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Xavier Gasull, Elisa Ferrer, Artur Llobet, Antonio Castellano, Jose M. Nicolás, Jordi Palés, Arcadi Gual; Cell Membrane Stretch Modulates the High-Conductance Ca2+-Activated K+ Channel in Bovine Trabecular Meshwork Cells. Invest. Ophthalmol. Vis. Sci. 2003;44(2):706-714. doi: 10.1167/iovs.02-0384.
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purpose. 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 Ca2+-activated K+ (BKCa) channels present in TM cells.
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 ([Ca2+]i) was measured in TM cells loaded with fura-2, using an epifluorescence microscope coupled to a charge-coupled device (CCD) camera.
Electrophysiological characterization of BKCa channels was in agreement with previous studies. In cell-attached patches, the open probability of the BKCa 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 BKCa channel was not mediated by increases in [Ca2+]i, because it was present in inside-out patches maintained at a constant Ca2+ concentration. Nevertheless, it cannot be ruled out that at low suction levels, a minimum Ca2+ concentration is necessary for channel activation. Whole-cell currents carried by BKCa 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 [Ca2+]i.
The data show that in TM cells, open probability of the BKCa channel is enhanced by membrane stretching as well as by membrane depolarization and [Ca2+]i. Changes in membrane tension induced by cell volume increase also activated whole-cell BKCa currents. Homeostatic mechanisms in TM cells may involve BKCa channel activation in response either to changes in cell volume or changes in IOP.
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