Multiple pathways for ligand internalization in rat hepatocytes II: Effect of hyperosmolarity and contribution of fluid‐phase endocytosis

In a companion report (Moss and Ward: J. Cell. Physiol. 149:313–318, 1991) evidence was presented for multiple pathways for insulin internalization based on differences between the internalization of insulin and that of two other ligands, asialofetuin (Afet) and epidermal growth factor (EGF), in the presence of several perturbations of endocytosis. In the present study we have explored the characteristics of three internalization pathways and the contribution of each to overall insulin uptake. Freshly isolated hepatocytes were incubated with radiolabeled ligands in the presence of hyperosmolar sucrose, treatment that is thought to inhibit the coated pit pathway of endocytosis. Insulin internalization was decreased approximately 39%, but much greater decreases were observed with Afet (86%) and EGF (62%). Competition between uptake of radiolabeled and unlabeled insulin was observed in hyperosmolar‐treated cells, suggestive of endocytosis by a receptor‐mediated noncoated‐pit pathway. Uptake of radiolabeled insulin that persisted in the presence of hyperosmolarity and high concentrations of unlabeled insulin suggested a third uptake pathway: fluid‐phase endocytosis. A rate of fluid‐phase endocytosis of 7.2 μL/hr/10⁶ cells was determined from the uptake of the fluid‐phase marker lucifer yellow. At high insulin concentrations (≥ 250 ng/ml), fluid‐phase endocytosis appears to be the predominant pathway for insulin uptake, but at lower insulin concentrations (physiological) the coated pit and noncoated pit pathways are the primary routes for insulin internalization.