Characterization of alpha-2D adrenergic receptor subtypes in bovine ocular tissue homogenates

The alpha-2 adrenergic receptors are known to be present in the mammalian eye and to mediate the effects of alpha-2 agonists used in the treatment of glaucoma. Little is known, however, regarding the relative densities of the three alpha-2 subtypes in the various tissues of the eye. We used receptor binding experiments with the radioligand [3H]RX821002 to characterize the alpha-2 adrenergic receptors in three tissues of the bovine eye, the ciliary body, retinal pigment epithelium/choriocapillaris and iris. The K(D) values in the three tissues were similar (0.12-0.14 nM), and the Bmax values ranged from 100 fmol/mg of protein for the ciliary body and retinal pigment epithelium/choriocapillaris to 200 fmol/mg of protein for the iris. The pharmacological characteristics of the alpha-2 receptors in all three tissues of the bovine eye, as assessed by competition studies, were essentially identical and were similar to the characteristics of the alpha-2A/D receptors of the bovine neurosensory retina. The correlation coefficients between the logarithms of the Ki values for the three tissues and the neurosensory retina for nine adrenergic agents were .98 to .99. We conclude that the alpha-2 adrenergic receptors in the ciliary body, iris and retinal pigment epithelium/choriocapillaris of the bovine eye are mainly alpha-2D.