p-( sup 125 I)iodoclonidine is a partial agonist at the alpha 2-adrenergic receptor

Journal Article · Wed Aug 01 04:00:00 EDT 1990 · Molecular Pharmacology; (USA)

OSTI ID:6795748

Gerhardt, M A; Wade, S M; Neubig, R R ^[1]

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1. Univ. of Michigan Medical School, Ann Arbor (USA)

The binding properties of p-(125I)iodoclonidine (( 125I)PIC) to human platelet membranes and the functional characteristics of PIC are reported. (125I)PIC bound rapidly and reversibly to platelet membranes, with a first-order association rate constant (kon) at room temperature of 8.0 +/- 2.7 x 10(6) M-1 sec-1 and a dissociation rate constant (koff) of 2.0 +/- 0.8 x 10(-3) sec-1. Scatchard plots of specific (125I)PIC binding (0.1-5 nM) were linear, with a Kd of 1.2 +/- 0.1 nM. (125I)PIC bound to the same number of high affinity sites as the alpha 2-adrenergic receptor (alpha 2-AR) full agonist (3H) bromoxidine (UK14,304), which represented approximately 40% of the sites bound by the antagonist (3H)yohimbine. Guanosine 5'-(beta, gamma-imido)triphosphate greatly reduced the amount of (125I)PIC bound (greater than 80%), without changing the Kd of the residual binding. In competition experiments, the alpha 2-AR-selective ligands yohimbine, bromoxidine, oxymetazoline, clonidine, p-aminoclonidine, (-)-epinephrine, and idazoxan all had Ki values in the low nanomolar range, whereas prazosin, propranolol, and serotonin yielded Ki values in the micromolar range. Epinephrine competition for (125I)PIC binding was stereoselective. Competition for (3H)bromoxidine binding by PIC gave a Ki of 1.0 nM (nH = 1.0), whereas competition for (3H)yohimbine could be resolved into high and low affinity components, with Ki values of 3.7 and 84 nM, respectively. PIC had minimal agonist activity in inhibiting adenylate cyclase in platelet membranes, but it potentiated platelet aggregation induced by ADP with an EC50 of 1.5 microM. PIC also inhibited epinephrine-induced aggregation, with an IC50 of 5.1 microM. Thus, PIC behaves as a partial agonist in a human platelet aggregation assay. (125I)PIC binds to the alpha 2B-AR in NG-10815 cell membranes with a Kd of 0.5 +/- 0.1 nM.

• OSTI ID: 6795748
• Journal Information: Molecular Pharmacology; (USA), Journal Name: Molecular Pharmacology; (USA) Vol. 38:2; ISSN 0026-895X; ISSN MOPMA
• Country of Publication: United States
• Language: English

Related Subjects

550201* -- Biochemistry-- Tracer Techniques
59 BASIC BIOLOGICAL SCIENCES
AROMATICS
AUTONOMIC NERVOUS SYSTEM AGENTS
AZAARENES
BETA DECAY RADIOISOTOPES
BIOCHEMICAL REACTION KINETICS
BIOLOGICAL MATERIALS
BLOOD
BLOOD CELLS
BLOOD COAGULATION
BLOOD PLATELETS
BODY FLUIDS
CELL CONSTITUENTS
CELL MEMBRANES
CHEMICAL PROPERTIES
CHROMATOGRAPHY
CYCLASES
DAYS LIVING RADIOISOTOPES
DRUGS
ELECTRON CAPTURE RADIOISOTOPES
ENZYMES
GUANOSINE
HETEROCYCLIC COMPOUNDS
INTERMEDIATE MASS NUCLEI
IODINE 125
IODINE ISOTOPES
ISOTOPE APPLICATIONS
ISOTOPES
KINETICS
LYASES
MATERIALS
MEMBRANE PROTEINS
MEMBRANES
NUCLEI
NUCLEOSIDES
NUCLEOTIDES
ODD-EVEN NUCLEI
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PARASYMPATHOMIMETICS
PROTEINS
PURINES
RADIOASSAY
RADIOISOTOPES
REACTION KINETICS
RECEPTORS
RIBOSIDES
SEPARATION PROCESSES
THIN-LAYER CHROMATOGRAPHY
TRACER TECHNIQUES