Regulation of α2-adrenoceptors in human vascular smooth muscle cells

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Abstract
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 (α2C> α2A, via RNase protection assay) and responded to α2-AR stimulation [5-bromo- N-(4,5-dihydro-1 H-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 (α2C>> α2A) were detected in aortas, α2C-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 α2A- and α2C-AR gene promoters in arteriolar compared with aortic VSMs. In arteriolar VSMs, serum increased expression of α2C-AR mRNA and protein but decreased expression of α2A-ARs. Serum induction of α2C-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 α2A-AR antagonist BRL-44408 (100 nM) but not by the α2C-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 α2C-ARs can be dramatically and selectively induced via a p38 MAPK-dependent pathway. Therefore, altered expression of α2C-ARs may contribute to pathological changes in vascular function.