Histoautoradiographic localization of (-)-3H-dihydroalprenolol in rabbit aorta

The distribution pattern of (-)-3H-dihydroalprenolol (3H-DHA) binding sites within the thoracic and abdominal aorta was studied using a histoautoradiographic technique. Frozen sections of rabbit aorta were incubated with 3H-DHA in the presence or absence of 1 mumol/l (-)-propranolol and of increasing concentrations of various adrenergic agents. After washing, the same secretions were placed in scintillation vials and counted to evaluate the pharmacological characteristics of 3H-DHA binding or processed for histoautoradiography. 3H-DHA was bound by sections of the rabbit aorta in a manner consistent with the presence of beta-adrenoceptors. The binding was reversible, saturable and of high affinity, with a dissociation constant Kd of 1.0 nmol/l and a maximum binding capacity Bmax of 17 fmol/mg tissue (wet weight). It was inhibited only by beta-adrenoceptor blocking drugs. Histoautoradiographic studies revealed that beta-adrenoceptors are located primarily in the media and then in the intima. The lowest density of 3H-DHA was found in the adventitia. The distribution pattern of beta-adrenoceptors in the media suggests the possible existence of two groups of beta-adrenoceptors within this layer of the aorta. The first group was found in the outer portions of the media and may be controlled by the adrenergic nerves that supply the artery. The second group was found in the portions of the media closest to the intima and is likely under the control of catecholamines coming from the lumen of the aorta.