The squid preparation as a general model for ionic and metabolic Na+/Ca2+ exchange interactions -: Physiopathological implications

We propose an integrated kinetic model for the squid nerve Na+/Ca2+ exchanger based on experimental evidences obtained in dialyzed axons. This model satisfactorily explains the interrelationship between ionic (Na-i(+)-H-i(+)-Ca-i(2+)) 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 Cai-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 mu M Ca-i(2+) pH(i) 8.0), ATP have no effect on the Cai-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 Cai-regulatory site plays a central role, accounts for the decrease in Na-i inactivation, at high pH(i), high Ca-i(2+) and MgATP. Furthermore, the model also predicts the net Ca2+ movements across the exchanger based on the exchanger complexes redistribution both during physiological and pathological conditions (ischemia).