IGF-I alleviates diabetes-induced RhoA activation, eNOS uncoupling, and myocardial dysfunction

IGF-I rescues diabetic heart defects and oxidative stress, although the underlying mechanism of action remains poorly understood. This study was designed to delineate the beneficial effects of IGF-I with a focus on RhoA, Akt, and eNOS coupling. Echocardiography was performed in normal or diabetic Friend Virus-B type (FVB) and IGF-I transgenic mice. Cardiomyocyte contractile properties were evaluated using peak shortening (PS), time-to-90% relengthening ([TR.sub.90]), and intracellular [Ca.sup.2+] rise and decay. Diabetes reduced fraction shortening, PS, and intracellular [Ca.sup.2+]; it increased chamber size, prolonged [TR.sub.90], and intracellular [Ca.sup.2+] decay. Levels of RhoA mRNA, active RhoA, and [O.sup.-.sub.2] were elevated, whereas nitric oxide (NO) levels were reduced in diabetes. Diabetes-induced [O.sup.-.sub.2] accumulation was ablated by the NO synthase (NOS) inhibitor nitro-Larginine methyl ester (L-NAME), indicating endothelial NOS (eNOS) uncoupling, all of which except heart size were negated by IGF-I. The IGF-I-elicited beneficial effects were mimicked by the Rho kinase inhibitor Y27632 and BH4. Diabetes depressed expression of [Kv.sub.1.2] and dihydrofolate reductase (DHFR), increased [beta]-myosin heavy-chain expression, stimulated p38 MAPK, and reduced levels of total Akt and phosphorylated Akt/eNOS, all of which with the exception of myosin heavy chain were attenuated by IGF-I. In addition, Y27632 and the eNOS coupler folate abrogated glucose toxicity-induced PS decline, [TR.sub.90] prolongation, while it increased [O.sup.-.sub.2] and decreased NO and [Kv.sub.1.2] levels. The DHFR inhibitor methotrexate impaired myocyte function, NO/[O.sup.-.sub.2] balance, and rescued Y27632-induced cardiac protection. These results revealed that IGF-I benefits diabetic hearts via Rho inhibition and antagonism of diabetes-induced decrease in pAkt, eNOS uncoupling, and [K.sup.+] channel expression. heart; Akt; [K.sup.+] channel; cardiomyocytes; nitric oxide; intracellular [Ca.sup.2+]