Hypotonicity- and Stretch-Stimulated Activity of Large Conductance Calcium-Activated Potassium Channels in Human Retinal Pigment Epithelial Cells

许淑娟1,2, Dan-Ning Hu3, Sheng-Nan Wu4
1Department of Ophthalmology, Kaohsiung Veterans General Hospital, Kaohsiung City, Taiwan,

Purpose: To identify the mechanisms by which hypotonicity and membrane stretch alters the activity of large-conductance Ca2+-activated K+ (BKCa) channels in human retinal pigment epithelial (RPE) cells. Methods: The effects of hypotonicity and membrane stretch on ion currents were investigated in human RPE R-50 cells with the aid of the patch-clamp technique. Results: A regulatory volume decrease in response to a hypotonic solution (200 mOsm/L) that could be blunted by paxilline was observed in human RPE cells. In whole-cell current recordings, hypotonic solution (200 mOsm/L) reversibly increased the amplitude of K+ outward currents (IK). The increase of IK in response to the hypotonic solution could be reversed by iberiotoxin (200 nM), paxilline (1 mM), and tetrandrine (5 mM), yet not by glibenclamide (10 mM), DIDS (100 mM), and dequalinium dichloride (10 mM). In human RPE cells preincubated with thapsigargin, aristolochic acid, or pertussis toxin, the increased amplitude of IK in response to hypotonicity remains unaltered.  In cell-attached patches, an increase in BKCa-channel activity was observed by either membrane stretch or hypotonicity-induced cell swelling. The enhanced channel activity elicited by these two maneuvers is mainly mediated by the increase in the number of long-lived openings. In inside-out patches, application of pipette suction did not modify single-channel conductance of BKCa channels; however, it did enhance the channel activity. Membrane stretch-induced channel activity is independent of the level of intracellular Ca2+. Membrane stretch also potentiated riluzole-stimulated BKCa-channel activity. Conclusions: This study provides the evidence that human RPE cells exhibit a membrane stretch-activated BKCa channel. Its open probability increases not only in response to membrane stretch induced by cell swelling, but also by changes in pipette pressure. Human RPE cells may sense volume swelling and transduce this signal to functional activity of BKCa channels.

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