LIGAND-BINDING PROPERTIES OF PUTATIVE BETA-3-ADRENOCEPTORS COMPARED IN BROWN ADIPOSE-TISSUE AND IN SKELETAL-MUSCLE MEMBRANES

1 The beta-adrenoceptor population was characterized in membrane preparations from rat brown adipose tissue (BAT) and from soleus muscle by use of the radioligand [I-125]-iodocyanopindolol ([I-125]-ICYP). In addition, atypical binding sites for [I-125]-ICYP found in both tissues were examined, and the relationship between these sites and the putative rat beta3-adrenoceptor is discussed. 2 It was established that BAT membranes host a mixed population of beta1- and beta2-adrenoceptors. Of these two sites, 55% showed a high affinity for the beta1-selective ligand CGP 20712A (pK 8.5), and 45% showed a high affinity for the beta2-selective antagonist ICI 118551 (pK 8.6). Soleus muscle membranes were found to host a population Of beta2-adrenoceptors, characterized by a high affinity for ICI 118551 (pK 9.1), but beta1-adrenoceptors could not be detected in this preparation. 5-Hydroxytryptamine receptors were not detected in either preparation. 3 In addition to beta1- and beta2-adrenoceptors, atypical binding sites were identified in both tissues using high concentrations of radioligand (0.5-0.6 nm) and in the presence of 1 muM (-)-propranolol. The atypical sites were abundant, representing 80 and 81% of the total [I-125]-ICYP binding sites in BAT and soleus muscle respectively. When the pK values for 11 ligands were compared, the correlation coefficient for atypical sites in BAT and soleus muscle was 0.94. 4 The atypical binding sites showed a moderate affinity for (+/-)-cyanopindolol (pK 7.3-7.7), poor stereoselectivity for the (+)- and (-)-enantiomers of alprenolol (<10 fold), and a low affinity for beta-adrenoceptor antagonists and partial agonists in the order: (+/-)-cyanopindolol>(-)-alprenolol> (-)-propranolol = (+/-)-ICI 118551>>(+/-)-CGP20712A. The affinity of these ligands for the atypical sites reflects their behaviour in functional studies of putative beta3-adrenoceptors in rat BAT, white adipose tissue, intestine and colon. 5 The atypical sites labelled by [I-125]-ICYP were resistant to agonist binding, and while the order of affinity of the agonists BRL 37344 > isoprenaline > noradrenaline matches their order of potency at putative beta3-adrenoceptors, none of these compounds caused displacement of the radioligand at concentrations below 10 muM. 6 It is concluded that the atypical binding sites for [I-125]-ICYP found in rat BAT and soleus muscle membranes are the same, and that these sites show some relationship to the putative rat beta3-adrenoceptor identified in functional studies using antagonists. However, under the conditions used in the present study, pK values obtained for beta3-agonist binding are not useful.