Role of receptor cycling in the regulation of angiotensin II surface receptor number and angiotensin II uptake in rat vascular smooth muscle cells.

In vivo data on the factors controlling angiotensin II (AII) cell surface binding are conflicting. We studied the specific effects of AII on AII binding in rat mesenteric artery vascular smooth muscle cells in culture. Incubation with unlabeled AII at 21 degrees C resulted in time- and concentration-dependent decreases in AII surface binding at 4 degrees C, with a 30% reduction after exposure to 300 nM AII for 15 min. Reductions in cell surface binding were due to decrements in receptor number rather than changes in binding affinity. Loss of surface receptors was mediated by receptor internalization as maneuvers that blocked ligand internalization (cold temperature and phenylarsine oxide [PAO]) attenuated AII-induced loss of surface receptors. After removal of AII, recovery of surface binding was rapid (t1/2 = 15 min) and was mediated by reinsertion of a preexisting pool of receptors into the surface membrane rather than by new receptor synthesis. To determine the role of receptor cycling on AII-induced surface receptor loss, cells were incubated with the endosomal inhibitor chloroquine during exposure to AII at 21 degrees C. Incubation with AII plus chloroquine resulted in a 70% greater loss of surface binding than after incubation with AII alone. To determine the role of receptor cycling on uptake of ligand, cells were incubated with PAO or endosomal inhibitors during exposure to AII at 4 and 21 degrees C. Compared with buffer these agents did not alter AII uptake at 4 degrees C, but decreased uptake by 12-50% at 21 degrees C. These results indicate that after binding AII receptors cycle and that receptor cycling attenuates AII-induced losses of surface receptors and enhances ligand uptake by providing a continuous source of receptors to the cell surface.