Dual effect of Nai+ on Ca2+ influx through the Na+/Ca2+ exchanger in dialyzed squid axons. Experimental data confirming the validity of the squid axon kinetic model

• Published in: American Journal of Physiology: Cell Physiology Vol. 294; no. 1; pp. C118 - C125
• Main Authors: Beauge, Luis, DiPolo, Reinaldo
• Format: Journal Article
• Language: English
• Published: United States 01.01.2008
• Subjects: Adenosine Triphosphate - metabolism; Animals; Axons - metabolism; Calcium - metabolism; Dialysis; Hydrogen-Ion Concentration; In Vitro Techniques; Kinetics; Loligo; Membrane Potentials; Models, Neurological; Neural Conduction; Reproducibility of Results; Sodium - metabolism; Sodium-Calcium Exchanger - metabolism
• ISSN: 0363-6143; 1522-1563
• DOI: 10.1152/ajpcell.00341.2007

1 Laboratorio de Biofísica, Instituto de Investigación Médica Mercedes y Martín Ferreyra, Consejo Nacional de Investigaciones Científicas y Técnicas, Córdoba, Argentina; 2 Laboratorio de Fisiología Celular, Centro de Biofísica y Bioquímica, Instituto Venezolano de Investigaciones Científicas, Caracas, Venezuela; and 3 Marine Biological Laboratory, Woods Hole, Massachusetts Submitted 1 August 2007 ; accepted in final form 24 October 2007 We propose a steady-state kinetic model for the squid Na + /Ca 2+ exchanger that differs from other current models of regulation in that it takes into account, within a single kinetic scheme, all ionic [intracellular Ca 2+ (Ca i 2+ )-intracellular Na + (Na i + )-intracellular H i + ] and metabolic (ATP) regulations of the exchanger in which the Ca i 2+ -regulatory pathway plays the central role in regulation. Although the integrated ionic-metabolic model predicts all squid steady-state experimental data on exchange regulation, a critical test for the validity of it is the predicted dual effect of Na i + on steady-state Ca 2+ influx through the exchanger. To test this prediction, an improved technique for the estimation of isotope fluxes in squid axons was developed, which allows sequential measurements of ion influx and effluxes. With this method, we report here two novel observations of the squid axon Na + /Ca 2+ exchanger. First, at intracellular pH (7.0) and in the absence of MgATP, Na i + has a dual effect on Ca 2+ influx: inhibition at low concentrations followed by stimulation at high Na i + concentrations, reaching levels higher than those seen without Na i + . Second, in the presence of MgATP, the biphasic response to Na i + disappears and is replaced by a sigmoid activation. Furthermore, the model predicts that Ca 2+ efflux is monotonically inhibited by Na i + , more pronouncedly without than with MgATP. These results are predicted by the proposed kinetic model. Although not fully applicable to all exchangers, this scheme might provide some insights on expected net Ca 2+ movements in other tissues under a variety of intracellular ionic and metabolic conditions. sodium/calcium exchanger Address for reprint requests and other correspondence: R. DiPolo, Laboratorio de Fisiología Celular, Centro de Biofísica y Bioquímica, Instituto Venezolano de Investigaciones Científicas, Apartado Postal 21827, Caracas 1020-A, Venezuela (e-mail: dipolor{at}ivic.ve )