Background: The cellular localization of the α1D-adrenergic receptor (α1D-AR) is controversial. Studies in heterologous cell systems have shown that this receptor is expressed in intracellular compartments. Other studies show that dimerization with other ARs promotes the cell surface expression of the α1D-AR. To assess the cellular localization in vascular smooth muscle cells, we developed an adenoviral vector for the efficient expression of a GFP labeled α1D-AR. We also measured cellular localization with immunocytochemistry. Intracellular calcium levels, measurement of reactive oxygen species and contraction of the rat aorta were used as measures of functional activity.
Results: The adenovirally expressed α1D-AR was expressed in intracellular compartments in human aortic smooth muscle cells. The intracellular localization of the α1D-AR was also demonstrated with immunocytochemistry using an α1D-AR specific antibody. RT-PCR analysis detected mRNA transcripts corresponding to the α1A-α1B- and α1D-ARs in these aortic smooth muscle cells. Therefore, the presence of the other α1-ARs, and the potential for dimerization with these receptors, does not alter the intracellular expression of the α1D-AR. Despite the predominant intracellular localization in vascular smooth muscle cells, the α1D-AR remained signaling competent and mediated the phenylephrine-induced increases in intracellular calcium. The α1D-AR also was coupled to the generation of reactive oxygen species in smooth muscle cells. There is evidence from heterologous systems that the α1D-AR heterodimerizes with the β2-AR and that desensitization of the β2-AR results in α1D-AR desensitization. In the rat aorta, desensitization of the β2-AR had no effect on contractile responses mediated by the α1D-AR.
Conclusion: Our results suggest that the dimerization of the α1D-AR with other ARs does not alter the cellular expression or functional response characteristics of the α1D-AR.
Digital Object Identifier (DOI)
Funding for Study: HL-38120-14
MTP, MLG, JLS, KAO and DFM: HL-38120-14
RWH and LAS: Natl Heart Lung and Blood Institute: 2 R01 HL056910-5A1
For manuscript: HL-38120-14
García-Cazarín, Mary L.; Smith, Jennifer L.; Olszewski, Kyle A.; McCune, Dan F.; Simmerman, Linda A.; Hadley, Robert W.; Kraner, Susan D.; and Piascik, Michael T., "The α1D-Adrenergic Receptor Is Expressed Intracellularly and Coupled to Increases in Intracellular Calcium and Reactive Oxygen Species in Human Aortic Smooth Muscle Cells" (2008). Pharmacology and Nutritional Sciences Faculty Publications. 14.