title: Interaction of clonidine and clonidine analogues with alpha-adrenergic receptors of neuroblastoma X glioma hybrid cells and rat brain: comparison of ligand binding with inhibition of adenylate cyclase.
abstract: Clonidine and several analogues of clonidine are shown to be useful probes for alpha 2-adrenergic receptors in a comparative study of ligand binding and inhibition of adenylate cyclase. The alpha-adrenergic properties of a new potential probe, N-(4-hydroxyphenacetyl)-4-aminoclonidine hydrochloride, are described. [3H]Clonidine binds to alpha-receptors of NG108-15 neuroblastoma X glioma hybrid cell membranes with Kd values of 1.7 and 33 nM for putative high-affinity and low-affinity sites, respectively. p-Aminoclonidine and hydroxyphenacetyl aminoclonidine displace [3H]clonidine from the high-affinity sites with Kd values of 2.3 and 5.8 nM, respectively. Rat brain alpha 2-receptors also exhibit high affinity toward clonidine, p-aminoclonidine, and hydroxyphenacetyl aminoclonidine, as determined by displacement of specifically bound [3H]clonidine. Clonidine, p-amino-clonidine, and hydroxyphenacetyl aminoclonidine elicit modest inhibition (up to 24%) of NG108-125 adenylate cyclase by interaction with alpha 2-receptors (Kd,app 300, 30, and 130 nM, respectively); these compounds also partially reverse the inhibition elicited by (--)-norepinephrine. Components of the adenylate cyclase assay mixture, particularly ATP, GTP, sodium ions, and a nucleoside-triphosphate-regenerating system, decrease the high-affinity [3H]clonidine binding to NG108-15 membranes; in the presence of these components, alpha-receptors possess only low affinity (Kd 43 nM) for [3H]clonidine. The results are consistent with the concept that certain components required for the receptor-mediated inhibition of adenylate cyclase convert alpha 2-receptors from a high-affinity inactive state to a low-affinity active state.
mesh_terms: D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D044482:Benzeneacetamides; D001921:Brain / Q000378:metabolism*; D003000:Clonidine / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology; D005260:Female; D005910:Glioma / Q000378:metabolism*; D006822:Hybrid Cells / Q000378:metabolism; D007700:Kinetics; D009374:Neoplasms, Experimental / Q000378:metabolism; D009447:Neuroblastoma / Q000378:metabolism*; D051381:Rats; D011941:Receptors, Adrenergic / Q000378:metabolism*; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*
chemical_list: D000067956:Adenylyl Cyclase Inhibitors; D044482:Benzeneacetamides; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; C029376:hydroxyphenacetyl aminoclonidine; C016986:apraclonidine; D003000:Clonidine