Role of Ca2+ for protein turnover in isolated rat hepatocytes

Experiments with bivalent-cation chelators (EGTA and EDTA), a Ca2+ ionophore (A23187) and a Ca2+-channel blocker (verapamil) indicate that Ca2+ is required for the lysosomal degradation of endogenous protein in hepatocytes. A distinction is made between lysosomal and non-lysosomal degradation by using the lysosomotropic agent methylamine. As Ca2+ does not appear to be required for the lysosomal degradation of endocytosed asialo-fetuin, the Ca2+-dependence for the degradation of endogenous protein is probably connected with the formation of autophagic vacuoles or the fusion of autophagic vacuoles with lysosomes. EGTA and EDTA had a slight inhibitory effect on the non-lysosomal degradation. This effect could be due to the activity of non-lysosomal Ca2+-dependent thiol proteinases. Together with previous experiments with thiol-proteinase inhibitors, the present experiments indicate that these proteinases have a very limited impact on the bulk protein degradation in the isolated hepatocytes.