Acta Pharmacologica Sinica (2009) 30: 1585–1593; doi: 10.1038/aps.2009.160

Aim: It is unclear why α_1D-adrenergic receptors (α_1D-ARs) play a critical role in the mediation of peripheral vascular resistance and blood pressure in situ but function inefficiently when studied in vitro.  The present study examined the causes for these inconsistencies in native α₁-adrenergic functional performance between the vascular smooth muscle and myocytes.  

Methods: The α₁-adrenergic mediated contraction, Ca²⁺ signaling and the subcellular receptor distribution were evaluated using the Fluo-4, BODIPY-FL prazosin and subtype-specific antibodies. 

Results: Rat aortic rings and freshly dissociated myocytes displayed contractile and increased intracellular Ca²⁺ responses to stimulation with phenylephrine (PE, 10 µmol), respectively.  However, the PE-induced responses disappeared completely in cultured aortic myocytes, whereas PE-enhanced Ca²⁺ transients were seen in cultured rat cardiac myocytes.  Further studies indicated that α_1D-ARs, the major receptor subtype responsible for the α₁-adrenergic regulation of aortic contraction, were distributed both intracellularly and at the cell membrane in freshly dispersed aortic myocytes, similar to the α _1A -AR subcellular localization in the cultured cardiomyocytes.  In the cultured aortic myocytes, however, in addition to a marked decrease in their protein expression relative to the aorta, most labeling signals for α_1D-ARs were found in the cytoplasm.  Importantly, treating the culture medium with charcoal/dextran caused the reappearance of α_1D-ARs at the cell surface and a partial restoration of the Ca²⁺ signal response to PE in approximately 30% of the cultured cells. 

Conclusion: Reduction in α_1D-AR total protein expression and disappearance from the cell surface contribute to the insensitivity of cultured vascular smooth muscle cells to α₁-adrenergic receptor activation.

Keywords: a_1D-adrenergic receptor; vascular smooth muscle cells; receptor sensitivity; receptor distribution; Ca²⁺ signaling