Abstract
We investigated the characteristics of β-adrenoceptors (β-ARs) in rat white adipose tissues (WAT) with a radioligand receptor binding assay using (-)-[3H]-CGP12177. Scatchard analysis revealed that there are high- and low-affinity sites for (-)-[3H]-CGP12177 in WAT. The (-)-[3H]-CGP12177 bound to a high-affinity site was displaced by 1 μM propranolol. The rank of pKi values of catecholamines for the site was isoproterenol>epinephrine>norepinephrine. By contrast, BRL37344A, BRL35135A and SR59230A, β3-selective agonists had high affinity for the low-affinity site of (-)-[3H]-CGP12177, whereas (-)-[3H]-CGP12177 bound to a low-affinity site was completely displaced by 100 μM bupranolol but not 1 μM propranolol. The pKi values of the catecholamines (isoproterenol, norepinephrine, epinephrine) for this site were very low. In addition, the correlation between the pKi values of various β-agonists for the low-affinity site of rat WAT and those obtained from rat cloned β3-ARs was significant, but those of human cloned β3-ARs were not. Consequently, the results suggested that the high- and low-affinity sites were β2-ARs and β3-ARs in rat WAT, respectively.
