Expression of alpha-adrenoceptor subtypes by smooth muscle cells and adventitial fibroblasts in rat aorta and in cell culture

Previous radioligand binding reports of vascular a-adrenoceptor (AR) densit y have been limited to total alpha (1)- or alpha (2)-ARs. Studies using who le blood vessel homogenates have not differentiated among receptor or mRNA expression by medial smooth muscle cells (SMCs) versus adventitial fibrobla sts (AFBs). Therefore, we used quantitative reverse transcription-polymeras e chain reaction and radioligand binding to measure a-AR subtypes in media, adventitia, and cultured SMCs and AFBs from rat aorta. Both media and adve ntitia expressed alpha (1A)-, alpha (1B)-, alpha (1D)-, and alpha (2D)-AR m RNAs, but in markedly different abundances. Total alpha (1)-AR density was the same for media and adventitia (B-max = 101 +/- 10 versus 96 +/- 16 fmol /mg of protein). However, densities for alpha (1A)-, alpha (1B)-, and alpha (1D)-AR subtypes in media were 19 +/-2, 26 +/-4, and 55 +/-2%, and in adve ntitia were 44 +/-3, 37 +/-5, and 19 +/-2%. No alpha (2B)- or alpha (2C)-AR transcripts were detected in either layer or in cultured SMCs or AFBs. Tot al alpha (1)-AR densities in cultured SMCs and AFBs (B-max = 111 +/-4 and 4 8 +/-6 fmol/mg of protein, respectively) were similar to media and adventit ia, with alpha (1B)- and alpha (1D)-AR transcript levels and receptors larg ely sustained. However, alpha (1A)- and alpha (2D)-AR expression in culture d SMCs and AFBs was strongly reduced, compared with media and adventitia, a n effect not prevented by 30 different culture conditions. Like SMCs, expos ure of AFBs to norepinephrine induced protein synthesis and proliferation o f AFBs. This is the first study to quantitate a-AR subtype expression in me dia and adventitia and in cultured SMCs and AFBs. In addition, we report th e intriguing finding that AFBs express alpha (1)-ARs in similar abundance a s medial SMCs and that norepinephrine induced them to proliferate.