Direct demonstration of β1‐ and evidence against β2‐ and β3‐adrenoceptors, in smooth muscle cells of rat small mesenteric arteries

Recent evidence supports additional subtypes of vasodilator β-adrenoceptor (β-AR) besides the ‘classical’ β2. The aim of this study was to investigate the distribution of β-ARs in the wall of rat mesenteric resistance artery (MRA), to establish the relative roles of β-ARs in smooth muscle and other cell types in mediating vasodilatation and to analyse this in relation to the functional pharmacology. We first examined the vasodilator β-AR subtype using ‘subtype-selective’ agonists against the, commonly employed, phenylephrine-induced tone. Concentration-related relaxation was produced by isoprenaline (pEC50: 7.70±0.1) (β1 and β2). Salbutamol (β2), BRL 37344 (β3) and CGP 12177 (atypical β) caused relaxation but were 144, 100 and 263 times less potent than isoprenaline; the ‘β3-adrenoceptor agonist’ CL 316243 was ineffective. In arteries precontracted with 5-HT or U 46619, isoprenaline produced concentration-related relaxation but salbutamol, BRL 37344, CGP 12177 and CL 316243 did not. SR 59230A, CGP 12177 and BRL 37344 caused a parallel rightward shift in the concentration–response curve to phenylephrine indicating competitive α1-AR antagonism, explaining the false-positive ‘vasodilator’ action against phenylephrine-induced tone. Endothelial denudation but not L-NAME slightly attenuated isoprenaline-mediated vasodilatation in phenylephrine and U 46619 precontracted MRA. The β-AR fluorescent ligand BODIPY TMR-CGP 12177 behaved as an irreversible β1-AR antagonist in MRA and bound to the surface and inside vascular smooth muscle cells in intact vascular wall. β-ARs in smooth muscle cells were observed in a perinuclear location, consistent with the location of Golgi and endoplasmic reticulum. Binding of BODIPY TMR-CGP 12177 was inhibited by BAAM (1 μM) in all three vascular tunics, confirming the presence of β-ARs in adventitia, media and intima. Binding in adventitia was observed in both neuronal and non-neuronal cell types. Lack of co-localisation with a fluorescent ligand for α-ARs confirms the selectivity of BODIPY TMR-CGP 12177 for β-ARs over α-ARs. Our results support the presence of functional vasodilator β1-ARs and show that they are mainly located in smooth muscle cells. Furthermore, we have demonstrated, for the first time, the usefulness of BODIPY TMR-CGP 12177 for identifying β-AR distribution in the ‘living’ vascular wall. British Journal of Pharmacology (2005) 146, 679–691. doi:10.1038/sj.bjp.0706369