Characterization of unique ADTN-catecholamine binding sites in the iris root-ciliary body of rabbits

Abstract

A binding site for tritiated 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN) has been partially characterized in the rabbit iris root-ciliary body. Binding of ADTN is proportional to protein content and requires at least 60 minutes to reach equilibrium. Binding is saturable, with a Kd of 27 ± 1 nM and a Bmax of 2.1 ±. 3 pmol/mg protein (mean ± SEM). Dopamine competes for this site with a Ki of 100 nM and apomorphine with a Ki of 180 nM. This site is not blocked by Ltimolol, phenoxybenzamine, or by several DA1 and DA2 antagonists. It appears to be a new type of catecholamine binding site, of a type not observed outside the anterior eye. It is possible that some of the effects of dopamine on intraocular pressure are mediated through this binding site.