RECEPTOR-MEDIATED ENDOCYTOSIS OF INSULIN IN LOWER-VERTEBRATES - INTERNALIZATION AND INTRACELLULAR PROCESSING OF I-125 INSULIN IN ISOLATED HEPATOCYTES OF LAMPREY AND FROG

The binding of I-125-insulin to cellular insulin receptors and the internalization of insulin-receptor complexes have been studied in isolated hepatocytes of frog and lamprey. Two classes of binding sites (K-d 10(-9) and 10(-8) M) were found in cells of both species. The molecular weight of the insulin receptor a-subunit was 130 kDa in both species. Internalization of bound I-125-insulin in both species was found in the temperature range 0 to 20 degrees Cells ''loaded'' with I-125-insulin were used to estimate the fate of the internalized ligand. Release of internalized ligand from frog cells increased at temperatures ranging from 0 to 20 degrees. At 0 degrees the degraded I-125-insulin was 5%, at 5 degrees 7%, and at 20 degrees 17% of total radioactivity accumulated in the medium. In lamprey hepatocytes there was neither radioactivity accumulation in the incubation medium nor release from cells at all temperatures studied. The intracellular degradation of internalized I-125-insulin in frog hepatocytes was much lower than that in lamprey cells. In frog hepatocytes the specific binding of I-125-insulin was increased twofold in the presence of the lysosomal inhibitor chloroquine. In contrast no increase was found in lamprey hepatocytes. In conclusion, the processing pathways of internalized insulin in the cells of ectothermal and endothermal vertebrates are generally similar but in ectothermal animals all events take place at lower temperatures and at lower rates. The peculiarities of insulin processing in lamprey hepatocytes most likely result from the transformation of hepatocytes during the nonfeeding prespawning period. (C) 1995 Academic Press, Inc.