[Receptor-mediated endocytosis in the cells of cold-blooded animals. II. The fate of internalized 125I-insulin in the isolated hepatocytes of the lamprey and the frog].

The 125I-insulin outflow from isolated hepatocytes of the frog and lamprey "loaded" with the labeled hormone has been studied. It is shown that the ligand outflow from the frog cells increased with the increase in the incubation temperature from 0 up to 20 degrees C. The curves of the rest cell radioactivity were reciprocal to those of the radioactivity accumulated in the medium at the corresponding temperatures. At 0.5 and 20 degrees C the degraded 125I-insulin made 5.7 and 17% of the whole hormone accumulated in the medium. In the lamprey hepatocytes, neither accumulation in the incubation medium nor outflow of the radioactivity from cell was seen at all temperatures studied. The intracellular degradation of 125I-insulin in the frog hepatocytes was no more than 7% of the internalized ligand, compared to about 25% in the lamprey cells. The specific binding of 125I-insulin was twice increased in the presence of lysosomal inhibitor chloroquin; contrary to this, no increase was found in the lamprey hepatocytes. The results of experiments on the frog hepatocytes lead us to a conclusion that the processing pathway of internalized insulin in cold-blooded vertebrate cells is similar mainly to that in cells of warm-blooded species, but takes place at lower temperatures and with slower rates. The peculiarities of processing in the lamprey hepatocytes (extralysosomal ligand degradation, the inability to release the internalized ligand and its degradation products) are dependent on a deep transformation of hepatocytes during prespawning migration period.