The Influence of Hypotonicity on Large-Conductance Calcium-Activated Potassium Channels in Human Retinal Pigment Epithelial Cells

The aim of this study was to characterize the effects of hypotonicity on the activity of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in human retinal pigment epithelial (RPE R-50) cells. Effects of hypotonicity on ion currents were investigated with the aid of the patch-clamp technique. A regulatory volume decrease in response to a hypotonic solution (200 mOsm/L) was observed that could be blunted by paxilline. In wholecell current recordings, a hypotonic solution (200 mOsm/L) reversibly increased the amplitude of K⁺ outward currents (I_K). The increase of I_K could be reversed by iberiotoxin (200 nM), paxilline (1 µM), or tetrandrine (5 µM), but not by glibenclamide (10 µM), disulphonic acid (DIDS) (100 µM), or dequalinium dichloride (10 µM). In RPE R-50 cells pretreated with thapsigargin, aristolochic acid, or pertussis toxin, the increased amplitude of I_K in response to hypotonicity was unaltered. In cell-attached patches, an increase in BK_Ca-channel activity was observed during hypotonicity-induced cell swelling. The enhanced channel activity elicited under this condition was mainly mediated by an increase in the number of long-lived openings. These findings support the evidence for the coupling of volume swelling to the functional activity of BK_Ca channels.