Receptor-mediated 125I-labelled insulin degradation in the rat hepatocyte. Studies using chloroquine

Degradation of insulin during incubation with target cells occurs via receptor-mediated processes. In this study, receptor-mediated degradation of 125I-labelled insulin was investigated in rat hepatocytes, using the agent chloroquine. Chloroquine increased specific cell-associated 125I-labelled insulin at 37 degrees. The increased radioactivity with chloroquine was intracellular (53.3 +/- 1.2% of initially bound label was displaced by excess cold insulin; in control cells 67.0 +/- 2.1% was displaced, P less than 0.005). The effect of chloroquine was prevented by adding label at 15 degrees or by pre-treatment with 5 mM KCN, 5 mM NaN3 or 1 g/l bacitracin, which indicated a post-internalization site of action. Chloroquine had no effect on degradation of 125I-labelled insulin in buffer alone or in buffer previously incubated with cells. Specific studies of receptor-mediated degradation at 37 degrees showed that more 125I-labelled insulin remained associated with hepatocytes when chloroquine was present (P less than 0.0005 after 60 min). Analysis of chloroquine's effect on the intactness of 125I-labelled insulin released during processing of surface-bound label at 37 degrees showed that chloroquine-sensitive mechanisms accounted for at least 50% of receptor-mediated insulin degradation in rat hepatocytes.