Characterization of α2-adrenoceptor binding sites in rabbit ciliary body membranes

Y. Jin, A. Verstappen, Thomas Yorio

Research output: Contribution to journalArticle

14 Citations (Scopus)

Abstract

Purpose. This study sought to identify and characterize subtypes of α2- adrenoceptors in rabbit ciliary body. Methods. Radioligand binding assays were performed with the α2-agonist ligand [125I]-p-iodoclonidine ([125I]PIC) and the antagonist ligand [3H]rauwolscine. Results. Both [125I]PIC and [3H]rauwolscine bound to a single population of receptors in membrane preparations of rabbit ciliary body. The densities for the two ligands were the same (about 640 fmol/mg). However, the affinity of [125I]PIC (dissociation constant, K(d) = 1.91 ± 0.24 nM) was about threefold higher than that of [3H]rauwolscine (dissociation constant = 6.79 ± 1.5 nM), and binding of [125I]PIC exhibited guanine nucleotide sensitivity. Inhibition of [125I]PIC binding by epinephrine, idazoxan, and amiloride was examined to differentiate between α2-adrenoceptors and the imidazoline-preferring receptor. Epinephrine and idazoxan competed for all of the [125I]PIC binding; relative potency was epinephrine > idazoxan >> amiloride. Subtypes of α2-adrenoceptors were further studied by competition for [125I]PIC binding by subtype-selective compounds. [125I]PIC binding sites showed the pharmacologic characteristics of an α(2A)-adrenoceptor (oxymetazoline > chlorpromazine >> prazosine), and competition by oxymetazoline and chlorpromazine was best fit by a one-site model. Likewise, the relative potency of inhibition of [3H]rauwolscine binding was oxymetazoline > chlorpromazine. However, inhibition of [3H]rauwolscine- binding by oxymetazoline was better fit by a two-site model, which was converted to a one-site model in the presence of 100 μM 5'- guanylimidodiphosphate. Conclusions. A large number of α2-adrenoceptors are present in rabbit ciliary body. They are not the imidazoline-preferring receptor but are α2-adrenergic-specific receptors of the α(2A) subtype.

Original languageEnglish
Pages (from-to)2500-2508
Number of pages9
JournalInvestigative Ophthalmology and Visual Science
Volume35
Issue number5
StatePublished - 1 Jan 1994

Fingerprint

Ciliary Body
Oxymetazoline
Yohimbine
Adrenergic Receptors
Binding Sites
Idazoxan
Rabbits
Membranes
Chlorpromazine
Epinephrine
Amiloride
Ligands
Imidazoline Receptors
Radioligand Assay
Guanine Nucleotides
Population

Keywords

  • α-adrenoceptors
  • [H]rauwolscine
  • rabbit ciliary body
  • radioligand binding assay [I]-p-iodoclonidine

Cite this

@article{61557c6f9fd540c5933aefd6ba540087,
title = "Characterization of α2-adrenoceptor binding sites in rabbit ciliary body membranes",
abstract = "Purpose. This study sought to identify and characterize subtypes of α2- adrenoceptors in rabbit ciliary body. Methods. Radioligand binding assays were performed with the α2-agonist ligand [125I]-p-iodoclonidine ([125I]PIC) and the antagonist ligand [3H]rauwolscine. Results. Both [125I]PIC and [3H]rauwolscine bound to a single population of receptors in membrane preparations of rabbit ciliary body. The densities for the two ligands were the same (about 640 fmol/mg). However, the affinity of [125I]PIC (dissociation constant, K(d) = 1.91 ± 0.24 nM) was about threefold higher than that of [3H]rauwolscine (dissociation constant = 6.79 ± 1.5 nM), and binding of [125I]PIC exhibited guanine nucleotide sensitivity. Inhibition of [125I]PIC binding by epinephrine, idazoxan, and amiloride was examined to differentiate between α2-adrenoceptors and the imidazoline-preferring receptor. Epinephrine and idazoxan competed for all of the [125I]PIC binding; relative potency was epinephrine > idazoxan >> amiloride. Subtypes of α2-adrenoceptors were further studied by competition for [125I]PIC binding by subtype-selective compounds. [125I]PIC binding sites showed the pharmacologic characteristics of an α(2A)-adrenoceptor (oxymetazoline > chlorpromazine >> prazosine), and competition by oxymetazoline and chlorpromazine was best fit by a one-site model. Likewise, the relative potency of inhibition of [3H]rauwolscine binding was oxymetazoline > chlorpromazine. However, inhibition of [3H]rauwolscine- binding by oxymetazoline was better fit by a two-site model, which was converted to a one-site model in the presence of 100 μM 5'- guanylimidodiphosphate. Conclusions. A large number of α2-adrenoceptors are present in rabbit ciliary body. They are not the imidazoline-preferring receptor but are α2-adrenergic-specific receptors of the α(2A) subtype.",
keywords = "α-adrenoceptors, [H]rauwolscine, rabbit ciliary body, radioligand binding assay [I]-p-iodoclonidine",
author = "Y. Jin and A. Verstappen and Thomas Yorio",
year = "1994",
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Characterization of α2-adrenoceptor binding sites in rabbit ciliary body membranes. / Jin, Y.; Verstappen, A.; Yorio, Thomas.

