Quinine and quinidine inhibit and reveal heterogeneity of K-Cl cotransport in low K sheep erythrocytes.

Low K (LK) sheep red blood cells (SRBCs) serve as a model to study K-Cl cotransport which plays an important role in cellular dehydration in human erythrocytes homozygous for hemoglobin S. Cinchona bark derivatives, such as quinine (Q) and quinidine (QD), are effectively used in the treatment of malaria. In the present study, we investigated in LK SRBCs, the effect of various concentrations of Q and QD on Cl-dependent K efflux and Rb influx (K(Rb)-Cl flux), activated by either swelling in hyposmotic media, thiol alkylation with N-ethylmaleimide (NEM), or by cellular Mg (Mgi) removal through A23187 in the presence of external chelators. K efflux or Rb influx were determined in Cl and NO3 medium and K(Rb)-Cl flux was defined as the Cl-dependent (Cl minus NO3) component. K(Rb)-Cl flux stimulated by all three interventions was inhibited by both Q and QD in a dose-dependent manner. Maximum inhibition of K(Rb)-Cl flux occurred at Q and QD concentrations > or = 1 mM. The inhibitory effect of Q was manifested in Cl, but not in NO3, whereas QD reduced K and Rb fluxes both in Cl and NO3 media. The mean 50% inhibitory concentration (IC50) of Q and QD to inhibit K(Rb)-Cl flux varied between 0.23 and 2.24 mM. From determinations of the percentages of inhibition of the different components of K and Rb fluxes, we found that SRBCs possess a Cl-dependent QD-sensitive and a Cl-dependent QD-insensitive K efflux and Rb influx. These two components vary in magnitude depending on the manipulation and directional flux, but in average they are about 50% of the total Cl-dependent flux. This study raises the possibility that, in SRBCs, the Cl-dependent K(Rb) fluxes are heterogeneous.