Receptor Autoratiographic Evidence for Specific 125I-Endothelin-1 Binding Sites in the Rat Eye

Specific 125I-endothelin-1 (125I-ET-1) binding sites were investigated in the rat eye, using receptor autoradiographic techniques. 125I-ET-1 was specifically bound to the rat eye sections, with a slow association rate, as evidenced by kinetic experiments. The radioligand binding reached a maximum at 48hr of incubation and a plateau was maintained for up to 72hr. No degradation of 125I-ET-1 during incubation was observed at 72hr. Specific 125I-ET-1 binding sites were localized in areas corresponding anatomically to the cornea, the iris, the retina, the choroid and the sclera. 125I-ET-1 binding to these sections was monophasically inhibited by unlabeled ET-1 with dissociation constant (Kd) of 128pM, whereas unlabeled ET-3, a member of the ET family peptides, biphasically inhibited binding with low affinity, inhibition constant (Ki) of 7.06nM, and high affinity, Ki of 53pM. This evidence for specific 125I-ET-1 binding sites supports the physiological significance of the ET family peptides in the rat eye.