Foreign anions modulate volume set point of sheep erythrocyte K-Cl cotransport

Preequilibration at 37 degrees C in isosmotic media with Cl replaced by lyotropic (foreign) anions reversibly increased Cl-dependent K efflux and Rb influx, the inhibition by furosemide, and thus K-Cl cotransport in low-K but not in high-K sheep erythrocytes with the following order of effectiveness: SCN greater than I greater than NO3 greater than Cl = Br. This effect depended on time, temperature, and anion concentration and was reversible. Preincubation in isosmotic SCN at 37 degrees C stimulated K-Cl flux in anisosmotic Cl media (370-240 mosM) by increasing the volume sensitivity through shifting the point of zero K-Cl flux by approximately 100 mosmol. Thus even shrunken cells exhibited K-Cl cotransport. Preincubation in hyperosmotic SCN or Cl (440 mosM) followed by K flux in hyposmotic Cl (240 mosM) caused a 30-min lag phase that was absent when cells were swollen only. Hence, foreign anions increased the K flux rate in Cl, suggesting upregulation of K-Cl cotransport through new sites or higher turnover per transporter. The anions must act directly on proteins and/or lipids as the accompanying intracellular pH (pHi) changes were too small to attribute the K-Cl flux activation to cellular acidification. After thiol alkylation, which also activates K-Cl cotransport, SCN preexposure at 37 degrees C became ineffective. Carbethoxylation significantly reduced the foreign anion enhancement of K-Cl cotransport and abolished K efflux in Br. It is concluded that interaction of anions through carbethoxylation-sensitive sites with thiols may determine the level of K-Cl cotransport activity.