Angiotensin II receptor internalization and signaling in isolated rat hepatocytes

Since angiotensin II (Ang II)-induced receptor internalization is required to maintain the production of certain intracellular signals in some target cells, we investigated the relationships between Ang II receptor endocytosis and the generation of second messengers in rat hepatocytes. The results of the present study demonstrate that in response to exposure of hepatocytes to Ang II, a decrease in surface Ang II receptors occurred, consistent with a rapid endocytosis of the receptor-bound hormone complex. Pretreatment of cells with okadaic acid (OA) did not have any effect on receptor-mediated internalization. In contrast, a marked reduction of the Ang II receptor endocytosis process occurred after treatment of hepatocytes with phenylarsine oxide (PAO), indicating that cysteine residues could be involved in receptor-mediated endocytosis. Stimulation of cells with Ang II blocked the generation of cyclic adenosine monophosphate (cAMP), which follows the stimulation of hepatocytes with forskolin. Moreover, Ang II increased both inositol 4,5-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3) generation, and enhanced intracellular calcium concentration ([Ca2+]i). Exposure of cells to PAO did not alter the effect of Ang II on the accumulation of cAMP after forskolin stimulation, indicating that endocytosis of the agonist-receptor complex is not involved in adenylate cyclase inhibition. Conversely, PAO and OA markedly reduced IP2 and IP3 synthesis, and the plateau phase of Ang II-induced Ca2+ mobilization. The relationship between Ang II-induced endocytosis and the generation of phosphoinositols and increment in [Ca2+]i indicates that sequestration of the Ang II receptor is necessary to maintain the production of these intracellular signals in rat hepatocytes.