Binding and processing of ¹²⁵I-ACTH by isolated rat splenic lymphocytes

The effect of incubation temperature and ligand competition was tested for ¹²⁵I-ACTH binding to isolated rat lymphocytes. αMSH but not Agouti-like peptide was an effective competitive inhibitor for cell surface binding at 4°C. Cells incubated with ¹²⁵I-ACTH at 37°C rapidly associated ligand for 10 min and then gradually lost the radioactivity with time. Cells incubated with ¹²⁵I-ACTH at 4°C accumulated ligand to only about half the maximal amount when compared to cells incubated at 37°C for 10 min. Temperatures below 20°C and toxins that block lysosomal degradation blocked the loss of cell-associated radioactivity. These results suggest the lymphocyte ACTH receptor is the Melanocortin 5 receptor and the receptor is internalized by endocytosis to deliver ligand to the lysosome.