Radioligand binding characteristics of β2–adrenoceptors of cultured melanoma cells

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Abstract
The existence of .beta.-adrenoceptors on intact cells of the human malignant melanoma cell line A-375 was investigated using the binding properties of the tritiated radioligand (-)-[3H]CGP-12177, a hydrophilic non-selective .beta.-adrenoceptor antagonist. Displacement experiments of the radioligand from its binding site were performed with antagonists and agonists to determine the .beta.-adrenoceptor subtype selectivity. The binding of (-)-[3H]CGP-12177 was saturable, of high affinity (KD = 0.025 nmol/l, n = 12) and was rapid and readily reversible. The maximal number of binding sites (Bmax) was 33.5 .+-. 1.9 fmol/107 cells or 2018 .+-. 114 receptors per cell. .beta.-adrenoceptor antagonists inhibited binding of the radioligand with monophasic displacement curves. IC50 values were (nmol/l): propranolol (non-selective) 2.82, alprenolol (non-selective) 2.0, ICI 118,551 (.beta.2-selective) 2200. Agonists inhibited binding in the order of potency of isoprenaline > adrenaline > noradrenaline. It is concluded that cells of the melanoma cell line A-375 contain a homogeneous population of .beta.2 adrenoceptors.