Characterization of propranolol‐resistant (−)‐[125I]‐cyanopindolol binding sites in rat soleus muscle

Abstract

The characteristics of a propranolol‐resistant (−)‐[125I]‐cyanopindolol (CYP) binding site in rat soleus muscle were determined. Saturation studies performed on homogenates of rat soleus muscle showed two phases of (−)‐[125I]‐CYP binding, a high affinity site (KD1 30.5 ± 16.3 pm, Bmax 9.4 ± 1.38 fmol mg−1 protein) and a lower affinity site (KD2 522.5 ± 29.1 pm, Bmax 62.19 ± 11.76 fmol mg−1 protein, n = 4). In rat soleus muscle homogenates labelled with (−)‐[125I]‐CYP (500 pm), (−)‐propranolol competition curves were biphasic with pKD values of 8.30 ± 0.19, and 5.33 ± 0.08, n = 7. Competition between (−)‐[125I]‐CYP (500 pm) and (±)‐tertatolol, (±)‐nadolol, (±)‐alprenolol, (±)‐CYP, and (−) and (+)‐pindolol showed that these compounds competed for binding at the propranolol‐resistant site with affinities lower than those displayed at typical β‐adrenoceptors. The atypical β‐adrenoceptor agonists BRL 37344, SR58611A and ICI D7114 and the partial agonist (±)‐CGP 12177 also competed for (−)‐[125I]‐CYP binding. Stereoselectivity was demonstrated for the stereoisomers of alprenolol and tertatolol. The (−)‐isomers of alprenolol and tertatolol had higher affinity than their corresponding (+)‐isomers (3.1 and 2.6 fold respectively). These low stereoselectivity values are a characteristic of atypical β‐adrenoceptors. The β‐adrenoceptor agonists, (−)‐adrenaline, (−)‐isoprenaline and (−)‐noradrenaline, all showed lower affinity than the atypical β‐adrenoceptor agonists and competition curves appeared biphasic in nature. These results confirm the presence of a propranolol‐resistant (−)‐[125I]‐CYP binding site in rat soleus muscle. The affinities of the tested compounds at the propranolol‐resistant (−)‐[125I]‐CYP binding site show similarities to their affinities at ‘atypical’ β‐adrenoceptors in adipocytes and gastrointestinal tissues and at the cloned β3‐adrenoceptor. 1993 British Pharmacological Society