Abstract
To determine the cellular location, capacity, and nutritional sensitivity of insulin-like growth factor (IGF) receptors, we measured the in vitro binding of [125I]-IGFs to skeletal muscle using light microscopic autoradiography. Muscle was collected from 8-month lambs that had received high or low nutrition diets (3% and 1.25% of body weight/day in pellets, respectively). Half of each group had also received growth hormone (0.25 mg/kg/day). Cryosections were incubated with [125I]-IGF alone or with unlabeled IGF-1, IGF-2, or insulin to characterize binding sites as probable Type 1 IGF, Type 2 IGF, or insulin receptors. [125I]-IGF-1 was found to bind to blood vessels and Type 1 receptors in connective tissue (p < or = 0.001), but not to muscle fiber or nerves. In muscle from 6-month lambs that were fed or fasted, [125I]-IGF-1 bound to Type 1 receptors in connective tissue (p < or = 0.01 fed; p < or = 0.05 fasted) and muscle fiber (p < or = 0.05). The binding to connective tissue was also greater in fasted than in fed animals (p < or = 0.05). Binding of [125I]-IGF-2 to the Type 2 receptor was located in blood vessels and connective tissue (p < or = 0.01) and did not alter with fasting. Therefore, these experiments have demonstrated that Type 1 and Type 2 receptors vary in their distribution and nutritional sensitivity in skeletal muscle.