Characterization of alpha(2) adrenergic receptor subtypes in human ocular tissue homogenates

PURPOSE. To determine the predominant a, adrenergic receptor subtypes prese nt in the human eye. METHODS. Saturation- and competition-receptor-binding experiments were perf ormed with the radioligand [(3)-H]RX821002 in human ciliary body, retinal p igmented epithelium- choriocapillaris, iris, and neurosensory retina. The a ffinities of various adrenergic antagonists in these ocular tissues were co mpared with their affinities for the cloned alpha(2A), alpha(2B), and alpha (2C), adrenergic receptor subtypes. RESULTS. The density of alpha(2) adrenergic receptors was highest in the ir is (440 femtomoles/mg protein), lowest in the neurosensory retina (14 femto moles/mg protein), and intermediate in the other two tissues (approximately 90 fmol/mg protein). The drug affinities in all four human ocular tissues were highly correlated (correlation coefficients between 0.94 and 0.37) wit h the affinities for the human cut, adrenergic receptor subtype and poorly correlated (correlation coefficients between 0.15 and 0.66) with the alpha( 2B) and alpha(2C) subtypes. CONCLUSIONS. In agreement with previous studies in several animal species, the alpha(2) adrenergic receptors in the human ciliary body, retinal pigmen ted epithelium- choriocapillaris, iris, and neurosensory retina are predomi nately of the alpha(2A) subtype.