The effect of growth hormone on rat myocardial collagen.

Growth hormone (GH) can increase cardiac performance, but conditions with GH excess, such as acromegaly, are associated with hypertrophy and fibrosis of the heart. The aim of this study was to examine the effect of GH administration on rat myocardial collagen. Female rats were injected with GH (5 mg/kg/day) for 80 days. The weight of the right ventricle (RV) and the left ventricle (LV) was increased in the GH-treated group compared with the control group (P< 0.001). No differences in the ratio of heart weight/body weight or ventricle weight/body weight were found. The total amount of RV and LV collagen was increased in the GH-treated group (P< 0.001), but the collagen concentration was decreased (P< 0.001). Histomorphometry showed that the area fraction of collagen relative to myocytes remained unchanged. The composition of ventricular collagen in the GH-injected group did not differ from that of the control group concerning the relative amounts of collagen types I and III and pyridinoline, a mature collagen cross-link. We conclude that GH induced a substantial, but proportionate growth of the myocardium without formation of fibrosis. GH actually decreased the collagen concentration, and did not change the composition of myocardial collagen.