Inhibition of P-glycoprotein-mediated transport by a hydrophobic contaminant in commercial gluconate salts

The substitution of gluconate for Cl^- is commonly used to characterize Cl^- transport or Cl^--dependent transport mechanisms. We evaluated the effects of substituting gluconate for Cl^- on the transport of the P- glycoprotein substrate rhodamine 123 (R123). The replacement of Ringer solution containing Cl^- (Cl^-Ringer) with gluconate-Ringer inhibited R123 efflux, whereas the replacement of Cl^- by other anions (sulfate or cyclamate) had no effect. The inhibition of R123 efflux by gluconate-Ringer was absent after chloroform extraction of the sodium gluconate salt. The readdition of the sodium gluconate-chloroform extract to the extracted gluconate-Ringer or to cyclamate-Ringer inhibited R123 efflux, whereas its addition to Cl^--Ringer had no effect. These observations indicate that the inhibition of P-glycoprotein-mediated R123 transport by gluconate is due to one or more chloroform-soluble contaminants and that the inhibition is absent in the presence of Cl^-. The results are consistent with the fact that P- glycoprotein substrates are hydrophobic. Care should be taken when replacing ions to evaluate membrane transport mechanisms because highly pure commercial preparations may still contain potent contaminants that affect transport.