Impaired cardiac function and IGF-I response in myocytes from calmodulin-diabetic mice: role of Akt and RhoA

This study characterized the cardiac contractile function and IGF-I response in a transgenic diabetic mouse model. Mechanical properties were evaluated in cardiac myocytes from OVE26 diabetic and FVB wild-type mice, including peak shortening (PS), time to PS (TPS), time to 90% relengthening (T[R.sub.90]) and maximal velocity of shortening/relengthening ([+ or -]dL/dt). Intracellular [Ca.sup.2+] was evaluated as [Ca.sup.2+]-induced [Ca.sup.2+] release [difference in fura 2 fluorescent intensity ([DELTA]FFI)] and fluorescence decay rate ([tau]). Sarco(endo)plasmic reticulum [Ca.sup.2+]-ATPase (SERCA)2a, phospholamban (PLB), [Na.sup.+]-[Ca.sup.2+] exchanger (NCX), GLUT4, and the serine-threonine kinase Akt were assessed by Western blot. RhoA and IGF-I/IGF-I receptor mRNA levels were determined by RT-PCR and Northern blot. OVE26 myocytes displayed decreased PS, [+ or -]dL/dt, and ([DELTA]FFI associated with prolonged TPS, T[R.sub.90], and [tau]. SERCA2a, NCX, and Akt activation were reduced, whereas PLB and RhoA were enhanced in OVE26 hearts. GLUT4 was unchanged. IGF-I enhanced PS and [DELTA]FFI in FVB but not OVE26 myocytes. IGF-I mRNA was increased, but IGF-I receptor mRNA was reduced in OVE26 hearts and livers. These results validate diabetic cardiomyopathy in OVE26 mice due to reduced SERCA2, NCX, IGF-I response, and Akt activation associated with enhanced RhoA level, suggesting a therapeutic potential for Akt and RhoA. diabetic mouse; ventricular myocyte; excitation-contraction coupling; insulin-like growth factor I; sarco(endo)plasmic reticulum [Ca.sup.2+]-ATPase; sodium-calcium exchanger