Net K and Cl effluxes induced by valinomycin or by gramicidin have been determined directly at varied external K, denoted by [K]o, in the presence and absence of the anion transport inhibitors DIDS (4,4'-diiso-thiocyano-2,2'-disulfonic acid stilbene), and its less potent analogue SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid). The results confirm that pretreatment with 10 microM DIDS, or 100 microM SITS, for 30 min at 23 degrees C inhibits conductive Cl efflux, measured in the continued presence of the inhibitors at 1 mM [K]o, by only 59-67%. This partial inhibition by 10 microM DIDS at 1 mM [K]o remains constant when the concentration of DIDS, or when the temperature or pH during pretreatment with DIDS, are increased. Observations of such partial inhibition previously prompted the postulation of two Cl conductance pathways in human red blood cells: a DIDS-sensitive pathway mediated by capnophorin (band 3 protein), and a DIDS-insensitive pathway. The present experiments demonstrate that at [K]o corresponding to values of EK between -35 and 0 mV the DIDS-insensitive component of net Cl efflux is negligible, being < or = 0.1 muMol/g Hb/min, both with valinomycin (1 microM) and with gramicidin (0.06 microgram/ml). At lower [K]o, where EK is below approximately -35 mV, the DIDS-insensitive fraction of net Cl efflux increases to 2.6 muMol/g Hb/min with valinomycin (1 microM), and to 4.8 muMol/g Hb/min with gramicidin (0.06 microgram/ml). With net fluxes determined from changes in mean cell volume, and with membrane potentials measured from changes in the external pH of unbuffered red cell suspensions, a current-voltage curve for DIDS-insensitive Cl conductance has been deduced. While specific effects of varied [K]o on net Cl efflux are unlikely but cannot strictly be ruled out, the results are consistent with the hypothesis that DIDS-insensitive Cl conductance turns on at an Em of approximately -40 mV.

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