Regulation of the cellular localization and signaling properties of the α(1B)- and α(1D)-adrenoceptors by agonists and inverse agonists

The regulation of the cellular distribution and intracellular signaling properties of the α(1B)- and α(1D)- adrenoceptor (α₁-AR) subtypes was examined in stably transfected Rat 1 fibroblasts. In unstimulated cells, α(1B)-AR expression was noted primarily on the cell surface. Treatment with phenylephrine induced internalization of the α(1B)-AR and promoted association with arrestin 2. The internalized α(1B)-AR colocalized with the transferrin receptor, an endosomal marker. In unstimulated fibroblasts, the α(1D)-AR was detected in a perinuclear orientation and was colocalized with arrestin 2 in a compartment also containing the transferrin receptor. After treatment with prazosin, which exhibits inverse agonist properties, the α(1D)-AR was redistributed from intracellular sites to the cellular periphery and was no longer associated with the transferrin receptor or arrestin 2. α(1D)-AR-expressing cells exhibited a high degree of basal activity for both inositol phosphate formation and extracellular signal regulated kinase (ERK), which was reduced by treatment with prazosin. In these cells, phenylephrine induced a dose-dependent increase in inositol phosphate formation but had no effect on ERK activity. In α(1B) -AR- expressing cells, phenylephrine stimulated both inositol phosphate formation and ERK activity. These data show that: 1) there are differences in the cellular localization of the α₁-AR subtypes; 2) the α(1B)-AR exhibits expected G protein-coupled receptor activity regarding cellular localization, agonist-mediated internalization, and coupling to second messengers; and 3) the α(1D)-AR is constitutively active and, as a result, is localized to intracellular compartments involved in receptor recycling.