Interactions of full and partial agonists with HT29 cell alpha 2-adrenoceptor: comparative study of (/sup 3/H)UK-14,304 and (/sup 3/H)clonidine binding

Journal Article · Wed Mar 01 04:00:00 EST 1989 · Mol. Pharmacol.; (United States)

OSTI ID:6257778

Paris, H; Galitzky, J; Senard, J M

The HT29 cell line expresses alpha 2-adrenoceptors that are negatively coupled to the adenylate cyclase system and is, in this respect, a valuable model for in vitro study of alpha 2-adrenergic receptivity in a tissue from human origin. In these cancerous cells, UK-14,304 is a full agonist of the alpha 2-adrenergic-mediated inhibition of the vasoactive intestinal peptide-induced cyclic AMP accumulation, whereas clonidine acts only as a partial agonist. In the present report, we used (3H)UK-14,304 as radioligand and compared its binding characteristics with those of (3H)clonidine in order to better understand the difference between full and partial agonism on the basis of agonist/receptor interactions. (3H)UK-14,304 labeled with high affinity (KD = 0.39 +/- 0.05 nM) a single class of sites having the pharmacological specificity of an alpha 2-adrenoceptor. Comparison of (3H)UK-14,304, (3H)clonidine, and (3H)yohimbine Bmax proved that both 3H-agonists labeled the same number of sites (172 +/- 14 versus 179 +/- 21 fmol/mg of protein), whereas the 3H-antagonist recognized more sites (246 +/- 22 fmol/mg of protein). Inhibition of (3H)yohimbine by the two agonists was consistent with the existence of an heterogeneous population of receptors and analysis of the data according a two-site inhibition model showed (1) that the KiL/KiH ratio was higher for UK-14,304 than for clonidine and (2) that the percentages of high affinity state receptor recognized by both agonists were identical (56 +/- 4% with UK-14,304 and 59 +/- 5% with clonidine). Kinetics of (3H)UK-14,304 and (3H)clonidine binding indicated more complex agonist-receptor interactions than equilibrium data did. Association as well as dissociation of both radioligands appeared to be biphasic, suggesting a relative heterogeneity of 3H-agonist binding sites.

• Research Organization: Universite Paul Sabatier, Toulouse (France)
• OSTI ID: 6257778
• Journal Information: Mol. Pharmacol.; (United States), Journal Name: Mol. Pharmacol.; (United States) Vol. 35:3; ISSN MOPMA
• Country of Publication: United States
• Language: English

Related Subjects

550201* -- Biochemistry-- Tracer Techniques
59 BASIC BIOLOGICAL SCIENCES
ADRENAL HORMONES
ADRENALINE
ALKALI METALS
AMP
ANIMAL CELLS
ANTIHYPERTENSIVE AGENTS
AUTONOMIC NERVOUS SYSTEM AGENTS
BIOCHEMICAL REACTION KINETICS
CARDIOTONICS
CARDIOVASCULAR AGENTS
COMPARATIVE EVALUATIONS
CYCLASES
DOSE-RESPONSE RELATIONSHIPS
DRUGS
ELEMENTS
ENZYMES
ISOTOPE APPLICATIONS
KINETICS
LABELLED COMPOUNDS
LIGANDS
LYASES
MEMBRANE PROTEINS
METALS
NEUROREGULATORS
NUCLEOTIDES
ORGANIC COMPOUNDS
PEPTIDES
PROTEINS
REACTION KINETICS
RECEPTORS
SODIUM
SYMPATHOLYTICS
SYMPATHOMIMETICS
TRACER TECHNIQUES
TRITIUM COMPOUNDS
TUMOR CELLS