Autoradiographic analysis of the distribution of β‐adrenoceptors in the dog splenic vasculature

Abstract

1 The technique of in vitro labelling and autoradiography has been used to localize β-adrenoceptors in sections of the splenic vascular bundle of the dog. 2 Binding of (−)-[¹²⁵I]-cyanopindolol (Cyp) to sections of splenic vascular bundle equilibrated within 150 min and slowly dissociated after addition of (−)-propranolol. The process was saturable with a dissociation constant (K_D) of 40.3 ± 4.4 pM and B_max of 18.9 ± 1.7 fM (in 6 sections). 3 Binding to sections was stereoselective, the (−)-isomer of propranolol being 90 times more effective than the (+)-isomer in competing for (−)-[¹²⁵I]-Cyp binding. 4 Delineation of β-adrenoceptor subtypes using the selective antagonists betaxolol (β₁) and ICI 118,551 (β₂) indicated that the receptors present were almost exclusively of the β₂-subtype. 5 Autoradiographic studies under the conditions evaluated in the biochemical experiments showed that β-adrenoceptors are unevenly distributed in the dog splenic vein, artery and associated nerve bundles. High concentrations of receptors are associated with the splenic nerves and lower but still significant concentrations in the vasculature. 6 Higher resolution studies with nuclear emulsion coated coverslips revealed concentrations of β-adrenoceptors over cells adjacent to the lumen in veins. In arteries most β-adrenoceptors were found associated with the medial layer with fewer receptors towards the intima or adventitia. Serial sections of either artery or vein incubated with (−)-[¹²⁵I]-Cyp in the presence of (−)-propranolol showed low levels of non-localized binding.