High glucose induces cardiac insulin-like growth factor I resistance in ventricular myocytes: role of Akt and ERK activation

Objective: Cardiac resistance to IGF-I occurs in diabetes and is attributed to cardiac dysfunction in diabetes. However, the mechanism of action responsible for cardiac lGF-I resistance is Still unknown. This study was designed to examine the impact of high glucose on lGF-1-induced contractile response and activation of serine-threonine kinase Akt as well as extracellular signal-regulated kinase (ERK1/2) in cardiac myocytes. Methods: Isolated adult rat vermicular myocytes were Cultured for 12-18 h in a serum-free medium containing either normal (NG. 5.5 mM) or high (HG, 25.5 HIM) glucose. Mechanical properties were evaluated using an lonOptix MyoCam(R) system. Myocytes were electrically stimulated at 0.5 Hz and contractile properties analyzed included peak shortening (PS), time-to-PS (TPS) and time-to-90% relengthening (TR90). Intracellular Ca2+-induced Ca2+ release was measured as fura-2 fluorescence intensity change (DeltaFFI). Protein levels of total and phosphorylated Akt and ERK1/2, indicators of Akt and ERK1/2 activation, IGF- I receptors (pro-IGF-IR and IGF-IRalpha) as well as the glucose transporter GLUT4 were assessed by Western blot. Results: IGF-I (10(-10)-10(-6) M) elicited a close-dependent increase in PS and DeltaFF1 in myocytes maintained in NG medium. However, IGF-I induced a negative response on PS and DeltaFFI in HG myocytes. The lGF-1-induced responses in NG or HG myocytes were blunted by the IGF-I receptor antagonist H-1356. Western blot analysis revealed that IGF-1Ralpha but not pro-IGF-IR was reduced in HG myocytes. While IGF-I 10(-6) M) upregulated total Akt protein levels in both NG and HG myocytes, it only induced a significant activation of Akt in NG but not HG myocytes. IGF-I elicited comparable ERK1/2 activation in both NG and HG inyocytes. Conclusion: These results suggest that the cardiac IGF-I resistance in diabetes is likely attributed, at least in part. to reduced IGF-IR and attenuated IGF-I-induced Akt phosphorylation under elevated extracellular glucose. (C) 2003 European Society of Cardiology. Published by Elsevier Science B.V. All rights reserved.