Localization and characterization of two propranolol resistant (-) [¹²⁵I]cyanopindolol binding sites in rat skeletal muscle

Autoradiographic studies were performed in sections of rat gastrocnemius, plantaris and soleus muscle bundles with (-)-[¹²⁵I]cyanopindolol (59-69 pM) in the presence of (-)-propranolol (1 μM) to block β₁- and β₂-adrenoceptors. Two distinct populations of binding sites remained, one evenly distributed over the muscle bundles and the other localized in discrete patches. Evenly distributed binding was highest in the soleus muscle and inhibited (±), (-)- and (+)-alprenolol (20 μM), tertatolol (1 μM), BRL 37344 (2-20 μM), (-)-isoprenaline (100 μM), phentolamine (10 μM) and haloperidol (250 μM) but not ICI 118,551 (70 nM), CGP 20712A (100 nM), (+)-isoprenaline (100 μM), pindolol (2 μM), cimaterol (100 μM) or serotonin (10 μM). Stereoselectivity for the optical isomers of alprenolol was displayed in the soleus muscle only. Highly localized binding was inhibited by serotonin (10 μM), (-)- and (+)-isoprenaline (100 μM) and phentolamine (10 μM).