Regulation of α2-adrenoceptors in human vascular smooth muscle cells

This study analyzed the regulation of alpha(2)- adrenoceptors (alpha(2)- ARs) in human vascular smooth muscle cells ( VSMs). Saphenous veins and dermal arterioles or VSMs cultured from them expressed high levels of alpha(2)-ARs (alpha(2C) > alpha(2A), via RNase protection assay) and responded to alpha(2)- AR stimulation [ 5- bromo- N-( 4,5- dihydro- 1H- imidazol- 2- yl)- 6- quinoxalinamine ( UK-14,304, 1 muM)] with constriction or calcium mobilization. In contrast, VSMs cultured from aorta did not express alpha(2)- ARs and neither cultured cells nor intact aorta responded to UK- 14,304. Although alpha(2)- ARs (alpha(2C) > > alpha(2A)) were detected in aortas, alpha(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 alpha(2)- AR stimulation. Reporter constructs demonstrated higher activities for alpha(2A) and alpha(2C)- AR gene promoters in arteriolar compared with aortic VSMs. In arteriolar VSMs, serum increased expression of alpha(2C)- AR mRNA and protein but decreased expression of alpha(2A)- ARs. Serum induction of alpha(2C)- ARs was reduced by inhibition of p38 mitogen- activated protein kinase ( MAPK) with 2 muM SB- 202190 or dominant- negative p38 MAPK. UK- 14,304 ( 1 muM) caused calcium mobilization in control and serum-stimulated cells: in control VSMs, the response was inhibited by the alpha(2A)- AR antagonist BRL- 44408 ( 100 nM) but not by the alpha(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 alpha(2)- ARs in human VSMs that reflects differential activity of alpha(2)- 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 alpha(2C)- ARs can be dramatically and selectively induced via a p38 MAPK- dependent pathway. Therefore, altered expression of alpha(2C)- ARs may contribute to pathological changes in vascular function.