Characterization of the K+(Na+)/H+ monovalent cation exchanger in the human red blood cell membrane: Effects of transport inhibitors

The (ouabain + bumetanide + EGTA)-insensitive K+ influx (defined as residua l K+ influx) in the human erythrocyte was investigated with respect to the characterization of the recently identified K+ (Na+)/H+ exchanger (Richter et al. 1997). In particular, the effects of selected ion transport inhibito rs on this flux in physiological ionic strength (high ionic strength, HIS) as well as low ionic strength (LIS) solutions were qstudied. The stimulatio n of the Kf influx observed in LIS medium was further enhanced when DIDS, p hloretin, eosin-5-maleimide, furosemide, DIOA, NPPB, or DCDPC was present a t a concentration of 0.1 mmol/l. This paradoxical, inhibitor-induced increa se of the K+ influx was more pronounced in LIS media where chloride (7.5 mm ol/l) was replaced by nitrate. For DNDS, niflumic acid, and MK-196 (0.1 mmo l/l) an enhanced K+ transport could only be observed in nitrate-containing LIS solution. Bumetamide and purine riboside, at a concentration of 0.1 mmol/l, did not c ause significant changes of the K+ influx in either chloride- or nitrate-co ntaining LIS media. Dipyridamole and ruthenium red (0.1 mmol/l), which are positively charged, significantly reduced the K+ influx in both chloride- a nd nitrate-containing LIS media. In nitrate-containing HTS solution only di pyridamole inhibited the K+ influx. The residual K+ influx in LIS solution was significantly increased by removing internal [Mg2+], and decreased by q uinacrine (1 mmol/l). In RIS solution, no effect of altering intracellular Mg2+ occurred but a stimulation of the flux by quinacrine was observed. The results are discussed in terms of a more general surface charge effect of the used inhibitors on the K+(Na+)/H+ exchanger.