CHARACTERIZATION AND REGULATION OF BETA-1-ADRENERGIC RECEPTORS IN A HUMAN NEUROEPITHELIOMA CELL-LINE

Intact human neuroepithelioma SK-N-MC cells bound the beta-adrenergic antagonist (-)-[H-3]-CGP 12177 with a K(D) of 0.13 nM and a B(max) of 17,500 sites/cell. When the cells were exposed to beta-adrenergic agonists, they accumulated cyclic AMP in the following order of potency: isoproterenol >> norepinephrine > epinephrine, which is indicative of a beta-1-subtype receptor. Membranes prepared from the cells bound (-)-3-[I-125]iodocyanopindolol with a K(D) of 11.5 pM. Inhibition of agonist-stimulated cyclic AMP production and competition binding experiments indicated that the beta-1-selective antagonists CGP 20712A and ICI 89,406 were much more potent than the beta-2-selective antagonist ICI 118,551. Analysis of the displacement curves indicated that the cells contained only beta-1-adrenergic receptors. Northern blot analysis of SK-N-MC mRNA using cDNA probes for the beta-1- and beta-2-adrenergic receptors revealed the presence of a very strong beta-1-adrenergic receptor mRNA signal, while under the same conditions no beta-2-adrenergic receptor mRNA was observed. Thus, SK-N-MC cells appear to express a pure population of beta-1-adrenergic receptors. When the cells were exposed to isoproterenol, there was no observable desensitization during the first hour. After longer exposure, desensitization slowly occurred and the receptors slowly down-regulated to 50% of control levels by 24 h. Other agents that elevate cyclic AMP levels, such as forskolin, cholera toxin, and cyclic AMP analogues, caused no or little substantial receptor loss.