AGONIST-INDUCED INTERNALIZATION OF THE ANGIOTENSIN II-BINDING SITES FROM PLASMA-MEMBRANES OF ISOLATED RAT HEPATOCYTES

Angiotensin II (Ang II) provokes rapid internalisation of its receptor from plasma membranes in isolated rat hepatocytes. After 10 min stimu lation with Ang II, plasma membrane lost about 60% of its I-125-Ang II -binding capacity. Internalisation was blocked by phenylarsine oxide ( PhAsO), whereas okadaic acid, which markedly reduced the sustained pha se of calcium mobilization, did not have a preventive effect on Ang II -receptor complex sequestration. These data suggest that Ang II recept or internalisation is probably independent of a phosphorylation/dephos phorylation cycle of critical serine/threonine residues in the recepto r molecule. To establish a relationship between sequestration of the A ng II receptor and the physical properties of the Ang II-binding sites , I-125-Ang II-receptor complex profiles were analysed by isoelectric focusing. In plasma membrane preparations two predominant Ang II-bindi ng sites, migrating to pI 6.8 and 6.5, were found. After exposure to A ng II, cells lost I-125-Ang II-binding capacity to the Ang II-receptor complex migrating at pI 6.8 which was prevented in PhAsO-treated cell s. Pretreatment of hepatocytes with okadaic acid did not modify Ang II -receptor complex profiles, indicating that the binding sites correspo nding to pI 6.5 and pI 6.8 do not represent a phosphorylated and/or no n-phosphorylated form of the Ang II receptor. The results show that th e Ang II-receptor complex isoform at pI 6.8 represents a functional fo rm of the type-1 Ang II receptor. Further studies are necessary to ide ntify the Ang II-related nature of the binding sites corresponding to pI 6.5.