Glibenclamide blocks volume-sensitive Cl- channels by dual mechanisms

To study the mechanisms of glibenclamide actions onvolume-sensitive Clchannels, whole cell patch-clamp studies were performed at various pHlevels in human epithelial Intestine 407 cells. Extracellular application of glibenclamide reversibly suppressed volume-sensitive Cl currents in the entirerange of voltage examined (100 to +100 mV) and accelerated thedepolarization-induced inactivation at pH 7.5. When glibenclamide wasapplied from the intracellular side, in contrast, no effect wasobserved. At acidic pH, at which the weak acid glibenclamide existslargely in the uncharged form, the instantaneous current was, in avoltage-independent manner, suppressed by the extracellular drug atmicromolar concentrations without significantly affecting thedepolarization-induced inactivation. At alkaline pH, at which almostall of the drug is in the charged form, glibenclamide speeded theinactivation time course and induced a leftward shift of thesteady-state inactivation curve at much higher concentrations. Thus itis concluded that glibenclamide exerts inhibiting actions onswelling-activated Clchannels from the extracellular side and that the uncharged form ismainly responsible for voltage-independent inhibition of instantaneous currents, whereas the anionic form facilitates voltage-dependent channel inactivation in human epithelial Intestine 407 cells.