Interaction of Clonidine and Clonidine Analogues with a-Adrenergic Receptors of Neuroblastoma Glioma Hybrid Cells and Rat Brain : Comparison of Ligand Binding with Inhibition of Adenylate Cyclase

Daphne ATLAS; Steven, L. SABOL

Febs Journal 113(3): 521-529

1981


ISSN/ISBN: 1742-464X
DOI: 10.1111/j.1432-1033.1981.tb05094.x
Accession: 061627833

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Abstract
Clonidine and several analogues of clonidine are shown to be useful probes for α2-adrenergic receptors in a comparative study of ligand binding and inhibition of adenylate cyclase. The α-adrenergic properties of a new potential probe, N-(4-hydroxyphenacetyl)-4-aminoclonidine hydrochloride, are described. [3H]Clonidine binds to α-receptors of NG108-15 neuroblastoma × glioma hybrid cell membranes with Kd values of 1.7 and 33 n M 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 n M, respectively. Rat brain α2-receptors also exhibit high affinity toward clonidine, p-aminoclonidine, and hydroxyphenacetyl aminoclonidine, as determined by displacement of specifically bound [3H]clonidine. Clonidine, p-aminoclonidine, and hydroxyphenacetylaminoclonidine elicit modest inhibition (up to 24%) of NG108-15 adenylate cyclase by interaction with α2-receptors (Kd,app 300, 50, and 130 n M, 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, α-receptors possess only low affinity (Kd 43 n M) for [3H]clonidine. These results are consistent with the concept that certain components required for the receptor-mediated inhibition of adenylate cyclase convert α2-receptors from a high-affinity inactive state to a low-affinity active state.