Insulin resistance induces hyperleptinemia, cardiac contractile dysfunction but not cardiac leptin resistance in ventricular myocytes

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Abstract
Insulin resistance is a metabolic syndrome commonly seen in obesity. Leptin, the obese gene product, plays a role in the regulation of cardiac function. Elevated leptin levels have been demonstrated under insulin-resistant states such as obesity and hypertension, although their role in cardiac dysfunction is unknown. This study was designed to determine the impact of prediabetic insulin resistance on leptin levels and leptin-induced cardiac contractile response. Whole-body insulin resistance was generated with a 10-week dietary sucrose feeding. Contractile and intracellular Ca2+ properties were evaluated in ventricular myocytes using an IonOptix™ system. The contractile indices analyzed included peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR90), maximal velocity of shortening/relengthening (dL/dt), fura-fluorescence intensity change (FFI) and decay rate (). Sucrose-fed rats displayed significantly elevated body weight and plasma leptin levels, depressed PS, dL/dt, shortened TPS, prolonged TR90 and , as well as reduced FFI compared to the starch-fed control group. Leptin (1–1000 nM) elicited a concentration-dependent depression of PS and FFI in myocytes from both starch and sucrose groups. Leptin-induced contractile depression was abolished by the nitric oxide synthase inhibitor N-nitro-L-arginine methyle ester, elevation of the extracellular Ca2+ concentration, the Janus activated kinase 2 inhibitor AG-490 or the mitogen activated protein kinase inhibitor SB203580 in myocytes from both sucrose and starch groups. Moreover, AG-490 and SB203580 unmasked a positive response of PS in myocytes from both groups. These data indicate that insulin resistance directly induces hyperleptinemia and cardiac contractile dysfunction, without affecting leptin-mediated cardiac contractile function at the myocyte level.