Insulin signaling in the heart is impaired by growth hormone: a direct and early event.

Growth hormone (GH) exerts major actions in cardiac growth and metabolism. Considering the important role of insulin in the heart and the well-established anti-insulin effects of GH, cardiac insulin resistance may play a role in the cardiopathology observed in acromegalic patients. As conditions of prolonged exposure to GH are associated with a concomitant increase of circulating GH, IGF-1 and insulin levels, to dissect the direct effects of GH, in this study we evaluated the activation of insulin signaling in the heart using four different models: 1) transgenic mice overexpressing GH, with chronically elevated GH, IGF-1 and insulin circulating levels, 2) liver IGF-1-deficient mice, with chronically elevated GH and insulin but decreased IGF-1 circulating levels, 3) mice treated with GH for a short period of time, and 4) primary culture of rat cardiomyocytes incubated with GH. Despite the differences in the development of cardiomegaly and in the metabolic alterations among the three experimental mouse models analysed, exposure to GH was consistently associated with a decreased response to acute insulin stimulation in the heart at the receptor level and through the PI3K/Akt pathway. Moreover, a blunted response to insulin stimulation of this signaling pathway was also observed in cultured cardiomyocytes of neonatal rats incubated with GH. Therefore, the key novel finding of this work is that impairment of insulin signaling in the heart is a direct and early event observed as a consequence of exposure to GH, which may play a major role in the development of cardiac pathology.