MOLECULAR AND PHARMACOLOGICAL IDENTITY OF THE A ADRENERGIC-RECEPTOR SUBTYPE IN BOVINE RETINA AND ITS PHOTORECEPTORS

The rat cA2-47 gene encodes the pharmacologically defined alpha(2D)-ad renergic receptor (alpha(2D)-AR) subtype. Previously, the expression o f its mRNA was shown in bovine retina by amplification through the rev erse transcription-polymerase chain reaction (RT-PCR) of a region corr esponding to the rat alpha(2D)-AR, amino acid (aa) residues 382-439, i ndicating the presence of this subtype in this neural tissue. In the p resent study, the structure of this gene has been probed and the encod ed receptor subtype has been characterized in bovine retina and its ph otoreceptor cells. The deduced aa sequence of the two bovine gene frag ments, aa residues 290-375 and aa residues 392-434, demonstrates 77% o verall identity with the rat alpha(2D)-AR subtype and 80% overall iden tity with the mouse alpha(2D)-AR. The receptor encoded by the bovine g ene was expressed in the retina and its photoreceptors with the typica l pharmacological characteristics established for the rat alpha(2D)-AR subtype: The receptor bound rauwolscine with a K-D of 14 nM in the re tina and with that of 19 nM in the photoreceptor cells; the binding as sociation 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 [H-3]-ra uwolscine with an EC(50) value of 85 nM, while SK & F 104078, and praz osin displaced the bound [H-3]-rauwolscine with the respective IC50 va lues of 900 nM and 3000 nM. The other alpha(2)-AR subtypes -alpha(2A)- AR, alpha(2B)-AR, alpha(2C)-AR-were not detected in the retina and its photoreceptors. Thus, this study shows that the bovine alpha(2D)-AR g ene is a structural variant of the rat and mouse genes, that the bovin e gene encodes the typical pharmacologically defined alpha(2D)-AR subt ype, that this subtype is present in its exclusive form in the bovine retina and its photoreceptors, where it may be presynaptic in nature.