Stimulation of Ca2+-gated Cl- currents by the calcium-dependent K+ channel modulators NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] and isopimaric acid

Because chloride (Cl-) channel blockers such as niflumic acid enhance large-conductance Ca2+-activated potassium channels (BKCa), the aim of this study was to determine whether there is a reciprocal modification of Ca2+-activated chloride Cl- currents (I-ClCa) by two selective activators of BKCa. Single smooth muscle cells were isolated by enzymatic digestion from murine portal vein and rabbit pulmonary artery. The BKCa activators NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] and isopimaric acid (IpA) augmented macroscopic I-ClCa elicited by pipette solutions containing [Ca2+](i) > 100 nM without any alteration in current kinetics. Enhanced currents recorded in the presence of NS1619 or IpA reversed at the theoretical Cl- equilibrium potential, which was shifted by approximately -40 mV upon replacement of the external anion with the more permeable thiocyanate anion. NS1619 increased the sensitivity of calcium-activated chloride channel (Cl-Ca) to Ca2+ (similar to 100 nM at +60 mV) and induced a leftward shift in their voltage dependence (similar to 80 mV with 1 mu M Ca2+). Single-channel experiments revealed that NS1619 increased the number of open channels times the open probability of small-conductance (1.8-3.1 pS) Cl-Ca without any alteration in their unitary amplitude or number of observable unitary levels of activity. These data, in addition to the established stimulatory effects of niflumic acid on BKCa, show that there is similarity in the pharmacology of calcium-activated chloride and potassium channels. Although nonspecific interactions are possible, one alternative hypothesis is that the channel underlying vascular I ClCa shares some structural similarity to the BKCa or that the latter K+ channel physically interacts with Cl-Ca.