β-Adrenoreceptors of multiple affinities in a clonal capillary endothelial cell line and its functional implication

Abstract

β-Adrenoreceptor has been studied in a clonal capillary endothelial cell line established from the vascular bed of the bovine adrenal medulla. [³H]Dihydroalprenolol ([³H]DHA) binding to the isolated plasma membranes from these cells has demonstrated the presence of β-adrenoreceptors with two different affinities. the dissociation constants (K_d) have been found to be 0.27±0.09×10⁻⁹ M and 2.96±0.31×10⁻⁹ M, respectively with the corresponding B_max of 5.1±0.05 and 70.0±0.2 pmol/mg protein, respectively. Inhibition of [³H]DHA binding to the β-receptor by atenolol (a β₁-antagonist) and ICI 118,551 (a β₁-antagonist) has suggested that the IC_50cor (=K_i) for atenolol and ICI 118,551 for high affinity site are 0.08±0.03×10⁻¹² M and 0.25±0.08×10⁻¹² M, respectively. This, therefore, indicates that both atenolol and ICI 118,551 are able to displace the bound ligand effectively but the β₁-selective antagonist atenolol is 3 times more potent than its β₂ counterpart, ICI 118,551. Displacement of [³H]DHA binding to the endothelial cell plasma membrane by the agonists isoproterenol, epinephrine and norephinephrine has established a relative order of K_i for these agents as isoproterenol (0.56±0.19×10⁻⁹ M)<epinephrine (0.77±0.26⁻⁹ M)>-norepinephrine (0.71±0.24×10⁻⁹ M) for the high affinity site. The corresponding values for the low affinity site, however, are 4.62±0.64×10⁻⁹ M, 6.21±0.86×10⁻⁹ M and 5.90±0.82×10⁻⁹ M, respectively for the same agonists. Increased intracellular cAMP accompanied with cellular proliferation in the presence of isoproterenol has suggested not only the coupling of β-adrenoreceptors to the adenylate cyclase system but also its involvement in endothelial cell proliferation.