Different patterns of agonist-induced modulation of α1B-adrenoceptor density at different initial expression levels

We compared the norepinephrine (NE) induced α1B-adrenoceptor (α1B-AR) expression modulation between two transfected human embryonic kidney (MEK) 293 cell lines in which α1B-AR densities were (6 336 ± 913) and (773 ± 164) fmol mg1, respectively. Treatment of cells with NE (10 μmol L1) for 48 h decreased high-level expressed α1B-AR density, but increased low-level expressed α1B-AR density. The protein kinase C inhibitor Calphostin C or Ro-31-8220 reversed, and its activator PMA mimicked the NE-induced down-regulation of high-level expressed α1B-AR. Moreover, PMA induced a down-regulation of low-level expressed α1B-AR. The endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (CPA) and the calcium chelator BAPTA/AM did not affect the down-regulation of high-level expressed α1B-AR, but inhibited the up-regulation of low-level expression α1B-AR induced by NE. These results suggest that α1B-adrenoceptor densities at different initial expression levels are differentially regulated by NE and their signal transduction pathways are different.