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

Abstract

We propose a steady-state kinetic model for the squid Na+/Ca2+ exchanger that differs from other current models of regulation in that it takes into account, within a single kinetic scheme, all ionic [intracellular Ca2+ (Cai2+)-intracellular Na+ (Nai+)-intracellular Hi+] and metabolic (ATP) regulations of the exchanger in which the Cai2+-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 Nai+ on steady-state Ca2+ 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+/Ca2+ exchanger. First, at intracellular pH (7.0) and in the absence of MgATP, Nai+ has a dual effect on Ca2+ influx: inhibition at low concentrations followed by stimulation at high Nai+ concentrations, reaching levels higher than those seen without Nai+. Second, in the presence of MgATP, the biphasic response to Nai+ disappears and is replaced by a sigmoid activation. Furthermore, the model predicts that Ca2+ efflux is monotonically inhibited by Nai+, 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 Ca2+ movements in other tissues under a variety of intracellular ionic and metabolic conditions.