A23187, 4-BrA23187, and ionomycin transport several lanthanide series trivalent cations at efficiencies similar to Ca2+, when compared at cation concentrations of ~10-5 M, ionophore concentrations of ~10-6 M, and a pH of 7.00. Selectivity sequences and the range of relative rates are as follows: A23187, Nd3+ > La3+ > Eu3+ > Gd3+ > Er3+ > Yb3+ > Lu3+ (~34-fold); 4-BrA23187, Nd3+ > Eu3+ > Gd3+ > La3+ > Er3+ > Yb3+ > Lu3+ (~34-fold); ionomycin, La3+ > Yb3+ > Nd3+ > bu3+ > Er3+ > Eu3+ > Gd3+ (~4-fold). At concentrations between 9 and 250 μM, La3+ is transported by an electroneutral mechanism, predominately through mixed complexes of the type (ionophore)2La·OH (A23187 and 4-BrA23187) or (ionophore)La·OH (ionomycin), when no membrane potential is present. For all three ionophores, an induced potential of ~160 mV accelerates transport by ~50-100%. However, measured values of H+/La3+ exchange indicate that only 4-BrA23187 displays a significant electrogenic activity under these conditions. At a La3+ concentration of 17 mM, transport by all three ionophores is electroneutral and apparently occurs through complexes of type (ionophore)3La (A23187 and 4-BrA23187) or (ionophore)La·OH (ionomycin). Analysis of these patterns in a context of comproportionation equilibria involving the transporting species and free La3+ indicates that the species containing three ionophore molecules are formed on the membrane when aqueous phase solution conditions would strongly favor a 1:1 complex, based upon previous studies in solution. The implications of this and other findings are discussed.
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