In: Investigative Ophthalmology and Visual Science, Vol. 35, No. 5, 01.01.1994, p. 2500-2508.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Characterization of α2-adrenoceptor binding sites in rabbit ciliary body membranes

AU - Jin, Y.

AU - Verstappen, A.

AU - Yorio, Thomas

PY - 1994/1/1

Y1 - 1994/1/1

N2 - Purpose. This study sought to identify and characterize subtypes of α2- adrenoceptors in rabbit ciliary body. Methods. Radioligand binding assays were performed with the α2-agonist ligand [125I]-p-iodoclonidine ([125I]PIC) and the antagonist ligand [3H]rauwolscine. Results. Both [125I]PIC and [3H]rauwolscine bound to a single population of receptors in membrane preparations of rabbit ciliary body. The densities for the two ligands were the same (about 640 fmol/mg). However, the affinity of [125I]PIC (dissociation constant, K(d) = 1.91 ± 0.24 nM) was about threefold higher than that of [3H]rauwolscine (dissociation constant = 6.79 ± 1.5 nM), and binding of [125I]PIC exhibited guanine nucleotide sensitivity. Inhibition of [125I]PIC binding by epinephrine, idazoxan, and amiloride was examined to differentiate between α2-adrenoceptors and the imidazoline-preferring receptor. Epinephrine and idazoxan competed for all of the [125I]PIC binding; relative potency was epinephrine > idazoxan >> amiloride. Subtypes of α2-adrenoceptors were further studied by competition for [125I]PIC binding by subtype-selective compounds. [125I]PIC binding sites showed the pharmacologic characteristics of an α(2A)-adrenoceptor (oxymetazoline > chlorpromazine >> prazosine), and competition by oxymetazoline and chlorpromazine was best fit by a one-site model. Likewise, the relative potency of inhibition of [3H]rauwolscine binding was oxymetazoline > chlorpromazine. However, inhibition of [3H]rauwolscine- binding by oxymetazoline was better fit by a two-site model, which was converted to a one-site model in the presence of 100 μM 5'- guanylimidodiphosphate. Conclusions. A large number of α2-adrenoceptors are present in rabbit ciliary body. They are not the imidazoline-preferring receptor but are α2-adrenergic-specific receptors of the α(2A) subtype.

AB - Purpose. This study sought to identify and characterize subtypes of α2- adrenoceptors in rabbit ciliary body. Methods. Radioligand binding assays were performed with the α2-agonist ligand [125I]-p-iodoclonidine ([125I]PIC) and the antagonist ligand [3H]rauwolscine. Results. Both [125I]PIC and [3H]rauwolscine bound to a single population of receptors in membrane preparations of rabbit ciliary body. The densities for the two ligands were the same (about 640 fmol/mg). However, the affinity of [125I]PIC (dissociation constant, K(d) = 1.91 ± 0.24 nM) was about threefold higher than that of [3H]rauwolscine (dissociation constant = 6.79 ± 1.5 nM), and binding of [125I]PIC exhibited guanine nucleotide sensitivity. Inhibition of [125I]PIC binding by epinephrine, idazoxan, and amiloride was examined to differentiate between α2-adrenoceptors and the imidazoline-preferring receptor. Epinephrine and idazoxan competed for all of the [125I]PIC binding; relative potency was epinephrine > idazoxan >> amiloride. Subtypes of α2-adrenoceptors were further studied by competition for [125I]PIC binding by subtype-selective compounds. [125I]PIC binding sites showed the pharmacologic characteristics of an α(2A)-adrenoceptor (oxymetazoline > chlorpromazine >> prazosine), and competition by oxymetazoline and chlorpromazine was best fit by a one-site model. Likewise, the relative potency of inhibition of [3H]rauwolscine binding was oxymetazoline > chlorpromazine. However, inhibition of [3H]rauwolscine- binding by oxymetazoline was better fit by a two-site model, which was converted to a one-site model in the presence of 100 μM 5'- guanylimidodiphosphate. Conclusions. A large number of α2-adrenoceptors are present in rabbit ciliary body. They are not the imidazoline-preferring receptor but are α2-adrenergic-specific receptors of the α(2A) subtype.

KW - α-adrenoceptors

KW - [H]rauwolscine

KW - rabbit ciliary body

KW - radioligand binding assay [I]-p-iodoclonidine

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