Characterization by 3HDihydroergocryptine Binding of Alpha‐Adrenergic Receptors in Neuroblastoma × Glioma Hybrid Cells

3HDihydroergocryptine (3HDHE) was shown to bind to sites in membranes from neuroblastoma X glioma hybrid cells (NG 108-15) that had the characteristics expected of alpha-adrenergic receptors. The binding was saturable with 0.3 pmol 3HDHE bound per mg of protein and of high affinity, with an apparent dissociation constant (KD) of 1.8 nM. The specificity of the binding site for various ligands was more similar to that of alpha 2 receptors than to that of alpha 1. No specific binding of 3HWB-4101 was found in the membranes derived from NG 108 cells. This finding also indicated that the 3HDHE binding site in the cell is the alpha 2 receptor. GTP lowered the affinity of agonists for the 3HDHE binding site, although the nucleotide hardly affected the affinity of antagonists including 3HDHE.