Functional Geometry of the Permeation Pathway of Ca-activated Cl Channels Inferred from Analysis of Voltage-dependent Block*

We examined the voltage-dependent block of Caactivated Cl channels by anthacene-9-carboxylic acid (A9C), diphenylamine-2-carboxylic acid (DPC), 4,4*-diisothiocyanostilbene-2,2*-disulfonic acid (DIDS), and niflumic acid (NFA) in excised inside-out and outside-out patches from Xenopus oocytes. The fraction of the voltage field (d) experienced by the blocking drug was determined from the voltage dependence of block. All the drugs blocked by entering the channel from the outside. d was 0.6 for A9C, 0.3 for DPC and DIDS, and <0.1 for NFA. Because the voltage dependence of the drugs differed, the order of potency was also voltage-dependent. At 1100 mV the order of potency was NFA > A9C > DIDS > DPC (Ki (mM) 5 10.1, 18.3, 48, and 111, respectively). Because the drugs are hydrophobic, they can cross the bilayer when applied from the inside and block the channel from the outside. The equilibrium geometries of the blockers were determined by molecular modeling and compared with their blocking positions (d). This analysis suggests that the channel is an elliptical cone with the largest opening facing the extracellular space. The selectivity filter has an apparent size of 0.33 3 0.75 nm, because C(CN)3 , which has these dimensions, permeates. The external opening is at least 0.60 3 0.94 nm, because DPC has these dimensions and penetrates the channel ;30%.