Transgenic overexpression of insulin-like growth factor I prevents streptozotocin-induced cardiac contractile dysfunction and β-adrenergic response in ventricular myocytes

Diabetic cardiomyopathy is characterized by cardiac dysfunction and altered level/function of insulin-like growth factor I (IGF-I). Both endogenous and exogenous IGF-I have been shown to effectively alleviate diabetes-induced cardiac dysfunction and oxidative stress. This study was designed to examine the effect of cardiac overexpression of IGF-I on streptozotocin (STZ)-induced cardiac contractile dysfunction in mouse myocytes. Both IGF-I heterozygous transgenic mice and their wild-type FVB littermates were made diabetic with a single injection of STZ (200 mg/kg, i.p.) and maintained for 2 weeks. The following mechanical indices were evaluated in ventricular myocytes: peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR₉₀) and maximal velocity of shortening/ relengthening (± dL/dt). Intracellular Ca²⁺ was evaluated as resting and peak intracellular Ca²⁺ levels, Ca²⁺-induced Ca²⁺ release and intracellular Ca²⁺ decay rate (tau). STZ led to hyperglycemia in FVB and IGF-I mice. STZ treatment prolonged TPS and TR₉₀, reduced Ca²⁺-induced Ca²⁺ release, increased resting intracellular Ca²⁺ levels and slowed tau associated with normal PS and ± dL/dt. All of which, except the elevated resting intracellular Ca²⁺, were prevented by the IGF-I transgene. In addition, myocytes from STZ-treated FVB mice displayed an attenuated contractile response to the β-adrenergic agonist isoproterenol, which was restored by the IGF-I transgene. Contractile response to the α-adrenergic agonist phenylephrine and angiotensin II was not affected by either STZ treatment or IGF-I. These results validate the beneficial role of IGF-I in diabetic cardiomyopathy, possibly due to an improved β-adrenergic response.