[3H]norepinephrine binding by rat glial cells in culture lack of correlation between binding and adenylate cyclase activation

Subcellular fractions prepared from rat glial cells in culture (clonal line C₆) were used in an attempt to characterize the adrenergic receptor involved in adenylate cyclase activation. Both ³H]norepinephrine binding and enzyme activation were measured under identical experimental conditions.Binding sites for norepinephrine could be detected; their main characteristics were: apparent K_m : 4 · 10⁻⁶ M, maximal capacity: 20 pmol/mg protein.Their stereospecificity towards structually related drugs was found to be different from the stereospecificity of the receptor involved in adenylate cyclase activation. Thus, 3-methoxydopamine (a competitive inhibitor of norepinephrine for adenylate cyclase activation), phenylephrine (a partial adrenergic agonist) and the blocking agent propranolol were unable to compete with ³H]norepinephrine for binding. On the other hand, several molecules like dopa bearing a catechol group and which are unable to interact with the adenylate cyclase as agonists or competitive inhibitors strongly inhibited ³H]norepinephrine binding.As in several other systems so far studied, the presence on the glial cell's membrane of a large number of “catechol-binding sites” makes it difficult to characterize the β-adrenergic receptor.