Chick embryo myotubes contain transferrin receptors and internalize and recycle transferrin

Embryonic chick skeletal myotubes grown in cell culture require transferrin to provide iron for proliferation and differentiation. We demonstrate here that cultured myotubes contain transferrin receptors as demonstrated by the finding of specific, saturable, and reversible high-affinity binding sites. Scatchard analysis of equilibrium binding data indicates an apparent Kd of 37 nM and one muscle cell equivalent contains 7,500 transferrin receptors. Myotubes exhibit a Kd 100 times higher for apotransferrin than for iron-saturated transferrin. Internalization of specifically bound transferrin is temperature dependent and occurs rapidly at 37 degrees C with a steady state reached after 10 min. Internalization studies using either 125I-ovotransferrin or 55Fe-ovotransferrin suggest that transferrin is internalized, depleted of iron, and recycled intact to the extracellular medium as shown in other cell systems. Autoradiography of muscle cell cultures incubated with 125I-ovotransferrin at 4 degrees C reveals clusters of receptors along the myotubes. The possible mechanisms by which transferrin is supplied to muscle in vivo are discussed in light of the evidence that motor neurons contain transferrin.