The Squid Axon Na⁺/Ca²⁺ Exchanger Shows Ping Pong Kinetics only when the Ca_i-regulatory Site is Saturated

In a previous work we demonstrated that, in dialyzed squid axons, an impairment of the Ca2+(i)-regulatory site affected the apparent affinities for external Na+ and Ca2+ in a way opposite to that predicted by the exiting (ping-pong) models for the exchangers. In the present work, we used model simulations and actual experiments where the Ca2+(i)-regulatory remained always saturated while [Ca2+](i) was either limiting or near saturating for the internal Ca2+ transport sites. Under these conditions, both the theoretical and experimental transport activation curves for external Na+ and Ca2+ were those expected from the current kinetic schemes. These observations have two important implications: on the one hand, they confirm the ping-pong translocation schemes for Na+/Ca2+ exchange. On the other, they call for caution in interpreting kinetic data in membrane transport systems possessing intracellular ionic and/or metabolic regulation.