Activation of α₂A-adrenergic receptors in human trabecular meshwork cells stimulates MAP kinase activity

Purpose: Previously, we have demonstrated the presence of α₂A-adrenergic receptors (AR) in primary cultures of human trabecular meshwork (HTM) cells by immunofluorescence microscopy and the inhibition of forskolin-stimulated adenylyl cyclase. Since it has been shown in transfected cells that the α₂A-AR can activate mitogen activated protein (MAP) kinase, we were interested in the possibility that the endogenous α₂A-ARs in HTM cells might be coupled to MAP kinase activity and possibly cellular proliferation. Methods: HTM cells were incubated in the presence of serum and bFGF for 48 hours and then were serum-starved for 24 hours. Cells were stimulated with either: phorbol ester myristate acetate (100nM), dexmedetomidine (DMT, 100 nM), DMT (100 nM) plus rauwolscine (10 μM) or DMT (100 nM) plus atipamezole (10 μM). Lysates were prepared and MAP kinase activity was measured by the incorporation of γ³²P-ATP into myelin basic protein. Results: Activation of α₂A-ARs by DMT resulted in a dose-dependent stimulation of MAP kinase activity. Preincubation of cells with α₂A-AR antagonists, rauwolscine or atipamezole, significantly reduced the activation of MAP kinase by DMT. Western blot further demonstrated that the amount of immunoreactive MAP kinase was the same in both control and agonist-treated cells which is consistent with an increase in the activity, rather than the amount of MAP kinase. Conclusions: Activation of α₂A-adrenergic receptors in human trabecular meshwork cells stimulates MAP kinase activity and, therefore, may be involved with the proliferation of these cells.