Does ionophore A23187 mediate Na transport in the absence of divalent cations?

Abstract

THE divalent cation-selective ionophore A23187 is known to form complexes with monovalent cations1, but it is not clear whether such complexes actually translocate across biological membranes. In the absence of Ca and Mg, the ionophore can transport K across the mitochondrial membrane in a manner reminiscent of the nigericin class of ionophores2. From spectrofluorometric and bulk organic phase partition data3, Pfeiffer and Lardy concluded that Na binds to the ionophore better than K (Li > > Na > K ≃ O) and it is surprising that no observations of this Na form of the ionophore crossing biological membranes have been reported. We have used a recently developed method3 designed to study the effect of intracellular Mg on the various ion fluxes mediated by the Na pump, and we report here that intact human red cells incubated in buffered isotonic NaCl media containing EDTA, no added divalent cations and the ionophore A23187, gain Na and swell. This effect was not accompanied by any change in the K permeability and could be prevented or reversed by adding Mg or Ca to the medium. Thus, in certain conditions A23187 may indeed mediate Na transport.