Regulation of α₂-adrenoceptors in human vascular smooth muscle cells

This study analyzed the regulation of α₂-adrenoceptors (α₂-ARs) in human vascular smooth muscle cells (VSMs). Saphenous veins and dermal arterioles or VSMs cultured from them expressed high levels of α₂-ARs (α_2C > α _2A, via RNase protection assay) and responded to α ₂-AR stimulation [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14,304, 1 μM)] with constriction or calcium mobilization. In contrast, VSMs cultured from aorta did not express α₂-ARs and neither cultured cells nor intact aorta responded to UK-14,304. Although α₂-ARs (α_2C ≫ α_2A) were detected in aortas, α_2C-ARs were localized by immunohistochemistry to VSMs of adventitial arterioles and not aortic media. In contrast with aortas, aortic arterioles constricted in response to α₂-AR stimulation. Reporter constructs demonstrated higher activities for α_2A- and α_2C-AR gene promoters in arteriolar compared with aortic VSMs. In arteriolar VSMs, serum increased expression of α_2C-AR mRNA and protein but decreased expression of α_2A-ARs. Serum induction of α_2C-ARs was reduced by inhibition of p38 mitogen-activated protein kinase (MAPK) with 2 μM SB-202190 or dominant-negative p38 MAPK. UK-14,304 (1 μM) caused calcium mobilization in control and serum-stimulated cells: in control VSMs, the response was inhibited by the α_2A-AR antagonist BRL-44408 (100 nM) but not by the α_2C-AR antagonist MK-912 (1 nM), whereas after serum stimulation, MK-912 (1 nM) but not BRL-44408 (100 nM) inhibited the response. These results demonstrate site-specific expression of α₂-ARs in human VSMs that reflects differential activity of α₂-AR gene promoters; namely, high expression and function in venous and arteriolar VSMs but no detectable expression or function in aortic VSMs. We found that α_2C-ARs can be dramatically and selectively induced via a p38 MAPK-dependent pathway. Therefore, altered expression of α _2C-ARs may contribute to pathological changes in vascular function.