Effect of Internal ATP-Mg2+ Concentration on the Human Red Blood Cell K+/Ca2+ Exchanger

Abstract

Little is known about the process of senescence of human erythrocytes, however it is accepted that the [Ca2 +]i increases with age. We have previously presented evidence of the existence of a novel transporter capable to account with the Ca2+ intake during the ageing process, the K+/ Ca2+ Exchanger (1)(2). The Activation process of the K+/ Ca2+ Exchanger has a sigmoidal voltage dependence and a permeability sequence: K+>Rb+>>Cs+ and Ca2+>Ba2+>> Mg2+. The ATP-Mg2+ complex is known to regulate the activity of different transporters. In order to determine the effect of the internal ATP-Mg2+ concentration on the K+/ Ca2+ Exchanger's permeability, activation and deactivation, we used Patch-clamp, changing the ATP concentration in the internal solution from 0 to 4mM. We found that exchanger currents increase in the presence of ATP-Mg2+. Interestingly, in the Ca2+ output mode the effect is dose-dependent, and no saturation is observed even at 4mM ATP-Mg2+, increasing from 48% to 200% (500µM to 4mM). On the other hand, in the Ca2+ entry mode, the permeability increases by 36% and this effect is already saturated at 500µM. Also, in this mode there is an increment of 20% in the time course of the deactivation, saturated at 500µM, whereas no such effect on the Ca2+ output mode was observed. These results suggest the existence of either two different binding sites for ATP, or one binding site that changes its affinity with the mode of exchange: Site A with low affinity in the order of mM, affecting permeability in the Ca2+ output mode, and site B, with high affinity in the order of µM, affecting permeability and the deactivation in the Ca2+ entry mode.(1)(2005) Biophys.J.88(1):593(2)(2006) Biophys.J.89(1):456