Insulin binding to cultured adult hepatocytes. Effects of bacitracin and chloroquine on the nature of cell-associated radioactivity.
Sephadex (G-50 fine grade)-gel chromatography and trichloroacetic acid (TCA) precipitation were used to investigate the effects of chloroquine and bacitracin on the nature of cell-associated radioactivity in studies on the binding and degradation of 125I-insulin in cultured rat hepatocytes. Sephadex peak I, eluted with the void volume, increased with hepatocyte incubation time and comprised 6% of total cell-bound radioactivity at 120 min. However, all radioactivity in this peak was due to unspecific binding. Peak II, corresponding to intact insulin, represented 95% of specifically cell-associated label at 5 min and decreased to 77% at 120 min. Peak III, containing the final low-Mr degradation products, increased with incubation time (22% of specifically bound label at 120 min). The TCA-precipitable and TCA-soluble fractions of hepatocytes extracted with 0.1% SDS were within 4-7% of the proportions of radioactivity in peaks II and III respectively. Scatchard plots based on insulin-binding data from Sephadex chromatography or TCA precipitation were identical. Dissociation studies revealed that at least 75% of the intact insulin associated with the hepatocytes was bound to receptors at the cell surface. Bacitracin increased the proportion of cell-associated intact hormone and decreased that of ligand degraded when analysed by either Sephadex chromatography or TCA precipitation. The proportion of surface-bound to internalized intact hormone remained unaltered, indicating that bacitracin acted predominantly at the cell surface. In the presence of chloroquine, which dramatically increased the contribution of peak I to specific binding, 'intact' insulin was substantially overestimated when determined as the TCA-precipitable fraction. In addition, all peak I material and 50% of cell-associated label in peak II was trapped intracellularly, thereby pointing to the lysosomal or prelysosomal site of action of this drug.
D000818:Animals; D001414:Bacitracin / Q000494:pharmacology*; D002478:Cells, Cultured; D011232:Chemical Precipitation; D002738:Chloroquine / Q000494:pharmacology*; D002850:Chromatography, Gel; D007328:Insulin / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D011869:Radioligand Assay; D051381:Rats; D011919:Rats, Inbred Strains; D011972:Receptor, Insulin / Q000187:drug effects*
D007328:Insulin; D001414:Bacitracin; D002738:Chloroquine; D011972:Receptor, Insulin