The Squid Preparation as a General Model for Ionic and Metabolic Na~+/Ca~(2+) Exchange Interactions Physiopathological Implications

摘要

We propose an integrated kinetic model for the squid nerve Na~+/Ca~(2+) exchanger based on experimental evidences obtained in dia-lyzed axons. This model satisfactorily explains the interrelationship between ionic (Na~+_i-H~+_i-Ca~(2+)_i) and metabolic (ATP, phosphoarginine (PA)) regulation of the exchanger. Data in dialyzed axons show that the Ca_i-regulatory site located in the large intracellular loop plays a central role in the modulation by ATP by antagonizing the inhibitory Na~+_i-H~+_i synergism. We have used the Na_o/Na_i exchange mode to unequivocally measure the affinity of the Caj-regulatory site. This allowed us to separate Ca_i-regulatory from Ca_i-transport sites and to estimate their respective affinities. In this work we show for the first time that under conditions of saturation of the Ca_i-regulatory site (10 μM Ca~(2+)_i, pH_i 8.0), ATP have no effect on the Ca_i-transport site. In addition, we have expanded our equilibrium kinetic model of ionic and metabolic interactions to a complete exchange cycle (circular model). This model, in which the Ca_i-regulatory site plays a central role, accounts for the decrease in Na_i inactivation, at high pH_i, high Ca~(2+)_i, and MgATP. Furthermore, the model also predicts the net Ca~(2+) movements across the exchanger based on the exchanger complexes redistribution both during physiological and pathological conditions (ischemia).