Peculiarities of distribution of 125I-insulin and 125I-insulin-like growth factor-I (IGF-I) at internalization in isolated rat hepatocytes

One of determining conditions of formation in vertebrate phylogenesis of hormonal systems of insulin and IGF-I—peptides common by origin, similar by structure and by biological action is temperature factor. In differentiation of functional roles of two related hormones an important place is ascribed to mechanism of their intracellular action. Study of formation of the two hormonal systems in vertebrate phylogenesis necessitates knowledge of peculiarities of internalization of two related hormones in mammals. On isolated rat hepatocytes at the identical maximal level of internalization of ^I25I-insulin and ¹²⁵I-IGF-I there are revealed marked differences of dynamics of their internalization. Besides, at internalization of ¹²⁵I-insulin or of ¹²⁵I-IGF-I, their peculiar distribution was observed in the cell and on the plasma membrane. At 37°C, only two thirds of ¹²⁵I-insulin relatively bound to receptors on the membrane were immersed into cell, whereas the portion of internalized ¹²⁵I-IGF-I turned out to be higher than the part located on the membrane. At 12°C, a decrease of ¹²⁵I-insulin inside the cell and its increase on the membrane indicated the interdependent label redistribution under these conditions. The form of the ¹²⁵I-IGF-I distribution at low temperature remained unchained. The pattern of established differences of internalization of ¹²⁵I-insulin and ¹²⁵I-IGF-I as well as different sensitivity of each of the processes to low temperature indicated that each of the peptides triggered individual mechanism of endocytosis of receptors. The high sensitivity of internalization of ¹²⁵I-insulin to low temperature and the temperature lability of the process in isolated rat hepatocytes agree with the earlier suggestion about the late formation of the regulatory insulin system in homoiothermal vertebrates.