Isolated human neutrophils possess three distinct pathways by which Cl- crosses the plasma membrane of steady state cells: anion exchange, active transport, and electrodiffusion. The purpose of the present work was to investigate the selectivity of each of these separate processes with respect to other external halide ions. (a) The bulk of total anion movements represents transport through an electrically silent anion-exchange mechanism that is insensitive to disulfonic stilbenes, but which can be competitively inhibited by alpha-cyano-4-hydroxycinnamate (CHC; Ki approximately 0.3 mM). The affinity of the external translocation site of the carrier for each of the different anions was determined (i) from substrate competition between Cl- and either Br-, F-, or I-, (ii) from trans stimulation of 36Cl- efflux as a function of the external concentrations of these anions, (iii) from changes in the apparent Ki for CHC depending on the nature of the replacement anion in the bathing medium, and (iv) from activation of 82Br- and 125I- influxes by their respective ions. Each was bound and transported at roughly similar rates (Vmax values all 1.0-1.4 meq/liter cell water.min); the order of decreasing affinities is Cl- greater than Br- greater than F- greater than I- (true Km values of 5, 9, 23, and 44 mM, respectively). These anions undergo 1:1 countertransport for internal Cl-. (b) There is a minor component of total Cl- influx that constitutes an active inward transport system for the intracellular accumulation of Cl- [( Cl-]i approximately 80 meq/liter cell water), fourfold higher than expected for passive distribution. This uptake is sensitive to intracellular ATP depletion by 2-deoxy-D-glucose and can be inhibited by furosemide, ethacrynic acid, and CHC, which also blocks anion exchange. This active Cl- uptake process binds and transports other members of the halide series in the sequence Cl- greater than Br- greater than I- greater than F- (Km values of 5, 8, 15, and 41 mM, respectively). (c) Electrodiffusive fluxes are small. CHC-resistant 82Br- and 125I- influxes behave as passive leak fluxes through low-conductance ion channels: they are nonsaturable and strongly voltage dependent. These anions permeate the putative Cl- channel in the sequence I- greater than Br- greater than Cl- with relative permeability ratios of 2.2:1.4:1, respectively, where PCl approximately 5 X 10(-9) cm/s.

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