Receptor-mediated endocytosis of angiotensin II in rat myometrial cells

The events involved in the processing of the angiotensin II (Ang II)-receptor complex were studied in primary cultures of rat myometrial cells. Ang II bound to rat myometrial cells in a specific, time- and temperature-dependent fashion. Pretreatment with cycloheximide did not interfere with binding up to 3 hr, but inhibited increases in binding observed over longer periods. The [3H]Ang II binding to intact cells was inhibited by dithiothreitol (DTT), and the rank order of potency of Ang II and nonpeptide antagonists to inhibit the [3H]Ang II binding was Ang II > Losartan >> PD 123319 or CGP 42112B, indicating the presence of the AT1 receptor type. Whereas most of the [3H]Ang II binding at 4 degrees was susceptible to acid or pronase treatment, binding at 35 degrees was resistant to both treatments, suggesting an internalization of the Ang II-receptor complex. Phenylarsine oxide (PAO) and N-ethylmaleimide (NEM) caused a concentration-dependent inhibition when the binding assay was performed at 35 degrees, but no effect was observed at 4 degrees, indicating that these agents did not alter cell-surface binding but actually prevented the internalization process. Simultaneous treatment with 1 mM DTT or beta-mercaptoethanol prevented the inhibitory effect of NEM, but only DTT could prevent the inhibition caused by PAO, indicating that two closely located sulfhydryl groups must be involved in the internalization process. Chloroquine (100 microM) inhibited the [3H]Ang II dissociation from cells, and monensin (25 microM) induced a 30% inhibition of [3H]Ang II binding (35 degrees, 3 hr), suggesting endosomal processing of the Ang II-receptor complex with receptor recycling to the cell surface. These results indicate that Ang II binding to AT1 receptors in rat myometrial cells is followed by internalization of the Ang II-receptor complex and recycling of the receptor to the cell surface.