Role of anion exchange and thiol groups in the regulation of potassium efflux by lead in human erythrocytes

Pb²⁺ is thought to enter erythrocytes through anion exchange (AE) and to remain in the cell by binding to thiol groups. To define the role of AE mechanism and thiol groups in Pb²⁺ toxicity, we studied the effects of drugs and conditions that modify AE and that modify thiol groups on the ability of Pb²⁺ to stimulate potassium efflux as measured with ⁸⁶Rb. The most potent stimulation of ⁸⁶Rb efflux by Pb²⁺ occurred when conditions were optimal for the AE mechanism - that is, when bicarbonate was included in the buffer or a buffer made with NaI or NaCl rather than NaClO₄ or NaNO₃ was used. Furthermore, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, potent inhibitors of the AE mechanism, completely inhibited stimulation of the ⁸⁶Rb efflux by Pb²⁺. These conditions or inhibitors did not affect stimulation of the ⁸⁶Rb efflux by ionomycin plus Ca²⁺. A role for Ca²⁺ channels was dismissed because the inorganic Ca²⁺ channel blockers, Cd²⁺ or Mn²⁺, did not prevent stimulation of ⁸⁶Rb efflux by Pb²⁺ but did inhibit stimulation by ionomycin plus Ca²⁺. ⁸⁶Rb efflux was more sensitive to Pb²⁺ if erythrocytes were treated for 15 min with thiol-modifying reagents that enter cells, such as iodoacetamide, N-ethylmaleimide, or dithiothreitol, than to reduced glutathione, a thiol-modifying reagent that is not permeable to the cell. Thus, in erythrocytes the AE mechanism and internal thiol groups are critical factors that affect the stimulation of a Ca²⁺-dependent process by Pb²⁺.