p-( sup 125 I)iodoclonidine, a novel radiolabeled agonist for studying central alpha 2-adrenergic receptors

Journal Article · Sat Sep 01 00:00:00 EDT 1990 · Molecular Pharmacology; (USA)

OSTI ID:6422762

Baron, B M; Siegel, B W ^[1]

---

1. Merrell Dow Research Institute, Cincinnati, OH (USA)

Unlabeled p-iodoclonidine was efficacious in attenuating forskolin-stimulated cAMP accumulation in SK-N-SH neuroblastoma cells. Maximal attenuation was 76 +/- 3%, with an EC50 of 347 +/- 60 nM. Comparable values of epinephrine were 72 +/- 3% and 122 +/- 22 nM. Responses to both agonists were abolished by 10 microM phentolamine. Therefore, p-iodoclonidine is an agonist in a cell culture model system of the neuronal alpha 2-adrenergic receptor. p-(125I)Iodoclonidine binding to membranes were measured using various regions of the rat brain. The agonist labeled a single population of sites present on cerebral cortical membranes, which was saturable (Bmax = 230 fmol/mg of protein) and possessed high affinity for the ligand (Kd = 0.6 nM). Binding was largely specific (93% at 0.6 nM). A variety of alpha 2-adrenergic agonists and antagonists were shown to compete for the binding of the radioligand. The binding of p-(125I)iodoclonidine was much less sensitive to agents that interact with alpha 1-adrenergic, serotonergic, and dopaminergic receptors. Approximately 65% of the binding was sensitive to guanine nucleotides. Association kinetics using 0.4 nM radioligand were biphasic (37% associate rapidly, with kobs = 0.96 min-1, with the remainder binding more slowly, with kobs = 0.031 min-1) and reached a plateau by 90 min at 25 degrees. Dissociation kinetics were also biphasic, with 30% of the binding dissociating rapidly (k1 = 0.32 min-1) and the remainder dissociating 50-fold more slowly (k2 = 0.006 min-1). Agonist binding is, therefore, uniquely complex and probably reflects the conformational changes that accompany receptor activation.

• OSTI ID: 6422762
• Journal Information: Molecular Pharmacology; (USA), Vol. 38:3; ISSN 0026-895X
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
ENZYME INHIBITORS
BIOCHEMICAL REACTION KINETICS
PARASYMPATHOMIMETICS
RECEPTORS
ADRENALINE
AMP
CELL MEMBRANES
IODINE 125
NERVE CELLS
RADIOASSAY
RATS
STRUCTURE-ACTIVITY RELATIONSHIPS
TRACER TECHNIQUES
TUMOR CELLS
ADRENAL HORMONES
ANIMAL CELLS
ANIMALS
AUTONOMIC NERVOUS SYSTEM AGENTS
BETA DECAY RADIOISOTOPES
CARDIOTONICS
CARDIOVASCULAR AGENTS
CELL CONSTITUENTS
DAYS LIVING RADIOISOTOPES
DRUGS
ELECTRON CAPTURE RADIOISOTOPES
INTERMEDIATE MASS NUCLEI
IODINE ISOTOPES
ISOTOPE APPLICATIONS
ISOTOPES
KINETICS
MAMMALS
MEMBRANE PROTEINS
MEMBRANES
NEUROREGULATORS
NUCLEI
NUCLEOTIDES
ODD-EVEN NUCLEI
ORGANIC COMPOUNDS
PROTEINS
RADIOISOTOPES
REACTION KINETICS
RODENTS
SOMATIC CELLS
SYMPATHOMIMETICS
VERTEBRATES
550201* - Biochemistry- Tracer Techniques