DIFFERENCES IN DYNAMICS OF INSULIN AND INSULIN-LIKE GROWTH I (IGF-I) RECEPTORS INTERNALIZATION IN ISOLATED RAT HEPATOCYTES
Insulin and IGF-I are two related peptides performing in the mammalian body functionally different roles of the metabolic and growth hormones, respectively. Internalization of the insulin-receptor complex (IRC) is the most important chain of mechanism of the action of hormone. To elucidate differences in the main stages of internalization of the two related hormones, the internalization dynamics of <sup>125</sup>I-insulin and <sup>125</sup>I-IGF-I was traced in isolated rat hepatocytes at 37 and 12 °С. There were established marked differences in the process of internalization of labeled hormones, which is stimulated by insulin and IGF-I. At 37 °С the insulin-stimulated internalization, unlike the process initiated by IGF-I, did not reach the maximal level for 1 h of incubation. However, essential differences in the internalization course of these two related peptide were obvious at the temperature of 12 °С. The internalization level of insulin receptors at 12 °С decreased by one third in spite of a significant increase of the insulin receptor binding on the hepatocytes plasma membrane. At 12 °С a slight decrease of the proportion of intracellular <sup>125</sup>I-IGF-I correlated with a decrease in the <sup>125</sup>I-IGF-I binding to receptors on the cell membrane. Internalization of IGF-I receptors was not affected by low temperature, as neither its level, nor the rate changed at 12 °С. The paradoxical decrease of the insulin-stimulated internalization at low temperature seems to represent a peculiar «inhibition mechanism» of immersion of IRC into the cell, which leads to accumulation of the complexes on the cell surface and possibly to a readjustment of the insulin biological activity. The resistance of internalization of the IGF-I receptor to cold seems to be related to the more ancient origin of this mechanism in the poikilothermal vertebrates.