Characterization and Regulation of β1‐Adrenergic Receptors in a Human Neuroepithelioma Cell Line

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Abstract
Intact human neuroepithelioma SK‐N‐MC cells bound the β‐adrenergic antagonist (–)‐[3H]‐CGP 12177 with a KD of 0.13 nM and a Bmax of 17,500 sites/cell. When the cells were exposed to β‐adrenergic agonists, they accumulated cyclic AMP in the following order of potency: isoproterenol norepinephrine > epinephrine, which is indicative of a β1‐subtype receptor. Membranes prepared from the cells bound (–)‐3‐[125I]iodocyanopindolol with a KD of 11.5 pM. Inhibition of agonist‐stimulated cyclic AMP production and competition binding experiments indicated that the β1‐selective antagonists CGP 20712A and ICI 89,406 were much more potent than the β2‐selective antagonist ICI 118,551. Analysis of the displacement curves indicated that the cells contained only β1‐adrenergic receptors. Northern blot analysis of SK‐N‐MC mRNA using cDNA probes for the β1‐ and β2‐adrenergic receptors revealed the presence of a very strong β1‐adrenergic receptor mRNA signal, while under the same conditions no β2‐adrenergic receptor mRNA was observed. Thus, SK‐N‐MC cells appear to express a pure population of β1‐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.