Human trabecular meshwork cell volume regulation

The volume of certain subpopulations of trabecular meshwork (TM) cells may modify outflow resistance of aqueous humor, thereby altering intraocular pressure. This study examines the contribution that Na⁺/H⁺, Cl^-/HCO^-₃ exchange, and K⁺-Cl^- efflux mechanisms have on the volume of TM cells. Volume, Cl^- currents, and intracellular Ca²⁺ activity of cultured human TM cells were studied with calcein fluorescence, whole cell patch clamping, and fura 2 fluorescence, respectively. At physiological bicarbonate concentration, the selective Na⁺/H⁺ antiport inhibitor dimethylamiloride reduced isotonic cell volume. Hypotonicity triggered a regulatory volume decrease (RVD), which could be inhibited by the Cl^- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), the K⁺ channel blockers Ba²⁺ and tetraethylammonium, and the K⁺-Cl^- symport blocker [(dihydroindenyl)oxy]alkanoic acid. The fluid uptake mechanism in isotonic conditions was dependent on bicarbonate; at physiological levels, the Na⁺/H⁺ exchange inhibitor dimethylamiloride reduced cell volume, whereas at low levels the Na⁺-K⁺-2Cl^- symport inhibitor bumetanide had the predominant effect. Patch-clamp measurements showed that hypotonicity activated an outwardly rectifying, NPPB-sensitive Cl^- channel displaying the permeability ranking Cl^- > methylsulfonate > aspartate. 2,3-Butanedione 2-monoxime antagonized actomyosin activity and both increased baseline [Ca²⁺] and abolished swelling-activated increase in [Ca²⁺], but it did not affect RVD. Results indicate that human TM cells display a Ca²⁺-independent RVD and that volume is regulated by swelling-activated K⁺ and Cl^- channels, Na⁺/H⁺ antiports, and possibly K⁺-Cl^- symports in addition to Na⁺-K⁺-2Cl^- symports.