β‐Adrenoceptor subtype expression and function in rat white adipocytes
Authors: Germack Renée , Starzec Anna B , Vassy Roger , Perret Gérard Y 
Published: August 2006

The pharmacological features of rat white adipocyte β‐adrenoceptor subtypes were investigated by saturation and β‐agonist competition studies with [3H]‐CGP 12177 and by lipolysis induced by β‐agonists as well as their inhibition by CGP 20712A (selective β1‐antagonist) and ICI 118551 (selective β2‐antagonist) in an attempt to establish a relationship between the functionality and binding capacity of β‐adrenoceptor subtypes.

Two populations of binding sites were identified on adipocyte membranes, one with high affinity (0.22±0.07 nm) and the other with low affinity (23±7 nm). The low affinity binding sites constituted 90% of the total binding sites.

The competition curves, with 15 nm [3H]‐CGP 12177, for the β‐agonists, isoprenaline (Iso), noradrenaline (NA) and adrenaline (Ad), and the selective β3‐agonist, BRL 37344 (BRL), were clearly biphasic (P<0.001). The rank orders of agonist potency (pKi) in competing for [3H]‐CGP 12177 high affinity and low affinity binding sites, respectively, were Iso (9.28±0.24)>NA (8.90±0.12)>Ad (8.65±0.12)>>BRL (4.53±0.17) and BRL (7.38±0.19)>>Iso (2.96±0.26)NA (2.80±0.17)>Ad (2.10±0.11) indicating the expression of β1‐ and β3‐adrenoceptor subtypes on rat white adipocytes, respectively. Inversely, competition studies with the selective β1‐agonist, xamoterol (Xam), provided evidence for a single homogeneous population of binding sites with low density (81±9 fmol mg−1) and high pKi value (7.23±0.26) confirming the presence of β1‐adrenoceptors.

To assess a possible contribution of the β2‐subtype, procaterol (Proc), a selective β2‐agonist, was used to compete with 2 nm [3H]‐CGP 12177. A single low affinity (4.61±0.07) population of binding sites was identified. The density of these sites (71±12 fmol mg−1) was similar to the one obtained with Xam, suggesting that Proc displaced [3H]‐CGP 12177 from the β1‐subtype.

The functional potency (pD2) order with BRL (9.07±0.20) and catecholamines (Iso: 7.26±0.06, NA: 6.89±0.02 and Ad: 6.32±0.07) was the same as that found for the low affinity binding sites in competition studies. Xam induced lipolysis with greater potency than dobutamine (Dob), 6.31±0.06 and 5.66±0.10, respectively. Proc stimulated lipolysis with a low potency (5.59±0.21).

The lipolytic response to 0.001 μm BRL was inhibited by both, selective β1‐ and β2‐antagonist, in a monophasic manner with low potencies (CGP 20712A pKi: <4.5 and ICI 118551 pKi: 5.57±0.13). Similar monophasic profiles were obtained for inhibition of Xam‐ and Dob‐induced lipolysis. In this case, CGP 20712A was more potent (>10 times) than ICI 118551. The monophasic inhibition was also observed with ICI 118551 in the presence of 0.05 μm Iso or 0.13 μm NA. In contrast, two populations of sites were identified with CGP 20712A in the presence of Iso as well as NA. The pKi values for the first sites were 8.41±0.09 and 8.58±0.17, respectively, and for the second population of sites 4.73±0.22 and 4.27±0.27, respectively. The proportion of the first sites was low: 19±4 and 22±5%, respectively. Biphasic curves were obtained with both antagonists using 2.5 μm Proc (CGP 20712A: pKi1: 8.17±0.08, site1: 23±6%, pKi2: 4.77±0.14; ICI 118551: pKi1: 7.78±0.03, site1: 37±2%, pKi2: 5.35±0.25).

Our results show that the radioligand [3H]‐CGP 12177 allows the characterization of β1‐ and β3‐adrenoceptor subtypes on rat white adipocytes. Lipolysis is highly dependent on β1‐ and β3‐adrenoceptors. Finally, binding and functional studies confirm that lipolysis is mainly driven by the β3‐subtype.

British Journal of Pharmacology (1997) 120, 201–210; doi:10.1038/sj.bjp.0700885