Expressed alpha(1)-adrenoceptors in adult rat brown adipocytes are primarily of alpha(1A) subtype

The main objective of this study was to characterize the alpha(1)-adrenoceptors expressed in adult rat brown adipocytes. For this purpose, membrane fractions were prepared from brown adipose tissue as well as from isolated brown adipocytes. The following are major findings: (i) BAT membranes were considerably enriched in alpha(1)-adrenoceptors (specific [H-3]prazosin binding, B-max, 79.49+/-16.77 fmol/mg protein; K-D, 0.24+/-0.04 nM); (ii) among the cells that comprise brown adipose tissue, brown adipocytes were enriched in alpha(1)-adrenoceptors; (iii) >95% of total alpha(1)-adrenoceptors were resistant to inactivation by 20 mu M chloroethylclonidine, which readily and essentially completely inactivated alpha(1B)-adrenoceptors in rat liver membranes; (iv) brown adipose tissue membrane alpha(1)-adrenoceptors showed high affinity towards 5-methyl urapidil (K-D 7.23+/-2.39 nM) and WB 4101 (K-D 0.66+/-0.30 nM) and low affinity towards BMY 7378 (K-D 0.34+/-0.03 mu M); essentially similar affinities for these drugs were seen for membranes prepared from brown adipocytes; and (v) EBDA/LIGAND analysis of 5-methyl urapidil, WB 4101, and BMY 7378 competition curves revealed the presence of a single binding site for these drugs. Recent work has documented that 5-methyl urapidil and WB 4101 interact with high affinity with alpha(1A)-adrenoceptors, while BMY 7378 interacts with high affinity with alpha(1D)-adrenoceptors. Taken together, these findings an consistent with the view that alpha(1)-adrenoceptors expressed in adult rat BAT are mainly of the alpha(1A) subtype.