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 o f AE mechanism and thiol groups in Pb2+ toxicity, we studied the effec ts of drugs and conditions that modify AE and that modify thiol groups on the ability of Pb2+ to stimulate potassium efflux as measured with Rb-86. The most potent stimulation of Rb-86 efflux by Pb2+ occurred w hen conditions were optimal for the AE mechanism-that is, when bicarbo nate was included in the buffer or a buffer made with NaI or NaCl rath er than NaClO4 or NaNO3 was used. Furthermore, 4,4'-diisothiocyanatost ilbene-2,2'-disulfonic acid and cetamido-4'-isothiocyanatostilbene-2,2 '-disulfonic acid, potent inhibitors of the AE mechanism, completely i nhibited stimulation of the Rb-86 efflux by Pb2+. These conditions or inhibitors did not affect stimulation of the Rb-86 efflux by ionomycin plus Ca2+. A role for Ca2+ channels was dismissed because the inorgan ic Ca2+ channel blockers, Cd2+ or Mn2+, did not prevent stimulation of Rb-86 efflux by Pb2+ but did inhibit stimulation by ionomycin plus Ca 2+. Rb-86 efflux was more sensitive to Pb2+ if erythrocytes were treat ed for 15 min with thiol-modifying reagents that enter cells, such as iodoacetamide, N-ethylmaleimide, or dithiothreitol, than to reduced gl utathione, a thiolmodifying 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 proce ss by Pb2+. (C) 1996 Wiley-Liss, Inc.