The squid axon Na/Ca2+ exchanger shows ping pong kinetics only when the Ca2+(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.
Beaugé
Luis
L
Laboratorio de Biofisica, Instituto de Investigación Médica 'Mercedes y Martin Ferreyra' (INIMEC-CONICET), Córdoba, Argentina.
DiPolo
Reinaldo
R
D000818:Animals; D001369:Axons / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D049832:Decapodiformes / Q000378:metabolism*; D007700:Kinetics; D008954:Models, Biological; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*
D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium