title: Cell membrane stretch modulates the high-conductance Ca2+-activated K+ channel in bovine trabecular meshwork cells.
abstract: OBJECTIVE
  Anterior chamber structures are subjected to changes in intraocular pressure (IOP). Several studies have pointed out that trabecular meshwork (TM) cells are sensitive to mechanical stretch and that cell-signaling mechanisms are activated in response to elevated pressure. Because membrane stretch has been shown to be a modulator of several ionic conductances, this study was conducted to determine its effects on the high-conductance Ca(2+)-activated K(+) (BK(Ca)) channels present in TM cells.
  
  
  METHODS
  Primary cultures of TM cells from bovine eyes were used. Patch-clamp recordings were performed in the cell-attached, inside-out, and whole-cell configurations. To stretch the cell membrane, both suction to the rear end of the patch pipette and hypotonic shock were used. Intracellular calcium concentration ([Ca(2+)](i)) was measured in TM cells loaded with fura-2, using an epifluorescence microscope coupled to a charge-coupled device (CCD) camera.
  
  
  RESULTS
  Electrophysiological characterization of BK(Ca) channels was in agreement with previous studies. In cell-attached patches, the open probability of the BK(Ca) channel (i.e., the amount of time the channel is open) increased consistently when 14- to 45-mm Hg suctions were applied at a constant depolarized voltage. At a constant pressure (25 or 45 mm Hg), channel openings increased when depolarizing pulses were applied to the patch. Stretch activation of the BK(Ca) channel was not mediated by increases in [Ca(2+)](i), because it was present in inside-out patches maintained at a constant Ca(2+) concentration. Nevertheless, it cannot be ruled out that at low suction levels, a minimum Ca(2+) concentration is necessary for channel activation. Whole-cell currents carried by BK(Ca) channels increased when the isotonic solution in the bath was exchanged with a hypotonic solution and were selectively blocked by iberiotoxin. In our conditions, the hypotonic shock did not modify [Ca(2+)](i).
  
  
  CONCLUSIONS
  The data show that in TM cells, open probability of the BK(Ca) channel is enhanced by membrane stretching as well as by membrane depolarization and [Ca(2+)](i). Changes in membrane tension induced by cell volume increase also activated whole-cell BK(Ca) currents. Homeostatic mechanisms in TM cells may involve BK(Ca) channel activation in response either to changes in cell volume or changes in IOP.
authors.affiliation: Laboratory of Neurophysiology, Department of Physiological Sciences I, Institute of Biomedical Investigations August Pi i Sunyer (IDIBAPS), Faculty of Medicine, University of Barcelona, Barcelona, Spain. xgasull@ub.edu
authors.forename: Xavier
authors.initials: X
authors.lastname: Gasull
authors.forename: Elisa
authors.initials: E
authors.lastname: Ferrer
authors.forename: Artur
authors.initials: A
authors.lastname: Llobet
authors.forename: Antonio
authors.initials: A
authors.lastname: Castellano
authors.forename: Jose M
authors.initials: JM
authors.lastname: Nicolás
authors.forename: Jordi
authors.initials: J
authors.lastname: Palés
authors.forename: Arcadi
authors.initials: A
authors.lastname: Gual
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002417:Cattle; D002462:Cell Membrane / Q000502:physiology*; D048429:Cell Size; D002478:Cells, Cultured; D016257:Fura-2 / Q000378:metabolism; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008856:Microscopy, Fluorescence; D018408:Patch-Clamp Techniques; D024681:Potassium Channels, Calcium-Activated / Q000378:metabolism*; D013314:Stress, Mechanical; D014129:Trabecular Meshwork / Q000166:cytology / Q000378:metabolism*
chemical_list: D051036:Large-Conductance Calcium-Activated Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D002118:Calcium; D016257:Fura-2