title: Human trabecular meshwork cell volume regulation.
abstract: 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 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.
authors.affiliation: Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6085, USA.
authors.forename: Claire H
authors.initials: CH
authors.lastname: Mitchell
authors.forename: Johannes C
authors.initials: JC
authors.lastname: Fleischhauer
authors.forename: W Daniel
authors.initials: WD
authors.lastname: Stamer
authors.forename: K
authors.initials: K
authors.lastname: Peterson-Yantorno
authors.forename: Mortimer M
authors.initials: MM
authors.lastname: Civan
mesh_terms: D001639:Bicarbonates / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D002712:Chlorides / Q000494:pharmacology; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D017136:Ion Transport / Q000502:physiology; D008698:Mesylates / Q000494:pharmacology; D008954:Models, Biological; D018408:Patch-Clamp Techniques; D017923:Sodium-Hydrogen Exchangers / Q000502:physiology; D027981:Symporters / Q000502:physiology; D014129:Trabecular Meshwork / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D000096922:K Cl- Cotransporters
chemical_list: D001639:Bicarbonates; D002118:Calcium; D002712:Chlorides; D008698:Mesylates; D017923:Sodium-Hydrogen Exchangers; D027981:Symporters; D000096922:K Cl- Cotransporters; C045880:methanesulfonic acid