NA-K-CL COTRANSPORT REGULATES INTRACELLULAR VOLUME AND MONOLAYER PERMEABILITY OF TRABECULAR MESHWORK CELLS

The trabecular meshwork (TM) of the eye plays a critical role in modul ating intraocular pressure (IOP) through regulation of aqueous humor o utflow, although the underlying mechanisms remain unknown. Ethacrynic acid, an agent known to inhibit Na-K-Cl cotransport of a number of cel l types, recently has been reported to increase aqueous outflow and lo wer IOP through an unknown effect on the TM. In vascular endothelial c ells and a variety of other cell types, the Na-K-Cl cotransporter func tions to regulate intracellular volume. The present study was conducte d to evaluate TM cells for the presence of Na-K-Cl cotransport activit y and to test the hypothesis that modulation of cotransport activity a lters intracellular volume and, consequently, permeability of the TM. We demonstrate here that bovine and human TM cells exhibit robust Na-K -Cl cotransport activity that is inhibited by bumetanide and by ethacr ynic acid. Our studies also show that TM cell Na-K-Cl cotransport is m odulated by a variety of hormones and neurotransmitters. Inhibition of the cotransporter either by bumetanide, ethacrynic acid, or inhibitor y hormones reduces TM intracellular volume, whereas stimulatory hormon es increase cell volume. In addition, shrinkage of the cells by hypert onic media stimulates cotransport activity and initiates a subsequent regulatory volume increase. Permeability of TM cell monolayers, assess ed as transmonolayer flux of [C-14]sucrose, is increased by hypertonic ity-induced cell shrinkage and by bumetanide. These findings suggest t hat Na-K-Cl cotransport of TM cells is of central importance to regula tion of intracellular volume and TM permeability. Defects of Na-K-Cl c otransport may underlie the pathophysiology of glaucoma.