Molecular and pharmacological identity of the α_2D-adrenergic receptor subtype in bovine retina and its photoreceptors

The rat cA2-47 gene encodes the pharmacologically defined α_2D-adrenergic receptor (α_2D-AR) subtype. Previously, the expression of its mRNA was shown in bovine retina by amplification through the reverse transcription-polymerase chain reaction (RT-PCR) of a region corresponding to the rat α_2D-AR, amino acid (aa) residues 382-439, indicating the presence of this subtype in this neural tissue. In the present study, the structure of this gene has been probed and the encoded receptor subtype has been characterized in bovine retina and its photoreceptor cells. The deduced aa sequence of the two bovine gene fragments, aa residues 290-375 and aa residues 392-434, demonstrates 77% overall identity with the rat α_2D-AR subtype and 80% overall identity with the mouse α_2D-AR. The receptor encoded by the bovine gene was expressed in the retina and its photoreceptors with the typical pharmacological characteristics established for the rat α_2D-AR subtype: The receptor bound rauwolscine with a K_D of 14 nM in the retina and with that of 19 nM in the photoreceptor cells; the binding association rate constant, k₊₁, for the ligand was 0.012 min^-1, the dissociation rate constant, k_-1 was 0.14 min^-1 and the half-time for dissociation was 5 min. Oxymetazoline displaced the bound [³H]-rauwolscine with an EC₅₀ value of 85 nM, while SK & F 104078, and prazosin displaced the bound [³H]-rauwolscine with the respective IC₅₀ values of 900 nM and 3000 nM. The other α₂-AR subtypes -α_2A-AR, α_2B-AR, α_2C-AR-were not detected in the retina and its photoreceptors. Thus, this study shows that the bovine α_2D-AR gene is a structural variant of the rat and mouse genes, that the bovine gene encodes the typical pharmacologically defined α_2D-AR subtype, that this subtype is present in its exclusive form in the bovine retina and its photoreceptors, where it may be presynaptic in nature.