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.
|Number of pages||9|
|Journal||Investigative Ophthalmology and Visual Science|
|State||Published - 1 Jan 1994|
- rabbit ciliary body
- radioligand binding assay [I]-p-iodoclonidine