Cell membrane stretch modulates the high-conductance Ca2+-activated K+ channel in bovine trabecular meshwork cells

PURPOSE. Anterior chamber structures are subjected to changes in intraocular pressure (IOP). Several studies have pointed out that trabecular meshwork (W) cells are sensitive to mechanical stretch and that cell-signaling mechanisms are activated in response to elevated pressure. Because membrane stretch has beers 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 T 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 tine cell membrane, both suction to the rear end of the path pipette and hypotonic shock were used. Intracellular calcium concentration ([Ca2+](i)) was measured in TM cells loaded with furs-2, using an epifluorescence microscope coupled to a charge-coupled device (CCD) camera. RESULTS Electrophysdological characterization of BKCa channels was in agreement with previous studies. In cell-attached patches, the open probability of the Bcannel (i.e., the amount of time the channel is open) increased consistently when 14- to 45 nun Hg suctions were applied at a constant depolarized voltage. At a constant pressure (25 or 45 mm), channel openings increased whet 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 stained at a constant Ca2+ concentration, Nevertheless, it cannot ruled out that at lour suction levels, a minfnum 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 neat mortify [Ca2+](i). CONCLUSIONS 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 invoke BKCa channel activation in response either to changes in cell volume or changes in IOP.