Cultured human trabecular meshwork cells express functional α₂A adrenergic receptors

Purpose. For the treatment of glaucoma, alpha-2 adrenergic receptor (α₂-AR) agonists are thought to lower intraocular pressure primarily by decreasing aqueous humor production. Effects on the outflow pathways, however, also may occur. To begin to examine this possibility, the authors characterized the α₂-AR subtypes present in cultures of human trabecular meshwork (HTM) cells using both immunofluorescence microscopy and functional measures of α₂-AR activation. Methods. For immunofluorescence microscopy, subtype-specific polyclonal antibodies that recognize each of the human α₂- AR subtypes (α₂A, α₂B, α₂C) were used. Functional studies involved the inhibition of forskolin-stimulated cyclic adenosine monophosphate (cAMP) production, the stimulation of mitogen-activated protein (MAP) kinase activity, and the stimulation of mitotic activity as reflected by the expression of proliferating cell nuclear antigen (PCNA). Results. From the immunofluorescence microscopy, there was evidence for the presence of the α₂A subtype, but not α₂B or α₂C subtype, on HTM cells. The administration of the α₂-agonist, dexmedetomidine, to HTM cells resulted in a 90% inhibition of forskolin-stimulated cAMP formation, a twofold stimulation of MAP kinase activity, and a threefold increase in the expression of PCNA. Additionally, preincubation of cells with either of the α₂-AR-selective antagonists, rauwolscine or atipamezole, reversed the functional effects of dexmedetomidine. Conclusions. Functional α₂A-ARs are present on HTM cells where they may affect the outflow pathway during the treatment of glaucoma with α₂-AR agonists.