Chloride channels in apical membrane patches of stellate cells of Malpighian tubules of Aedes aegypti

Stellate cells of Aedes aegypti Malpighian tubules were investigated using patch-clamp methods to probe the route of transepithelial Cl- secretion. Tw o types of Cl- channel were identified in excised, inside-out apical membra ne patches. The first Cl- channel, type I, had a conductance of 24 pS, an o pen probability of 0.816+/-0.067, an open time of 867+/-114 ms (mean +/- S. E.M., four patches) and the selectivity sequence I->Cl(-)much greater than isethionate>gluconate. The I-/Cl- permeability ratio was 1.48, correspondin g to Eisenman sequence I. The type I Cl- channel was blocked by 2,2'-iminod ibenzoic acid (DPC) and niflumic acid {2-[3-(trifluoromethyl)anilo]nicotini c acid}. The removal of Ca2+ from the Ringer's solution on the cytoplasmic side had no effect on channel activity. The second Cl- channel, type II, ha d a conductance of 8 pS, an open probability of 0.066+/-0.021 and an open t ime of 7.53+/-1.46 ms (mean +/- S.E.M., four patches). The high density and halide selectivity sequence of the type I Cl- channel is consistent with a role in transepithelial Cl- secretion under control conditions, but it rem ains to be determined whether these Cl- channels also mediate transepitheli al Cl- secretion under diuretic conditions in the presence of leucokinin.