Loss of surface galactosyl receptor activity on isolated rat hepatocytes induced by monensin or chloroquine requires receptor internalization via a clathrin coated pit pathway

We studied the effect of hyperosmotic inhibition of the clathrin coated pit cycle on the monensin- and chloroquine-dependent loss of surface galactosyl (Gal) receptor activity on isolated rat hepatocytes. Cells treated for 60 min without ligand at 37 degrees C with 25 microM monensin or 300 microM chloroquine in normal medium (osmolality congruent to 275 mmol/kg) bound 40-60% less 125I-asialo-orosomucoid (ASOR) at 4 degrees C than untreated cells. Cells exposed to monensin or chloroquine retained progressively more surface Gal receptor activity, however, when the osmolality of the medium was increased above 400 mmol/kg (using sucrose as osmolite) 10 min prior to and during drug treatment. Cells pretreated for 10 min with hyperosmolal media (600 mmol/kg) alone internalized less than or equal to 10% of surface-bound 125I-ASOR. Thus, the ligand-independent loss of surface Gal receptor activity on monensin- and chloroquine-treated hepatocytes requires internalization of constitutively recycling receptors via a coated pit pathway.