Molecular and pharmacological identity of the α_(2D)-adrenergic receptor subtype in bovine retina and its photoreceptors

摘要

The rat cA2-47 gene encodes the pharmacologically defined ct2D-adrenergic receptor (a2D-AR) subtype. Previously, the ex-pression of its mRNA was shown in bovine retina by amplification through the reverse transcription-polymerase chain reac-tion (RT-PCR) of a region corresponding to the rat a2D-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 frag-ments, aa residues 290-375 and aa residues 392-434, demonstrates 77% overall identity with the rat a2D-AR subtype and 80% overall identity with the mouse a2D-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 a2D-AR subtype: The receptor bound rauwolscine with a KD 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 mhr1, the dissociation rate constant, k_p was 0.14 mhr1 and the half-time for dissocia-tion 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 a2-AR subtypes — a2A-AR, a2B-AR, a2C-AR-were not detected in the retina and its photoreceptors. Thus, this study shows that the bovine a2D-AR gene is a structural variant of the rat and mouse genes, that the bovine gene encodes the typical pharmacologically defined a2D-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.