Angiotensin II receptors labelled with 125I-[Sar1, Ile8]-AII in albino rabbit ocular tissues

High affinity binding sites for the angiotensin II antagonist 125I-[Sar1,Ile8]-AII have been identified and characterized in membrane suspensions of ocular tissues of albino rabbits. Scatchard analysis of the binding indicated a single class of sites with Kd values of 186, 92, 152, 50, 102 pM for the iris + ciliary body, choroid, ciliary process, retina and cornea, respectively. The corresponding concentrations of binding sites were 22, 68, 35, 22 and 4 fmole/mg of protein. The order of potency for several AII analogs to compete with 125I-[Sar1,Ile8]-AII at its binding sites in iris + ciliary body membranes ([Sar1,Leu8]-AII = [Sar1,Ile8]-AII greater than AII = [Sar1, Ala8]-AII greater than AIII greater than AI) resembled the order of potency found for AII receptors in other tissues. The competition curves for this tissue using AII and AIII were best explained by the existence of two populations of binding sites. The addition of the guanine nucleotide, GppNHp, to the assay resulted in a 6.7-fold and 2.3-fold decrease in the respective affinities of AII and AIII for 125I-[Sar1,Ile8]-AII binding sites without a change in the slope of the competition curves. The GppNHp-induced effect was also observed in ciliary process membranes but not in retinal or choroidal membranes. These results indicate the presence of AII receptors regulated by a GTP-binding protein in both the ciliary process and the iris + ciliary body of the rabbit. They also suggest a difference in the guanine nucleotide regulation of AII receptors in different ocular tissues.