EFFECTS OF DEXAMETHASONE ON SODIUM-POTASSIUM-CHLORIDE COTRANSPORT IN TRABECULAR MESHWORK CELLS

Purpose. Previous studies in the authors' laboratory have shown that b ovine and human trabecular meshwork (TM) cells possess a robust sodium -potassium-chloride (Na-K-CI) cotransport system that functions in reg ulating intracellular volume and may play a central role in modulating outflow facility across the TM. Dexamethasone, which can induce ocula r hypertension, has been found to increase resistance to aqueous outfl ow across the TM. The current study was conducted to investigate the h ypothesis that alteration of TM cell Na-KCI cotransport function, regu lation, or both may be an underlying factor in steroid-induced glaucom a. To this end, the authors evaluated the effects of dexamethasone tre atment of TM cells on Na-K-Cl cotransport activity and cotransporter p rotein expression. Methods. Cultured bovine and human TM cell monolaye rs were exposed to dexamethasone (10(-9) to 10(-6) M) for varying time s, then evaluated for Na-K-CI cotransport activity or harvested for ce llular membrane proteins. Cotransport activity was assessed as bumetan ide-sensitive K influx. Cotransport protein expression was evaluated b y Western blot analysis of cellular proteins using a monoclonal antibo dy to the human colonic T84 epithelial cell Na-K-Cl cotransporter. Res ults. The authors found that 24- and 48-hour exposures of human and bo vine TM cells to dexamethasone stimulates Na-K-Cl cotransport activity (10(-8) to 10(-6) M dexamethasone in human cells; 10(-8) and 10(-7) M in bovine cells). The authors also found that dexamethasone (10(-8) M ) stimulates Na-K-CI cotransport activity of TM cells with exposure ti mes as early as 12 hours and up to 5 days. In addition, the authors fo und that the level of Na-K-CI cotransport protein expressed in TM cell s is modulated by dexamethasone. When bovine or human TM cells are exp osed to 10(-8) or 10(-6) M dexamethasone for 2 to 5 days, cotransporte r protein expression is increased. With longer exposures, however, cot ransporter protein levels decrease below control levels. Finally, the authors found that TM cells exposed to dexamethasone become unresponsi ve to regulation by hypertonicity and vasopressin. Conclusions, The au thors' findings suggest that dexamethasone map be exerting its effect, at least in part, through altering Na-K-CI cotransport function and r egulation in TM cells.