CHARACTERIZATION OF ALPHA-2D ADRENERGIC-RECEPTOR SUBTYPES IN BOVINE OCULAR TISSUE-HOMOGENATES

The alpha-2 adrenergic receptors are known to be present in the mammal ian eye and to mediate the effects of alpha-2 agonists used in the tre atment of glaucoma. Little is known, however, regarding the relative d ensities of the three alpha-2 subtypes in the various tissues of the e ye. We used receptor binding experiments with the radioligand [H-3]RX8 21002 to characterize the alpha-2 adrenergic receptors in three tissue s of the bovine eye, the ciliary body, retinal pigment epithelium/chor iocapillaris and iris. The K-D values in the three tissues were simila r (0.12-0.14 nM), and the B-max values ranged from 100 fmol/mg of prot ein for the ciliary body and retinal pigment epithelium/choriocapillar is to 200 fmol/mg of protein for the iris. The pharmacological charact eristics of the alpha-2 receptors in all three tissues of the bovine e ye, as assessed by competition studies, were essentially identical and were similar to the characteristics of the alpha-2A/D receptors of th e bovine neurosensory retina. The correlation coefficients between the logarithms of the K-i values for the three tissues and the neurosenso ry retina for nine adrenergic agents were .98 to .99. We conclude that the alpha-2 adrenergic receptors in the ciliary body, iris and retina l pigment epithelium/choriocapillaris of the bovine eye are mainly alp ha-2D.