for 2 weeks, body weight (BW) increased and LV weight/BW remained unchanged, the LV end-diastolic volume (EDV) and stroke volume increased (but not when normalized to BW), and the LV ejection fraction increased in rats with large infarctions. These findings suggested a beneficial rather than detrimental effect of such treatment, and we then studied the action of combined IGF-1 and GH starting after infarct healing at 4-weeks' post-MI. BW increased substantially and LVEDV/BW was lower in treated rats than in control rats, suggesting relatively less LV dilation with little remodeling in this setting; IGF-1/GH increased the cardiac output by 46%, systemic vascular resistance (SVR) fell and the cardiac index (CI) was significantly elevated in treated rats with a large MI. Recently, others have used the rat MI model to study the effects of 2-weeks' of GH started at 4-weeks' post-MI, as well as IGF/GH for 2-weeks in rats treated with an ACE inhibitor for 3-month's post-MI. In both studies, in conscious treated rats the BW increased, LV/BW was not different compared to the control rats, but the CI increased, SVR fell, and estimated LV dP/dtmax was significantly augmented. Preliminary data in our laboratory suggest that beneficial effects may also occur with GH administration in the setting of chronic angiotensin II receptor blockade (losartan) after MI in the rat. Thus, growth factor therapy appears to have favorable effects in heart failure early and late after MI in the rat. Additional cardiac hypertrophy occurs early after MI, but the later beneficial effects appear to relate primarily to systemic vasodilation, improved cardiac output, and enhanced myocardial contractility.

Elevated blood pressure and enhanced myocardial contractility in mice with severe IGF-1 deficiency

To circumvent the embryonic lethality of a complete deficiency in insulin-like growth factor 1 (IGF-1), we generated mice homozygous for a site-specific insertional event that created a mutant IGF-1 allele (igf1m). These mice have IGF-1 levels 30% of wild type yet survive to adulthood, thereby allowing physiological analysis of the phenotype. Miniaturized catheterization technology revealed elevated conscious blood pressure in IGF-1(m/m) mice, and measurements of left ventricular contractility were increased. Adenylyl cyclase activity was enhanced in IGF-1(m/m) hearts, without an increase in beta-adrenergic receptor density, suggesting that crosstalk between IGF-1 and beta-adrenergic signaling pathways may mediate the increased contractility. The hypertrophic response of the left ventricular myocardium in response to aortic constriction, however, was preserved in IGF-1(m/m) mice. We conclude that chronic alterations in IGF-1 levels can selectively modulate blood pressure and left ventricular function, while not affecting adaptive myocardial hypertrophy in vivo.

Recombinant growth hormone: a new cardiovascular drug therapy

GH has an important role in normal cardiovascular physiologic functioning, working indirectly through effects on IGF-1. An excess or deficiency of GH causes an increased rate of cardiovascular disease, including cardiomyopathy. A relative GH deficiency in older subjects may also increase cardiovascular morbidity and mortality risk. In replacement doses, GH can enhance myocardial contractility; can decrease peripheral vascular resistance; and can reduce total cholesterol and LDL-cholesterol values and fibrinogen and PAI levels. These effects of GH, coupled with the ability to improve skeletal muscle function and reduce adiposity, make it an attractive treatment for patients with CHF and a potential maintenance drug for elderly people. Clinical trials, including studies with GHRH that may reduce the adverse effects of GH therapy, such as hyperglycemia and hypertension, are now in progress.