Evidence Against a Direct Effect of Leptin on Glucose Transport in Skeletal Muscle and Adipocytes

Recently, it has been proposed that leptin, the ob gene product, influences some steps in the insulin-signaling cascade. The purpose of the present study was to determine whether leptin exerts direct effects on glucose transport in insulin target tissues. Epitrochlearis muscles or isolated adipocytes from male SD rats were incubated in the absence or presence of recombinant leptin (3–1,000 ng/ml), and in the absence or presence of submaximal or maximal insulin concentrations. In skeletal muscle, insulin increased 3-O-methylglucose transport (1.88 ± 0.21, 4.06 ± 0.59, and 9.35 ± 1.90 umol ·μmol · ml⁻¹ ·h⁻¹, for 0, 0.6, and 12.0 nmol/1 insulin, respectively). Leptin exposure (300 ng/ml) for 2 h did not alter the basal, submaximal, or maximal response of glucose transport to insulin in skeletal muscle (1.50 ± 0.14,4.76 ± 0.58, and 9.04 ± 1.09 umol · ml⁻¹ · h⁻¹ for 0, 0.6, and 12.0 nmol/l insulin, respectively). Insulin increased glucose transport in rat adipocytes (0.194 ± 0.007,1.059 ± 0.029, and 3.367 ± 0.143 pmol [14C]glucose · 0.5 ml⁻¹ cell suspension · min⁻¹ for 0,0.8, and 80 nmol/l insulin, respectively); in vitro exposure to leptin (300 ng/ml) did not alter glucose transport (0.220 ± 0.006,1.269 ± 0.046, and 3.221 ± 0.285 pmol [¹⁴C]glucose · 0.5 ml⁻¹ cell suspension · min⁻¹ for 0, 0.8, and 80 nmol/1 insulin, respectively). Similar to our findings in the epitrochlearis muscle, leptin had no direct effect on basal or insulin-stimulated glucose uptake in soleus muscle from ob/ob or lean mice or adipocytes from normal mice. In summary, in vitro exposure of skeletal muscle or adipocytes to recombinant leptin did not alter glucose transport in the absence of insulin, nor did it affect the sensitivity or responsiveness of the glucose transport system to insulin.