Expression of α-Adrenoceptor Subtypes by Smooth Muscle Cells and Adventitial Fibroblasts in Rat Aorta and in Cell Culture

Abstract

Previous radioligand binding reports of vascular α-adrenoceptor (AR) density have been limited to total α₁- or α₂-ARs. Studies using whole blood vessel homogenates have not differentiated among receptor or mRNA expression by medial smooth muscle cells (SMCs) versus adventitial fibroblasts (AFBs). Therefore, we used quantitative reverse transcription-polymerase chain reaction and radioligand binding to measure α-AR subtypes in media, adventitia, and cultured SMCs and AFBs from rat aorta. Both media and adventitia expressed α_1A-, α_1B-, α_1D-, and α_2D-AR mRNAs, but in markedly different abundances. Total α₁-AR density was the same for media and adventitia (B_max = 101 ± 10 versus 96 ± 16 fmol/mg of protein). However, densities for α_1A-, α_1B-, and α_1D-AR subtypes in media were 19 ± 2, 26 ± 4, and 55 ± 2%, and in adventitia were 44 ± 3, 37 ± 5, and 19 ± 2%. No α_2B- or α_2C-AR transcripts were detected in either layer or in cultured SMCs or AFBs. Total α₁-AR densities in cultured SMCs and AFBs (B_max = 111 ± 4 and 48 ± 6 fmol/mg of protein, respectively) were similar to media and adventitia, with α_1B- and α_1D-AR transcript levels and receptors largely sustained. However, α_1A- and α_2D-AR expression in cultured SMCs and AFBs was strongly reduced, compared with media and adventitia, an effect not prevented by 30 different culture conditions. Like SMCs, exposure of AFBs to norepinephrine induced protein synthesis and proliferation of AFBs. This is the first study to quantitate α-AR subtype expression in media and adventitia and in cultured SMCs and AFBs. In addition, we report the intriguing finding that AFBs express α₁-ARs in similar abundance as medial SMCs and that norepinephrine induced them to proliferate.