Quantitative and autoradiographic analyses of α-adrenergic and serotonergic receptors on aorta and coronary artery

Norepinephrine, epinephrine, and 5-hydroxytryptamine (5-HT) modulate the vascular tone via specific receptors on the vascular wall. Binding characteristics and localization of α-adrenergic and serotonergic receptors were determined in porcine aortas and coronary arteries. Radioligand binding studies were done using frozen sections of vascular tissue (10 μm thick), and the obtained data were compared with findings on microsomal fractions prepared from smooth muscle cells. Affinities [dissociation constant (K(d))] of ¹²⁵I-labeled 2-[β-(4-hydroxyphenyl)-ethylaminomethyl]-tetralone (BE 2254), an α₁-antagonist, for α-adrenergic receptors on aortic and coronary microsomal fractions were 190 and 224 pM, respectively. The K(d) of ¹²⁵I-labeled lysergic acid diethylamide (LSD), a serotonergic antagonist, to serotonergic receptors on the aorta was 403 pM, a value less than that obtained for the coronary artery (873 pM, P < 0.01). The K(d) of ¹²⁵I-BE 2254 on tissue sections from the aorta was 164 pM and did not significantly differ from that obtained for microsomal fractions. Inhibition constants of adrenergic agonists and antagonists for specific ¹²⁵I-BE 2254 binding on aortic sections were identical with those obtained for microsomal fractions. Receptor densities examined by ¹²⁵I-BE 2254 and ¹²⁵I-LSD were higher in the aorta than in the coronary artery. ¹²⁵I-BE 2254 bound specifically to α₁-adrenergic and ¹²⁵I-LSD to 5-HT₁-like receptors. Radiodensities of ¹²⁵I-BE 2254 and ¹²⁵I-LSD were homogenous macroscopically across the aortic media. Silver grains distributed homogenously over the smooth muscle cells across the media. There was no accumulation of silver grains to the intima. Radiodensity of ¹²⁵I-BE 2254 in the coronary artery was also homogeneous across the media, but that of ¹²⁵I-LSD was not detected. The characterization and autoradiographic visualization of adrenergic and serotonergic receptors across the aortas and coronary arteries should pave the way for a better understanding of these receptors.