Effects of vasopressin on receptor-mediated endocytosis of asialoglycoprotein by hepatocytes from normal and diabetic rats.

The hepatic asialoglycoprotein receptor is a membrane glycoprotein used as a model to study receptor-mediated endocytosis. In order to examine the ability of second messengers to modulate intracellular trafficking, we performed a comparative study on normal and diabetic rat hepatocytes exploring the effects of an in vivo modulation, streptozotocin-diabetes, and an in vitro modulator, vasopressin, which transduces signals via the phosphoinositide pathway. We studied three main experimental aspects: (1) constitutive endocytosis, (2) continuous ligand flux, and (3) a synchronous wave of ligand. In normal cells, vasopressin decreased ligand-binding capacity by 20%, without altering the mechanism of internalization, and decreased the level of degradation, without affecting the distribution of degradation products. Diabetic cells were characterized by a 50% decrease in cell-surface and intracellular receptor ligand-binding capacity, slowed internalization of a synchronous wave of ligand, and markedly reduced degradation with an altered distribution of degraded products. Vasopressin had no additive effect on the modification induced by diabetes. These results suggest that second messengers generated by hormones play a role in the regulation of receptor-mediated endocytosis. They also confirm that receptors are subdivided into those susceptible to modulation of any kind and those insensitive to modulation, although the boundary between the two subsets is variable.